Set Docker to use 3.11 for now (#3927)

[etc/vim.git] / src / black / __init__.py

diff --git a/src/black/__init__.py b/src/black/__init__.py
index 431ee027270d670086251d6acaba42fc304b0fa4..188a4f79f0e51f027f445525bda3b2e68fd93081 100644 (file)
--- a/src/black/__init__.py
+++ b/src/black/__init__.py
@@ -1,165 +1,92 @@
-import ast
-import asyncio
-from abc import ABC, abstractmethod
-from collections import defaultdict
-from concurrent.futures import Executor, ThreadPoolExecutor, ProcessPoolExecutor
-from contextlib import contextmanager
-from datetime import datetime
-from enum import Enum
-from functools import lru_cache, partial, wraps

 import io
 import io

-import itertools
-import logging
-from multiprocessing import Manager, freeze_support
-import os
-from pathlib import Path
-import pickle
-import regex as re
-import signal
+import json
+import platform
+import re

 import sys
 import sys

-import tempfile

 import tokenize
 import traceback
 import tokenize
 import traceback

+from contextlib import contextmanager
+from dataclasses import replace
+from datetime import datetime, timezone
+from enum import Enum
+from json.decoder import JSONDecodeError
+from pathlib import Path

 from typing import (
     Any,
 from typing import (
     Any,

-    Callable,
-    Collection,

     Dict,
     Generator,
     Dict,
     Generator,

-    Generic,
-    Iterable,

     Iterator,
     List,
     Iterator,
     List,

+    MutableMapping,

     Optional,
     Pattern,
     Sequence,
     Set,
     Sized,
     Tuple,
     Optional,
     Pattern,
     Sequence,
     Set,
     Sized,
     Tuple,

-    Type,
-    TypeVar,

     Union,
     Union,

-    cast,
-    TYPE_CHECKING,

 )
 )

-from mypy_extensions import mypyc_attr

 
 

-from appdirs import user_cache_dir
-from dataclasses import dataclass, field, replace

 import click
 import click

-import toml
-
-try:
-    from typed_ast import ast3, ast27
-except ImportError:
-    if sys.version_info < (3, 8):
-        print(
-            "The typed_ast package is not installed.\n"
-            "You can install it with `python3 -m pip install typed-ast`.",
-            file=sys.stderr,
-        )
-        sys.exit(1)
-    else:
-        ast3 = ast27 = ast
-
+from click.core import ParameterSource
+from mypy_extensions import mypyc_attr

 from pathspec import PathSpec
 from pathspec import PathSpec

-
-# lib2to3 fork
-from blib2to3.pytree import Node, Leaf, type_repr
-from blib2to3 import pygram, pytree
-from blib2to3.pgen2 import driver, token
-from blib2to3.pgen2.grammar import Grammar
-from blib2to3.pgen2.parse import ParseError
+from pathspec.patterns.gitwildmatch import GitWildMatchPatternError

 
 from _black_version import version as __version__
 
 from _black_version import version as __version__

+from black.cache import Cache
+from black.comments import normalize_fmt_off
+from black.const import (
+    DEFAULT_EXCLUDES,
+    DEFAULT_INCLUDES,
+    DEFAULT_LINE_LENGTH,
+    STDIN_PLACEHOLDER,
+)
+from black.files import (
+    find_project_root,
+    find_pyproject_toml,
+    find_user_pyproject_toml,
+    gen_python_files,
+    get_gitignore,
+    normalize_path_maybe_ignore,
+    parse_pyproject_toml,
+    wrap_stream_for_windows,
+)
+from black.handle_ipynb_magics import (
+    PYTHON_CELL_MAGICS,
+    TRANSFORMED_MAGICS,
+    jupyter_dependencies_are_installed,
+    mask_cell,
+    put_trailing_semicolon_back,
+    remove_trailing_semicolon,
+    unmask_cell,
+)
+from black.linegen import LN, LineGenerator, transform_line
+from black.lines import EmptyLineTracker, LinesBlock
+from black.mode import FUTURE_FLAG_TO_FEATURE, VERSION_TO_FEATURES, Feature
+from black.mode import Mode as Mode  # re-exported
+from black.mode import TargetVersion, supports_feature
+from black.nodes import (
+    STARS,
+    is_number_token,
+    is_simple_decorator_expression,
+    is_string_token,
+    syms,
+)
+from black.output import color_diff, diff, dump_to_file, err, ipynb_diff, out
+from black.parsing import InvalidInput  # noqa F401
+from black.parsing import lib2to3_parse, parse_ast, stringify_ast
+from black.report import Changed, NothingChanged, Report
+from black.trans import iter_fexpr_spans
+from blib2to3.pgen2 import token
+from blib2to3.pytree import Leaf, Node

 
 

-if sys.version_info < (3, 8):
-    from typing_extensions import Final
-else:
-    from typing import Final
-
-if TYPE_CHECKING:
-    import colorama  # noqa: F401
-
-DEFAULT_LINE_LENGTH = 88
-DEFAULT_EXCLUDES = r"/(\.direnv|\.eggs|\.git|\.hg|\.mypy_cache|\.nox|\.tox|\.venv|\.svn|_build|buck-out|build|dist)/"  # noqa: B950
-DEFAULT_INCLUDES = r"\.pyi?$"
-CACHE_DIR = Path(user_cache_dir("black", version=__version__))
-STDIN_PLACEHOLDER = "__BLACK_STDIN_FILENAME__"
-
-STRING_PREFIX_CHARS: Final = "furbFURB"  # All possible string prefix characters.
-
+COMPILED = Path(__file__).suffix in (".pyd", ".so")

 
 # types
 FileContent = str
 Encoding = str
 NewLine = str
 
 # types
 FileContent = str
 Encoding = str
 NewLine = str

-Depth = int
-NodeType = int
-ParserState = int
-LeafID = int
-StringID = int
-Priority = int
-Index = int
-LN = Union[Leaf, Node]
-Transformer = Callable[["Line", Collection["Feature"]], Iterator["Line"]]
-Timestamp = float
-FileSize = int
-CacheInfo = Tuple[Timestamp, FileSize]
-Cache = Dict[str, CacheInfo]
-out = partial(click.secho, bold=True, err=True)
-err = partial(click.secho, fg="red", err=True)
-
-pygram.initialize(CACHE_DIR)
-syms = pygram.python_symbols
-
-
-class NothingChanged(UserWarning):
-    """Raised when reformatted code is the same as source."""
-
-
-class CannotTransform(Exception):
-    """Base class for errors raised by Transformers."""
-
-
-class CannotSplit(CannotTransform):
-    """A readable split that fits the allotted line length is impossible."""
-
-
-class InvalidInput(ValueError):
-    """Raised when input source code fails all parse attempts."""
-
-
-class BracketMatchError(KeyError):
-    """Raised when an opening bracket is unable to be matched to a closing bracket."""
-
-
-T = TypeVar("T")
-E = TypeVar("E", bound=Exception)
-
-
-class Ok(Generic[T]):
-    def __init__(self, value: T) -> None:
-        self._value = value
-
-    def ok(self) -> T:
-        return self._value
-
-
-class Err(Generic[E]):
-    def __init__(self, e: E) -> None:
-        self._e = e
-
-    def err(self) -> E:
-        return self._e
-
-
-# The 'Result' return type is used to implement an error-handling model heavily
-# influenced by that used by the Rust programming language
-# (see https://doc.rust-lang.org/book/ch09-00-error-handling.html).
-Result = Union[Ok[T], Err[E]]
-TResult = Result[T, CannotTransform]  # (T)ransform Result
-TMatchResult = TResult[Index]

 
 
 class WriteBack(Enum):
 
 
 class WriteBack(Enum):


@@ -182,147 +109,10 @@ class WriteBack(Enum):
         return cls.DIFF if diff else cls.YES
 
 
         return cls.DIFF if diff else cls.YES
 
 

-class Changed(Enum):
-    NO = 0
-    CACHED = 1
-    YES = 2
-
-
-class TargetVersion(Enum):
-    PY27 = 2
-    PY33 = 3
-    PY34 = 4
-    PY35 = 5
-    PY36 = 6
-    PY37 = 7
-    PY38 = 8
-    PY39 = 9
-
-    def is_python2(self) -> bool:
-        return self is TargetVersion.PY27
-
-
-class Feature(Enum):
-    # All string literals are unicode
-    UNICODE_LITERALS = 1
-    F_STRINGS = 2
-    NUMERIC_UNDERSCORES = 3
-    TRAILING_COMMA_IN_CALL = 4
-    TRAILING_COMMA_IN_DEF = 5
-    # The following two feature-flags are mutually exclusive, and exactly one should be
-    # set for every version of python.
-    ASYNC_IDENTIFIERS = 6
-    ASYNC_KEYWORDS = 7
-    ASSIGNMENT_EXPRESSIONS = 8
-    POS_ONLY_ARGUMENTS = 9
-    RELAXED_DECORATORS = 10
-    FORCE_OPTIONAL_PARENTHESES = 50
-
-
-VERSION_TO_FEATURES: Dict[TargetVersion, Set[Feature]] = {
-    TargetVersion.PY27: {Feature.ASYNC_IDENTIFIERS},
-    TargetVersion.PY33: {Feature.UNICODE_LITERALS, Feature.ASYNC_IDENTIFIERS},
-    TargetVersion.PY34: {Feature.UNICODE_LITERALS, Feature.ASYNC_IDENTIFIERS},
-    TargetVersion.PY35: {
-        Feature.UNICODE_LITERALS,
-        Feature.TRAILING_COMMA_IN_CALL,
-        Feature.ASYNC_IDENTIFIERS,
-    },
-    TargetVersion.PY36: {
-        Feature.UNICODE_LITERALS,
-        Feature.F_STRINGS,
-        Feature.NUMERIC_UNDERSCORES,
-        Feature.TRAILING_COMMA_IN_CALL,
-        Feature.TRAILING_COMMA_IN_DEF,
-        Feature.ASYNC_IDENTIFIERS,
-    },
-    TargetVersion.PY37: {
-        Feature.UNICODE_LITERALS,
-        Feature.F_STRINGS,
-        Feature.NUMERIC_UNDERSCORES,
-        Feature.TRAILING_COMMA_IN_CALL,
-        Feature.TRAILING_COMMA_IN_DEF,
-        Feature.ASYNC_KEYWORDS,
-    },
-    TargetVersion.PY38: {
-        Feature.UNICODE_LITERALS,
-        Feature.F_STRINGS,
-        Feature.NUMERIC_UNDERSCORES,
-        Feature.TRAILING_COMMA_IN_CALL,
-        Feature.TRAILING_COMMA_IN_DEF,
-        Feature.ASYNC_KEYWORDS,
-        Feature.ASSIGNMENT_EXPRESSIONS,
-        Feature.POS_ONLY_ARGUMENTS,
-    },
-    TargetVersion.PY39: {
-        Feature.UNICODE_LITERALS,
-        Feature.F_STRINGS,
-        Feature.NUMERIC_UNDERSCORES,
-        Feature.TRAILING_COMMA_IN_CALL,
-        Feature.TRAILING_COMMA_IN_DEF,
-        Feature.ASYNC_KEYWORDS,
-        Feature.ASSIGNMENT_EXPRESSIONS,
-        Feature.RELAXED_DECORATORS,
-        Feature.POS_ONLY_ARGUMENTS,
-    },
-}
-
-
-@dataclass
-class Mode:
-    target_versions: Set[TargetVersion] = field(default_factory=set)
-    line_length: int = DEFAULT_LINE_LENGTH
-    string_normalization: bool = True
-    magic_trailing_comma: bool = True
-    experimental_string_processing: bool = False
-    is_pyi: bool = False
-
-    def get_cache_key(self) -> str:
-        if self.target_versions:
-            version_str = ",".join(
-                str(version.value)
-                for version in sorted(self.target_versions, key=lambda v: v.value)
-            )
-        else:
-            version_str = "-"
-        parts = [
-            version_str,
-            str(self.line_length),
-            str(int(self.string_normalization)),
-            str(int(self.is_pyi)),
-        ]
-        return ".".join(parts)
-
-

 # Legacy name, left for integrations.
 FileMode = Mode
 
 
 # Legacy name, left for integrations.
 FileMode = Mode
 
 

-def supports_feature(target_versions: Set[TargetVersion], feature: Feature) -> bool:
-    return all(feature in VERSION_TO_FEATURES[version] for version in target_versions)
-
-
-def find_pyproject_toml(path_search_start: Iterable[str]) -> Optional[str]:
-    """Find the absolute filepath to a pyproject.toml if it exists"""
-    path_project_root = find_project_root(path_search_start)
-    path_pyproject_toml = path_project_root / "pyproject.toml"
-    if path_pyproject_toml.is_file():
-        return str(path_pyproject_toml)
-
-    path_user_pyproject_toml = find_user_pyproject_toml()
-    return str(path_user_pyproject_toml) if path_user_pyproject_toml.is_file() else None
-
-
-def parse_pyproject_toml(path_config: str) -> Dict[str, Any]:
-    """Parse a pyproject toml file, pulling out relevant parts for Black
-
-    If parsing fails, will raise a toml.TomlDecodeError
-    """
-    pyproject_toml = toml.load(path_config)
-    config = pyproject_toml.get("tool", {}).get("black", {})
-    return {k.replace("--", "").replace("-", "_"): v for k, v in config.items()}
-
-

 def read_pyproject_toml(
     ctx: click.Context, param: click.Parameter, value: Optional[str]
 ) -> Optional[str]:
 def read_pyproject_toml(
     ctx: click.Context, param: click.Parameter, value: Optional[str]
 ) -> Optional[str]:


@@ -332,16 +122,18 @@ def read_pyproject_toml(
     otherwise.
     """
     if not value:
     otherwise.
     """
     if not value:

-        value = find_pyproject_toml(ctx.params.get("src", ()))
+        value = find_pyproject_toml(
+            ctx.params.get("src", ()), ctx.params.get("stdin_filename", None)
+        )

         if value is None:
             return None
 
     try:
         config = parse_pyproject_toml(value)
         if value is None:
             return None
 
     try:
         config = parse_pyproject_toml(value)

-    except (toml.TomlDecodeError, OSError) as e:
+    except (OSError, ValueError) as e:

         raise click.FileError(
             filename=value, hint=f"Error reading configuration file: {e}"
         raise click.FileError(
             filename=value, hint=f"Error reading configuration file: {e}"

-        )
+        ) from None

 
     if not config:
         return None
 
     if not config:
         return None


@@ -360,6 +152,16 @@ def read_pyproject_toml(
             "target-version", "Config key target-version must be a list"
         )
 
             "target-version", "Config key target-version must be a list"
         )
 

+    exclude = config.get("exclude")
+    if exclude is not None and not isinstance(exclude, str):
+        raise click.BadOptionUsage("exclude", "Config key exclude must be a string")
+
+    extend_exclude = config.get("extend_exclude")
+    if extend_exclude is not None and not isinstance(extend_exclude, str):
+        raise click.BadOptionUsage(
+            "extend-exclude", "Config key extend-exclude must be a string"
+        )
+

     default_map: Dict[str, Any] = {}
     if ctx.default_map:
         default_map.update(ctx.default_map)
     default_map: Dict[str, Any] = {}
     if ctx.default_map:
         default_map.update(ctx.default_map)


@@ -380,18 +182,34 @@ def target_version_option_callback(
     return [TargetVersion[val.upper()] for val in v]
 
 
     return [TargetVersion[val.upper()] for val in v]
 
 

+def re_compile_maybe_verbose(regex: str) -> Pattern[str]:
+    """Compile a regular expression string in `regex`.
+
+    If it contains newlines, use verbose mode.
+    """
+    if "\n" in regex:
+        regex = "(?x)" + regex
+    compiled: Pattern[str] = re.compile(regex)
+    return compiled
+
+

 def validate_regex(
     ctx: click.Context,
     param: click.Parameter,
     value: Optional[str],
 def validate_regex(
     ctx: click.Context,
     param: click.Parameter,
     value: Optional[str],

-) -> Optional[Pattern]:
+) -> Optional[Pattern[str]]:

     try:
         return re_compile_maybe_verbose(value) if value is not None else None
     try:
         return re_compile_maybe_verbose(value) if value is not None else None

-    except re.error:
-        raise click.BadParameter("Not a valid regular expression")
+    except re.error as e:
+        raise click.BadParameter(f"Not a valid regular expression: {e}") from None

 
 
 
 

-@click.command(context_settings=dict(help_option_names=["-h", "--help"]))
+@click.command(
+    context_settings={"help_option_names": ["-h", "--help"]},
+    # While Click does set this field automatically using the docstring, mypyc
+    # (annoyingly) strips 'em so we need to set it here too.
+    help="The uncompromising code formatter.",
+)

 @click.option("-c", "--code", type=str, help="Format the code passed in as a string.")
 @click.option(
     "-l",
 @click.option("-c", "--code", type=str, help="Format the code passed in as a string.")
 @click.option(
     "-l",


@@ -408,8 +226,9 @@ def validate_regex(
     callback=target_version_option_callback,
     multiple=True,
     help=(
     callback=target_version_option_callback,
     multiple=True,
     help=(

-        "Python versions that should be supported by Black's output. [default: per-file"
-        " auto-detection]"
+        "Python versions that should be supported by Black's output. By default, Black"
+        " will try to infer this from the project metadata in pyproject.toml. If this"
+        " does not yield conclusive results, Black will use per-file auto-detection."

     ),
 )
 @click.option(
     ),
 )
 @click.option(


@@ -420,6 +239,30 @@ def validate_regex(
         " when piping source on standard input)."
     ),
 )
         " when piping source on standard input)."
     ),
 )

+@click.option(
+    "--ipynb",
+    is_flag=True,
+    help=(
+        "Format all input files like Jupyter Notebooks regardless of file extension "
+        "(useful when piping source on standard input)."
+    ),
+)
+@click.option(
+    "--python-cell-magics",
+    multiple=True,
+    help=(
+        "When processing Jupyter Notebooks, add the given magic to the list"
+        f" of known python-magics ({', '.join(sorted(PYTHON_CELL_MAGICS))})."
+        " Useful for formatting cells with custom python magics."
+    ),
+    default=[],
+)
+@click.option(
+    "-x",
+    "--skip-source-first-line",
+    is_flag=True,
+    help="Skip the first line of the source code.",
+)

 @click.option(
     "-S",
     "--skip-string-normalization",
 @click.option(
     "-S",
     "--skip-string-normalization",


@@ -436,9 +279,14 @@ def validate_regex(
     "--experimental-string-processing",
     is_flag=True,
     hidden=True,
     "--experimental-string-processing",
     is_flag=True,
     hidden=True,

+    help="(DEPRECATED and now included in --preview) Normalize string literals.",
+)
+@click.option(
+    "--preview",
+    is_flag=True,

     help=(
     help=(

-        "Experimental option that performs more normalization on string literals."
-        " Currently disabled because it leads to some crashes."
+        "Enable potentially disruptive style changes that may be added to Black's main"
+        " functionality in the next major release."

     ),
 )
 @click.option(
     ),
 )
 @click.option(


@@ -465,6 +313,15 @@ def validate_regex(
     is_flag=True,
     help="If --fast given, skip temporary sanity checks. [default: --safe]",
 )
     is_flag=True,
     help="If --fast given, skip temporary sanity checks. [default: --safe]",
 )

+@click.option(
+    "--required-version",
+    type=str,
+    help=(
+        "Require a specific version of Black to be running (useful for unifying results"
+        " across many environments e.g. with a pyproject.toml file). It can be"
+        " either a major version number or an exact version."
+    ),
+)

 @click.option(
     "--include",
     type=str,
 @click.option(
     "--include",
     type=str,


@@ -481,15 +338,15 @@ def validate_regex(
 @click.option(
     "--exclude",
     type=str,
 @click.option(
     "--exclude",
     type=str,

-    default=DEFAULT_EXCLUDES,

     callback=validate_regex,
     help=(
         "A regular expression that matches files and directories that should be"
         " excluded on recursive searches. An empty value means no paths are excluded."
         " Use forward slashes for directories on all platforms (Windows, too)."
     callback=validate_regex,
     help=(
         "A regular expression that matches files and directories that should be"
         " excluded on recursive searches. An empty value means no paths are excluded."
         " Use forward slashes for directories on all platforms (Windows, too)."

-        " Exclusions are calculated first, inclusions later."
+        " Exclusions are calculated first, inclusions later. [default:"
+        f" {DEFAULT_EXCLUDES}]"

     ),
     ),

-    show_default=True,
+    show_default=False,

 )
 @click.option(
     "--extend-exclude",
 )
 @click.option(
     "--extend-exclude",


@@ -512,12 +369,23 @@ def validate_regex(
 @click.option(
     "--stdin-filename",
     type=str,
 @click.option(
     "--stdin-filename",
     type=str,

+    is_eager=True,

     help=(
         "The name of the file when passing it through stdin. Useful to make "
         "sure Black will respect --force-exclude option on some "
         "editors that rely on using stdin."
     ),
 )
     help=(
         "The name of the file when passing it through stdin. Useful to make "
         "sure Black will respect --force-exclude option on some "
         "editors that rely on using stdin."
     ),
 )

+@click.option(
+    "-W",
+    "--workers",
+    type=click.IntRange(min=1),
+    default=None,
+    help=(
+        "Number of parallel workers [default: BLACK_NUM_WORKERS environment variable "
+        "or number of CPUs in the system]"
+    ),
+)

 @click.option(
     "-q",
     "--quiet",
 @click.option(
     "-q",
     "--quiet",


@@ -536,7 +404,13 @@ def validate_regex(
         " due to exclusion patterns."
     ),
 )
         " due to exclusion patterns."
     ),
 )

-@click.version_option(version=__version__)
+@click.version_option(
+    version=__version__,
+    message=(
+        f"%(prog)s, %(version)s (compiled: {'yes' if COMPILED else 'no'})\n"
+        f"Python ({platform.python_implementation()}) {platform.python_version()}"
+    ),
+)

 @click.argument(
     "src",
     nargs=-1,
 @click.argument(
     "src",
     nargs=-1,


@@ -544,6 +418,7 @@ def validate_regex(
         exists=True, file_okay=True, dir_okay=True, readable=True, allow_dash=True
     ),
     is_eager=True,
         exists=True, file_okay=True, dir_okay=True, readable=True, allow_dash=True
     ),
     is_eager=True,

+    metavar="SRC ...",

 )
 @click.option(
     "--config",
 )
 @click.option(
     "--config",


@@ -560,7 +435,7 @@ def validate_regex(
     help="Read configuration from FILE path.",
 )
 @click.pass_context
     help="Read configuration from FILE path.",
 )
 @click.pass_context

-def main(
+def main(  # noqa: C901

     ctx: click.Context,
     code: Optional[str],
     line_length: int,
     ctx: click.Context,
     code: Optional[str],
     line_length: int,


@@ -570,20 +445,85 @@ def main(
     color: bool,
     fast: bool,
     pyi: bool,
     color: bool,
     fast: bool,
     pyi: bool,

+    ipynb: bool,
+    python_cell_magics: Sequence[str],
+    skip_source_first_line: bool,

     skip_string_normalization: bool,
     skip_magic_trailing_comma: bool,
     experimental_string_processing: bool,
     skip_string_normalization: bool,
     skip_magic_trailing_comma: bool,
     experimental_string_processing: bool,

+    preview: bool,

     quiet: bool,
     verbose: bool,
     quiet: bool,
     verbose: bool,

-    include: Pattern,
-    exclude: Pattern,
-    extend_exclude: Optional[Pattern],
-    force_exclude: Optional[Pattern],
+    required_version: Optional[str],
+    include: Pattern[str],
+    exclude: Optional[Pattern[str]],
+    extend_exclude: Optional[Pattern[str]],
+    force_exclude: Optional[Pattern[str]],

     stdin_filename: Optional[str],
     stdin_filename: Optional[str],

+    workers: Optional[int],

     src: Tuple[str, ...],
     config: Optional[str],
 ) -> None:
     """The uncompromising code formatter."""
     src: Tuple[str, ...],
     config: Optional[str],
 ) -> None:
     """The uncompromising code formatter."""

+    ctx.ensure_object(dict)
+
+    if src and code is not None:
+        out(
+            main.get_usage(ctx)
+            + "\n\n'SRC' and 'code' cannot be passed simultaneously."
+        )
+        ctx.exit(1)
+    if not src and code is None:
+        out(main.get_usage(ctx) + "\n\nOne of 'SRC' or 'code' is required.")
+        ctx.exit(1)
+
+    root, method = (
+        find_project_root(src, stdin_filename) if code is None else (None, None)
+    )
+    ctx.obj["root"] = root
+
+    if verbose:
+        if root:
+            out(
+                f"Identified `{root}` as project root containing a {method}.",
+                fg="blue",
+            )
+
+        if config:
+            config_source = ctx.get_parameter_source("config")
+            user_level_config = str(find_user_pyproject_toml())
+            if config == user_level_config:
+                out(
+                    "Using configuration from user-level config at "
+                    f"'{user_level_config}'.",
+                    fg="blue",
+                )
+            elif config_source in (
+                ParameterSource.DEFAULT,
+                ParameterSource.DEFAULT_MAP,
+            ):
+                out("Using configuration from project root.", fg="blue")
+            else:
+                out(f"Using configuration in '{config}'.", fg="blue")
+            if ctx.default_map:
+                for param, value in ctx.default_map.items():
+                    out(f"{param}: {value}")
+
+    error_msg = "Oh no! 💥 💔 💥"
+    if (
+        required_version
+        and required_version != __version__
+        and required_version != __version__.split(".")[0]
+    ):
+        err(
+            f"{error_msg} The required version `{required_version}` does not match"
+            f" the running version `{__version__}`!"
+        )
+        ctx.exit(1)
+    if ipynb and pyi:
+        err("Cannot pass both `pyi` and `ipynb` flags!")
+        ctx.exit(1)
+

     write_back = WriteBack.from_configuration(check=check, diff=diff, color=color)
     if target_version:
         versions = set(target_version)
     write_back = WriteBack.from_configuration(check=check, diff=diff, color=color)
     if target_version:
         versions = set(target_version)


@@ -594,75 +534,101 @@ def main(
         target_versions=versions,
         line_length=line_length,
         is_pyi=pyi,
         target_versions=versions,
         line_length=line_length,
         is_pyi=pyi,

+        is_ipynb=ipynb,
+        skip_source_first_line=skip_source_first_line,

         string_normalization=not skip_string_normalization,
         magic_trailing_comma=not skip_magic_trailing_comma,
         experimental_string_processing=experimental_string_processing,
         string_normalization=not skip_string_normalization,
         magic_trailing_comma=not skip_magic_trailing_comma,
         experimental_string_processing=experimental_string_processing,

+        preview=preview,
+        python_cell_magics=set(python_cell_magics),

     )
     )

-    if config and verbose:
-        out(f"Using configuration from {config}.", bold=False, fg="blue")
+

     if code is not None:
     if code is not None:

-        print(format_str(code, mode=mode))
-        ctx.exit(0)
-    report = Report(check=check, diff=diff, quiet=quiet, verbose=verbose)
-    sources = get_sources(
-        ctx=ctx,
-        src=src,
-        quiet=quiet,
-        verbose=verbose,
-        include=include,
-        exclude=exclude,
-        extend_exclude=extend_exclude,
-        force_exclude=force_exclude,
-        report=report,
-        stdin_filename=stdin_filename,
-    )
+        # Run in quiet mode by default with -c; the extra output isn't useful.
+        # You can still pass -v to get verbose output.
+        quiet = True

 
 

-    path_empty(
-        sources,
-        "No Python files are present to be formatted. Nothing to do 😴",
-        quiet,
-        verbose,
-        ctx,
-    )
+    report = Report(check=check, diff=diff, quiet=quiet, verbose=verbose)

 
 

-    if len(sources) == 1:
-        reformat_one(
-            src=sources.pop(),
-            fast=fast,
-            write_back=write_back,
-            mode=mode,
-            report=report,
+    if code is not None:
+        reformat_code(
+            content=code, fast=fast, write_back=write_back, mode=mode, report=report

         )
     else:
         )
     else:

-        reformat_many(
-            sources=sources, fast=fast, write_back=write_back, mode=mode, report=report
+        assert root is not None  # root is only None if code is not None
+        try:
+            sources = get_sources(
+                root=root,
+                src=src,
+                quiet=quiet,
+                verbose=verbose,
+                include=include,
+                exclude=exclude,
+                extend_exclude=extend_exclude,
+                force_exclude=force_exclude,
+                report=report,
+                stdin_filename=stdin_filename,
+            )
+        except GitWildMatchPatternError:
+            ctx.exit(1)
+
+        path_empty(
+            sources,
+            "No Python files are present to be formatted. Nothing to do 😴",
+            quiet,
+            verbose,
+            ctx,

         )
 
         )
 

+        if len(sources) == 1:
+            reformat_one(
+                src=sources.pop(),
+                fast=fast,
+                write_back=write_back,
+                mode=mode,
+                report=report,
+            )
+        else:
+            from black.concurrency import reformat_many
+
+            reformat_many(
+                sources=sources,
+                fast=fast,
+                write_back=write_back,
+                mode=mode,
+                report=report,
+                workers=workers,
+            )
+

     if verbose or not quiet:
     if verbose or not quiet:

-        out("Oh no! 💥 💔 💥" if report.return_code else "All done! ✨ 🍰 ✨")
-        click.secho(str(report), err=True)
+        if code is None and (verbose or report.change_count or report.failure_count):
+            out()
+        out(error_msg if report.return_code else "All done! ✨ 🍰 ✨")
+        if code is None:
+            click.echo(str(report), err=True)

     ctx.exit(report.return_code)
 
 
 def get_sources(
     *,
     ctx.exit(report.return_code)
 
 
 def get_sources(
     *,

-    ctx: click.Context,
+    root: Path,

     src: Tuple[str, ...],
     quiet: bool,
     verbose: bool,
     include: Pattern[str],
     src: Tuple[str, ...],
     quiet: bool,
     verbose: bool,
     include: Pattern[str],

-    exclude: Pattern[str],
+    exclude: Optional[Pattern[str]],

     extend_exclude: Optional[Pattern[str]],
     force_exclude: Optional[Pattern[str]],
     report: "Report",
     stdin_filename: Optional[str],
 ) -> Set[Path]:
     """Compute the set of files to be formatted."""
     extend_exclude: Optional[Pattern[str]],
     force_exclude: Optional[Pattern[str]],
     report: "Report",
     stdin_filename: Optional[str],
 ) -> Set[Path]:
     """Compute the set of files to be formatted."""

-
-    root = find_project_root(src)

     sources: Set[Path] = set()
     sources: Set[Path] = set()

-    path_empty(src, "No Path provided. Nothing to do 😴", quiet, verbose, ctx)
-    gitignore = get_gitignore(root)
+
+    using_default_exclude = exclude is None
+    exclude = re_compile_maybe_verbose(DEFAULT_EXCLUDES) if exclude is None else exclude
+    gitignore: Optional[Dict[Path, PathSpec]] = None
+    root_gitignore = get_gitignore(root)

 
     for s in src:
         if s == "-" and stdin_filename:
 
     for s in src:
         if s == "-" and stdin_filename:


@@ -673,9 +639,15 @@ def get_sources(
             is_stdin = False
 
         if is_stdin or p.is_file():
             is_stdin = False
 
         if is_stdin or p.is_file():

-            normalized_path = normalize_path_maybe_ignore(p, root, report)
+            normalized_path: Optional[str] = normalize_path_maybe_ignore(
+                p, root, report
+            )

             if normalized_path is None:
             if normalized_path is None:

+                if verbose:
+                    out(f'Skipping invalid source: "{normalized_path}"', fg="red")

                 continue
                 continue

+            if verbose:
+                out(f'Found input source: "{normalized_path}"', fg="blue")

 
             normalized_path = "/" + normalized_path
             # Hard-exclude any files that matches the `--force-exclude` regex.
 
             normalized_path = "/" + normalized_path
             # Hard-exclude any files that matches the `--force-exclude` regex.


@@ -690,8 +662,24 @@ def get_sources(
             if is_stdin:
                 p = Path(f"{STDIN_PLACEHOLDER}{str(p)}")
 
             if is_stdin:
                 p = Path(f"{STDIN_PLACEHOLDER}{str(p)}")
 

+            if p.suffix == ".ipynb" and not jupyter_dependencies_are_installed(
+                warn=verbose or not quiet
+            ):
+                continue
+

             sources.add(p)
         elif p.is_dir():
             sources.add(p)
         elif p.is_dir():

+            p_relative = normalize_path_maybe_ignore(p, root, report)
+            assert p_relative is not None
+            p = root / p_relative
+            if verbose:
+                out(f'Found input source directory: "{p}"', fg="blue")
+
+            if using_default_exclude:
+                gitignore = {
+                    root: root_gitignore,
+                    p: get_gitignore(p),
+                }

             sources.update(
                 gen_python_files(
                     p.iterdir(),
             sources.update(
                 gen_python_files(
                     p.iterdir(),


@@ -702,12 +690,17 @@ def get_sources(
                     force_exclude,
                     report,
                     gitignore,
                     force_exclude,
                     report,
                     gitignore,

+                    verbose=verbose,
+                    quiet=quiet,

                 )
             )
         elif s == "-":
                 )
             )
         elif s == "-":

+            if verbose:
+                out("Found input source stdin", fg="blue")

             sources.add(p)
         else:
             err(f"invalid path: {s}")
             sources.add(p)
         else:
             err(f"invalid path: {s}")

+

     return sources
 
 
     return sources
 
 


@@ -717,11 +710,39 @@ def path_empty(
     """
     Exit if there is no `src` provided for formatting
     """
     """
     Exit if there is no `src` provided for formatting
     """

-    if not src and (verbose or not quiet):
-        out(msg)
+    if not src:
+        if verbose or not quiet:
+            out(msg)

         ctx.exit(0)
 
 
         ctx.exit(0)
 
 

+def reformat_code(
+    content: str, fast: bool, write_back: WriteBack, mode: Mode, report: Report
+) -> None:
+    """
+    Reformat and print out `content` without spawning child processes.
+    Similar to `reformat_one`, but for string content.
+
+    `fast`, `write_back`, and `mode` options are passed to
+    :func:`format_file_in_place` or :func:`format_stdin_to_stdout`.
+    """
+    path = Path("<string>")
+    try:
+        changed = Changed.NO
+        if format_stdin_to_stdout(
+            content=content, fast=fast, write_back=write_back, mode=mode
+        ):
+            changed = Changed.YES
+        report.done(path, changed)
+    except Exception as exc:
+        if report.verbose:
+            traceback.print_exc()
+        report.failed(path, str(exc))
+
+
+# diff-shades depends on being to monkeypatch this function to operate. I know it's
+# not ideal, but this shouldn't cause any issues ... hopefully. ~ichard26
+@mypyc_attr(patchable=True)

 def reformat_one(
     src: Path, fast: bool, write_back: WriteBack, mode: Mode, report: "Report"
 ) -> None:
 def reformat_one(
     src: Path, fast: bool, write_back: WriteBack, mode: Mode, report: "Report"
 ) -> None:


@@ -744,15 +765,16 @@ def reformat_one(
             is_stdin = False
 
         if is_stdin:
             is_stdin = False
 
         if is_stdin:

+            if src.suffix == ".pyi":
+                mode = replace(mode, is_pyi=True)
+            elif src.suffix == ".ipynb":
+                mode = replace(mode, is_ipynb=True)

             if format_stdin_to_stdout(fast=fast, write_back=write_back, mode=mode):
                 changed = Changed.YES
         else:
             if format_stdin_to_stdout(fast=fast, write_back=write_back, mode=mode):
                 changed = Changed.YES
         else:

-            cache: Cache = {}
+            cache = Cache.read(mode)

             if write_back not in (WriteBack.DIFF, WriteBack.COLOR_DIFF):
             if write_back not in (WriteBack.DIFF, WriteBack.COLOR_DIFF):

-                cache = read_cache(mode)
-                res_src = src.resolve()
-                res_src_s = str(res_src)
-                if res_src_s in cache and cache[res_src_s] == get_cache_info(res_src):
+                if not cache.is_changed(src):

                     changed = Changed.CACHED
             if changed is not Changed.CACHED and format_file_in_place(
                 src, fast=fast, write_back=write_back, mode=mode
                     changed = Changed.CACHED
             if changed is not Changed.CACHED and format_file_in_place(
                 src, fast=fast, write_back=write_back, mode=mode


@@ -761,7 +783,7 @@ def reformat_one(
             if (write_back is WriteBack.YES and changed is not Changed.CACHED) or (
                 write_back is WriteBack.CHECK and changed is Changed.NO
             ):
             if (write_back is WriteBack.YES and changed is not Changed.CACHED) or (
                 write_back is WriteBack.CHECK and changed is Changed.NO
             ):

-                write_cache(cache, [src], mode)
+                cache.write([src])

         report.done(src, changed)
     except Exception as exc:
         if report.verbose:
         report.done(src, changed)
     except Exception as exc:
         if report.verbose:


@@ -769,114 +791,6 @@ def reformat_one(
         report.failed(src, str(exc))
 
 
         report.failed(src, str(exc))
 
 

-def reformat_many(
-    sources: Set[Path], fast: bool, write_back: WriteBack, mode: Mode, report: "Report"
-) -> None:
-    """Reformat multiple files using a ProcessPoolExecutor."""
-    executor: Executor
-    loop = asyncio.get_event_loop()
-    worker_count = os.cpu_count()
-    if sys.platform == "win32":
-        # Work around https://bugs.python.org/issue26903
-        worker_count = min(worker_count, 60)
-    try:
-        executor = ProcessPoolExecutor(max_workers=worker_count)
-    except (ImportError, OSError):
-        # we arrive here if the underlying system does not support multi-processing
-        # like in AWS Lambda or Termux, in which case we gracefully fallback to
-        # a ThreadPollExecutor with just a single worker (more workers would not do us
-        # any good due to the Global Interpreter Lock)
-        executor = ThreadPoolExecutor(max_workers=1)
-
-    try:
-        loop.run_until_complete(
-            schedule_formatting(
-                sources=sources,
-                fast=fast,
-                write_back=write_back,
-                mode=mode,
-                report=report,
-                loop=loop,
-                executor=executor,
-            )
-        )
-    finally:
-        shutdown(loop)
-        if executor is not None:
-            executor.shutdown()
-
-
-async def schedule_formatting(
-    sources: Set[Path],
-    fast: bool,
-    write_back: WriteBack,
-    mode: Mode,
-    report: "Report",
-    loop: asyncio.AbstractEventLoop,
-    executor: Executor,
-) -> None:
-    """Run formatting of `sources` in parallel using the provided `executor`.
-
-    (Use ProcessPoolExecutors for actual parallelism.)
-
-    `write_back`, `fast`, and `mode` options are passed to
-    :func:`format_file_in_place`.
-    """
-    cache: Cache = {}
-    if write_back not in (WriteBack.DIFF, WriteBack.COLOR_DIFF):
-        cache = read_cache(mode)
-        sources, cached = filter_cached(cache, sources)
-        for src in sorted(cached):
-            report.done(src, Changed.CACHED)
-    if not sources:
-        return
-
-    cancelled = []
-    sources_to_cache = []
-    lock = None
-    if write_back in (WriteBack.DIFF, WriteBack.COLOR_DIFF):
-        # For diff output, we need locks to ensure we don't interleave output
-        # from different processes.
-        manager = Manager()
-        lock = manager.Lock()
-    tasks = {
-        asyncio.ensure_future(
-            loop.run_in_executor(
-                executor, format_file_in_place, src, fast, mode, write_back, lock
-            )
-        ): src
-        for src in sorted(sources)
-    }
-    pending: Iterable["asyncio.Future[bool]"] = tasks.keys()
-    try:
-        loop.add_signal_handler(signal.SIGINT, cancel, pending)
-        loop.add_signal_handler(signal.SIGTERM, cancel, pending)
-    except NotImplementedError:
-        # There are no good alternatives for these on Windows.
-        pass
-    while pending:
-        done, _ = await asyncio.wait(pending, return_when=asyncio.FIRST_COMPLETED)
-        for task in done:
-            src = tasks.pop(task)
-            if task.cancelled():
-                cancelled.append(task)
-            elif task.exception():
-                report.failed(src, str(task.exception()))
-            else:
-                changed = Changed.YES if task.result() else Changed.NO
-                # If the file was written back or was successfully checked as
-                # well-formatted, store this information in the cache.
-                if write_back is WriteBack.YES or (
-                    write_back is WriteBack.CHECK and changed is Changed.NO
-                ):
-                    sources_to_cache.append(src)
-                report.done(src, changed)
-    if cancelled:
-        await asyncio.gather(*cancelled, loop=loop, return_exceptions=True)
-    if sources_to_cache:
-        write_cache(cache, sources_to_cache, mode)
-
-

 def format_file_in_place(
     src: Path,
     fast: bool,
 def format_file_in_place(
     src: Path,
     fast: bool,


@@ -892,23 +806,37 @@ def format_file_in_place(
     """
     if src.suffix == ".pyi":
         mode = replace(mode, is_pyi=True)
     """
     if src.suffix == ".pyi":
         mode = replace(mode, is_pyi=True)

+    elif src.suffix == ".ipynb":
+        mode = replace(mode, is_ipynb=True)

 
 

-    then = datetime.utcfromtimestamp(src.stat().st_mtime)
+    then = datetime.fromtimestamp(src.stat().st_mtime, timezone.utc)
+    header = b""

     with open(src, "rb") as buf:
     with open(src, "rb") as buf:

+        if mode.skip_source_first_line:
+            header = buf.readline()

         src_contents, encoding, newline = decode_bytes(buf.read())
     try:
         dst_contents = format_file_contents(src_contents, fast=fast, mode=mode)
     except NothingChanged:
         return False
         src_contents, encoding, newline = decode_bytes(buf.read())
     try:
         dst_contents = format_file_contents(src_contents, fast=fast, mode=mode)
     except NothingChanged:
         return False

+    except JSONDecodeError:
+        raise ValueError(
+            f"File '{src}' cannot be parsed as valid Jupyter notebook."
+        ) from None
+    src_contents = header.decode(encoding) + src_contents
+    dst_contents = header.decode(encoding) + dst_contents

 
     if write_back == WriteBack.YES:
         with open(src, "w", encoding=encoding, newline=newline) as f:
             f.write(dst_contents)
     elif write_back in (WriteBack.DIFF, WriteBack.COLOR_DIFF):
 
     if write_back == WriteBack.YES:
         with open(src, "w", encoding=encoding, newline=newline) as f:
             f.write(dst_contents)
     elif write_back in (WriteBack.DIFF, WriteBack.COLOR_DIFF):

-        now = datetime.utcnow()
-        src_name = f"{src}\t{then} +0000"
-        dst_name = f"{src}\t{now} +0000"
-        diff_contents = diff(src_contents, dst_contents, src_name, dst_name)
+        now = datetime.now(timezone.utc)
+        src_name = f"{src}\t{then}"
+        dst_name = f"{src}\t{now}"
+        if mode.is_ipynb:
+            diff_contents = ipynb_diff(src_contents, dst_contents, src_name, dst_name)
+        else:
+            diff_contents = diff(src_contents, dst_contents, src_name, dst_name)

 
         if write_back == WriteBack.COLOR_DIFF:
             diff_contents = color_diff(diff_contents)
 
         if write_back == WriteBack.COLOR_DIFF:
             diff_contents = color_diff(diff_contents)


@@ -927,53 +855,28 @@ def format_file_in_place(
     return True
 
 
     return True
 
 

-def color_diff(contents: str) -> str:
-    """Inject the ANSI color codes to the diff."""
-    lines = contents.split("\n")
-    for i, line in enumerate(lines):
-        if line.startswith("+++") or line.startswith("---"):
-            line = "\033[1;37m" + line + "\033[0m"  # bold white, reset
-        elif line.startswith("@@"):
-            line = "\033[36m" + line + "\033[0m"  # cyan, reset
-        elif line.startswith("+"):
-            line = "\033[32m" + line + "\033[0m"  # green, reset
-        elif line.startswith("-"):
-            line = "\033[31m" + line + "\033[0m"  # red, reset
-        lines[i] = line
-    return "\n".join(lines)
-
-
-def wrap_stream_for_windows(
-    f: io.TextIOWrapper,
-) -> Union[io.TextIOWrapper, "colorama.AnsiToWin32"]:
-    """
-    Wrap stream with colorama's wrap_stream so colors are shown on Windows.
-
-    If `colorama` is unavailable, the original stream is returned unmodified.
-    Otherwise, the `wrap_stream()` function determines whether the stream needs
-    to be wrapped for a Windows environment and will accordingly either return
-    an `AnsiToWin32` wrapper or the original stream.
-    """
-    try:
-        from colorama.initialise import wrap_stream
-    except ImportError:
-        return f
-    else:
-        # Set `strip=False` to avoid needing to modify test_express_diff_with_color.
-        return wrap_stream(f, convert=None, strip=False, autoreset=False, wrap=True)
-
-

 def format_stdin_to_stdout(
 def format_stdin_to_stdout(

-    fast: bool, *, write_back: WriteBack = WriteBack.NO, mode: Mode
+    fast: bool,
+    *,
+    content: Optional[str] = None,
+    write_back: WriteBack = WriteBack.NO,
+    mode: Mode,

 ) -> bool:
     """Format file on stdin. Return True if changed.
 
 ) -> bool:
     """Format file on stdin. Return True if changed.
 

+    If content is None, it's read from sys.stdin.
+

     If `write_back` is YES, write reformatted code back to stdout. If it is DIFF,
     write a diff to stdout. The `mode` argument is passed to
     :func:`format_file_contents`.
     """
     If `write_back` is YES, write reformatted code back to stdout. If it is DIFF,
     write a diff to stdout. The `mode` argument is passed to
     :func:`format_file_contents`.
     """

-    then = datetime.utcnow()
-    src, encoding, newline = decode_bytes(sys.stdin.buffer.read())
+    then = datetime.now(timezone.utc)
+
+    if content is None:
+        src, encoding, newline = decode_bytes(sys.stdin.buffer.read())
+    else:
+        src, encoding, newline = content, "utf-8", ""
+

     dst = src
     try:
         dst = format_file_contents(src, fast=fast, mode=mode)
     dst = src
     try:
         dst = format_file_contents(src, fast=fast, mode=mode)


@@ -987,11 +890,14 @@ def format_stdin_to_stdout(
             sys.stdout.buffer, encoding=encoding, newline=newline, write_through=True
         )
         if write_back == WriteBack.YES:
             sys.stdout.buffer, encoding=encoding, newline=newline, write_through=True
         )
         if write_back == WriteBack.YES:

+            # Make sure there's a newline after the content
+            if dst and dst[-1] != "\n":
+                dst += "\n"

             f.write(dst)
         elif write_back in (WriteBack.DIFF, WriteBack.COLOR_DIFF):
             f.write(dst)
         elif write_back in (WriteBack.DIFF, WriteBack.COLOR_DIFF):

-            now = datetime.utcnow()
-            src_name = f"STDIN\t{then} +0000"
-            dst_name = f"STDOUT\t{now} +0000"
+            now = datetime.now(timezone.utc)
+            src_name = f"STDIN\t{then}"
+            dst_name = f"STDOUT\t{now}"

             d = diff(src, dst, src_name, dst_name)
             if write_back == WriteBack.COLOR_DIFF:
                 d = color_diff(d)
             d = diff(src, dst, src_name, dst_name)
             if write_back == WriteBack.COLOR_DIFF:
                 d = color_diff(d)


@@ -1000,6 +906,19 @@ def format_stdin_to_stdout(
         f.detach()
 
 
         f.detach()
 
 

+def check_stability_and_equivalence(
+    src_contents: str, dst_contents: str, *, mode: Mode
+) -> None:
+    """Perform stability and equivalence checks.
+
+    Raise AssertionError if source and destination contents are not
+    equivalent, or if a second pass of the formatter would format the
+    content differently.
+    """
+    assert_equivalent(src_contents, dst_contents)
+    assert_stable(src_contents, dst_contents, mode=mode)
+
+

 def format_file_contents(src_contents: str, *, fast: bool, mode: Mode) -> FileContent:
     """Reformat contents of a file and return new contents.
 
 def format_file_contents(src_contents: str, *, fast: bool, mode: Mode) -> FileContent:
     """Reformat contents of a file and return new contents.
 


@@ -1007,20 +926,127 @@ def format_file_contents(src_contents: str, *, fast: bool, mode: Mode) -> FileCo
     valid by calling :func:`assert_equivalent` and :func:`assert_stable` on it.
     `mode` is passed to :func:`format_str`.
     """
     valid by calling :func:`assert_equivalent` and :func:`assert_stable` on it.
     `mode` is passed to :func:`format_str`.
     """

-    if not src_contents.strip():
+    if mode.is_ipynb:
+        dst_contents = format_ipynb_string(src_contents, fast=fast, mode=mode)
+    else:
+        dst_contents = format_str(src_contents, mode=mode)
+    if src_contents == dst_contents:

         raise NothingChanged
 
         raise NothingChanged
 

-    dst_contents = format_str(src_contents, mode=mode)
-    if src_contents == dst_contents:
+    if not fast and not mode.is_ipynb:
+        # Jupyter notebooks will already have been checked above.
+        check_stability_and_equivalence(src_contents, dst_contents, mode=mode)
+    return dst_contents
+
+
+def validate_cell(src: str, mode: Mode) -> None:
+    """Check that cell does not already contain TransformerManager transformations,
+    or non-Python cell magics, which might cause tokenizer_rt to break because of
+    indentations.
+
+    If a cell contains ``!ls``, then it'll be transformed to
+    ``get_ipython().system('ls')``. However, if the cell originally contained
+    ``get_ipython().system('ls')``, then it would get transformed in the same way:
+
+        >>> TransformerManager().transform_cell("get_ipython().system('ls')")
+        "get_ipython().system('ls')\n"
+        >>> TransformerManager().transform_cell("!ls")
+        "get_ipython().system('ls')\n"
+
+    Due to the impossibility of safely roundtripping in such situations, cells
+    containing transformed magics will be ignored.
+    """
+    if any(transformed_magic in src for transformed_magic in TRANSFORMED_MAGICS):

         raise NothingChanged
         raise NothingChanged

+    if (
+        src[:2] == "%%"
+        and src.split()[0][2:] not in PYTHON_CELL_MAGICS | mode.python_cell_magics
+    ):
+        raise NothingChanged
+

 
 

+def format_cell(src: str, *, fast: bool, mode: Mode) -> str:
+    """Format code in given cell of Jupyter notebook.
+
+    General idea is:
+
+      - if cell has trailing semicolon, remove it;
+      - if cell has IPython magics, mask them;
+      - format cell;
+      - reinstate IPython magics;
+      - reinstate trailing semicolon (if originally present);
+      - strip trailing newlines.
+
+    Cells with syntax errors will not be processed, as they
+    could potentially be automagics or multi-line magics, which
+    are currently not supported.
+    """
+    validate_cell(src, mode)
+    src_without_trailing_semicolon, has_trailing_semicolon = remove_trailing_semicolon(
+        src
+    )
+    try:
+        masked_src, replacements = mask_cell(src_without_trailing_semicolon)
+    except SyntaxError:
+        raise NothingChanged from None
+    masked_dst = format_str(masked_src, mode=mode)

     if not fast:
     if not fast:

-        assert_equivalent(src_contents, dst_contents)
-        assert_stable(src_contents, dst_contents, mode=mode)
-    return dst_contents
+        check_stability_and_equivalence(masked_src, masked_dst, mode=mode)
+    dst_without_trailing_semicolon = unmask_cell(masked_dst, replacements)
+    dst = put_trailing_semicolon_back(
+        dst_without_trailing_semicolon, has_trailing_semicolon
+    )
+    dst = dst.rstrip("\n")
+    if dst == src:
+        raise NothingChanged from None
+    return dst

 
 
 
 

-def format_str(src_contents: str, *, mode: Mode) -> FileContent:
+def validate_metadata(nb: MutableMapping[str, Any]) -> None:
+    """If notebook is marked as non-Python, don't format it.
+
+    All notebook metadata fields are optional, see
+    https://nbformat.readthedocs.io/en/latest/format_description.html. So
+    if a notebook has empty metadata, we will try to parse it anyway.
+    """
+    language = nb.get("metadata", {}).get("language_info", {}).get("name", None)
+    if language is not None and language != "python":
+        raise NothingChanged from None
+
+
+def format_ipynb_string(src_contents: str, *, fast: bool, mode: Mode) -> FileContent:
+    """Format Jupyter notebook.
+
+    Operate cell-by-cell, only on code cells, only for Python notebooks.
+    If the ``.ipynb`` originally had a trailing newline, it'll be preserved.
+    """
+    if not src_contents:
+        raise NothingChanged
+
+    trailing_newline = src_contents[-1] == "\n"
+    modified = False
+    nb = json.loads(src_contents)
+    validate_metadata(nb)
+    for cell in nb["cells"]:
+        if cell.get("cell_type", None) == "code":
+            try:
+                src = "".join(cell["source"])
+                dst = format_cell(src, fast=fast, mode=mode)
+            except NothingChanged:
+                pass
+            else:
+                cell["source"] = dst.splitlines(keepends=True)
+                modified = True
+    if modified:
+        dst_contents = json.dumps(nb, indent=1, ensure_ascii=False)
+        if trailing_newline:
+            dst_contents = dst_contents + "\n"
+        return dst_contents
+    else:
+        raise NothingChanged
+
+
+def format_str(src_contents: str, *, mode: Mode) -> str:

     """Reformat a string and return new contents.
 
     `mode` determines formatting options, such as how many characters per line are
     """Reformat a string and return new contents.
 
     `mode` determines formatting options, such as how many characters per line are


@@ -1050,35 +1076,57 @@ def format_str(src_contents: str, *, mode: Mode) -> FileContent:
         hey
 
     """
         hey
 
     """

-    src_node = lib2to3_parse(src_contents.lstrip(), mode.target_versions)
-    dst_contents = []
-    future_imports = get_future_imports(src_node)
+    dst_contents = _format_str_once(src_contents, mode=mode)
+    # Forced second pass to work around optional trailing commas (becoming
+    # forced trailing commas on pass 2) interacting differently with optional
+    # parentheses.  Admittedly ugly.
+    if src_contents != dst_contents:
+        return _format_str_once(dst_contents, mode=mode)
+    return dst_contents
+
+
+def _format_str_once(src_contents: str, *, mode: Mode) -> str:
+    src_node = lib2to3_parse(src_contents.lstrip(), mode.target_versions)
+    dst_blocks: List[LinesBlock] = []

     if mode.target_versions:
         versions = mode.target_versions
     else:
     if mode.target_versions:
         versions = mode.target_versions
     else:

-        versions = detect_target_versions(src_node)
+        future_imports = get_future_imports(src_node)
+        versions = detect_target_versions(src_node, future_imports=future_imports)
+
+    context_manager_features = {
+        feature
+        for feature in {Feature.PARENTHESIZED_CONTEXT_MANAGERS}
+        if supports_feature(versions, feature)
+    }

     normalize_fmt_off(src_node)
     normalize_fmt_off(src_node)

-    lines = LineGenerator(
-        mode=mode,
-        remove_u_prefix="unicode_literals" in future_imports
-        or supports_feature(versions, Feature.UNICODE_LITERALS),
-    )
-    elt = EmptyLineTracker(is_pyi=mode.is_pyi)
-    empty_line = Line(mode=mode)
-    after = 0
+    lines = LineGenerator(mode=mode, features=context_manager_features)
+    elt = EmptyLineTracker(mode=mode)

     split_line_features = {
         feature
         for feature in {Feature.TRAILING_COMMA_IN_CALL, Feature.TRAILING_COMMA_IN_DEF}
         if supports_feature(versions, feature)
     }
     split_line_features = {
         feature
         for feature in {Feature.TRAILING_COMMA_IN_CALL, Feature.TRAILING_COMMA_IN_DEF}
         if supports_feature(versions, feature)
     }

+    block: Optional[LinesBlock] = None

     for current_line in lines.visit(src_node):
     for current_line in lines.visit(src_node):

-        dst_contents.append(str(empty_line) * after)
-        before, after = elt.maybe_empty_lines(current_line)
-        dst_contents.append(str(empty_line) * before)
+        block = elt.maybe_empty_lines(current_line)
+        dst_blocks.append(block)

         for line in transform_line(
             current_line, mode=mode, features=split_line_features
         ):
         for line in transform_line(
             current_line, mode=mode, features=split_line_features
         ):

-            dst_contents.append(str(line))
+            block.content_lines.append(str(line))
+    if dst_blocks:
+        dst_blocks[-1].after = 0
+    dst_contents = []
+    for block in dst_blocks:
+        dst_contents.extend(block.all_lines())
+    if not dst_contents:
+        # Use decode_bytes to retrieve the correct source newline (CRLF or LF),
+        # and check if normalized_content has more than one line
+        normalized_content, _, newline = decode_bytes(src_contents.encode("utf-8"))
+        if "\n" in normalized_content:
+            return newline
+        return ""

     return "".join(dst_contents)
 
 
     return "".join(dst_contents)
 
 


@@ -1099,4846 +1147,55 @@ def decode_bytes(src: bytes) -> Tuple[FileContent, Encoding, NewLine]:
         return tiow.read(), encoding, newline
 
 
         return tiow.read(), encoding, newline
 
 

-def get_grammars(target_versions: Set[TargetVersion]) -> List[Grammar]:
-    if not target_versions:
-        # No target_version specified, so try all grammars.
-        return [
-            # Python 3.7+
-            pygram.python_grammar_no_print_statement_no_exec_statement_async_keywords,
-            # Python 3.0-3.6
-            pygram.python_grammar_no_print_statement_no_exec_statement,
-            # Python 2.7 with future print_function import
-            pygram.python_grammar_no_print_statement,
-            # Python 2.7
-            pygram.python_grammar,
-        ]
-
-    if all(version.is_python2() for version in target_versions):
-        # Python 2-only code, so try Python 2 grammars.
-        return [
-            # Python 2.7 with future print_function import
-            pygram.python_grammar_no_print_statement,
-            # Python 2.7
-            pygram.python_grammar,
-        ]
-
-    # Python 3-compatible code, so only try Python 3 grammar.
-    grammars = []
-    # If we have to parse both, try to parse async as a keyword first
-    if not supports_feature(target_versions, Feature.ASYNC_IDENTIFIERS):
-        # Python 3.7+
-        grammars.append(
-            pygram.python_grammar_no_print_statement_no_exec_statement_async_keywords
-        )
-    if not supports_feature(target_versions, Feature.ASYNC_KEYWORDS):
-        # Python 3.0-3.6
-        grammars.append(pygram.python_grammar_no_print_statement_no_exec_statement)
-    # At least one of the above branches must have been taken, because every Python
-    # version has exactly one of the two 'ASYNC_*' flags
-    return grammars
-
-
-def lib2to3_parse(src_txt: str, target_versions: Iterable[TargetVersion] = ()) -> Node:
-    """Given a string with source, return the lib2to3 Node."""
-    if not src_txt.endswith("\n"):
-        src_txt += "\n"
-
-    for grammar in get_grammars(set(target_versions)):
-        drv = driver.Driver(grammar, pytree.convert)
-        try:
-            result = drv.parse_string(src_txt, True)
-            break
-
-        except ParseError as pe:
-            lineno, column = pe.context[1]
-            lines = src_txt.splitlines()
-            try:
-                faulty_line = lines[lineno - 1]
-            except IndexError:
-                faulty_line = "<line number missing in source>"
-            exc = InvalidInput(f"Cannot parse: {lineno}:{column}: {faulty_line}")
-    else:
-        raise exc from None
-
-    if isinstance(result, Leaf):
-        result = Node(syms.file_input, [result])
-    return result
-
-
-def lib2to3_unparse(node: Node) -> str:
-    """Given a lib2to3 node, return its string representation."""
-    code = str(node)
-    return code
-
-
-class Visitor(Generic[T]):
-    """Basic lib2to3 visitor that yields things of type `T` on `visit()`."""
-
-    def visit(self, node: LN) -> Iterator[T]:
-        """Main method to visit `node` and its children.
-
-        It tries to find a `visit_*()` method for the given `node.type`, like
-        `visit_simple_stmt` for Node objects or `visit_INDENT` for Leaf objects.
-        If no dedicated `visit_*()` method is found, chooses `visit_default()`
-        instead.
-
-        Then yields objects of type `T` from the selected visitor.
-        """
-        if node.type < 256:
-            name = token.tok_name[node.type]
-        else:
-            name = str(type_repr(node.type))
-        # We explicitly branch on whether a visitor exists (instead of
-        # using self.visit_default as the default arg to getattr) in order
-        # to save needing to create a bound method object and so mypyc can
-        # generate a native call to visit_default.
-        visitf = getattr(self, f"visit_{name}", None)
-        if visitf:
-            yield from visitf(node)
-        else:
-            yield from self.visit_default(node)
-
-    def visit_default(self, node: LN) -> Iterator[T]:
-        """Default `visit_*()` implementation. Recurses to children of `node`."""
-        if isinstance(node, Node):
-            for child in node.children:
-                yield from self.visit(child)
-
-
-@dataclass
-class DebugVisitor(Visitor[T]):
-    tree_depth: int = 0
-
-    def visit_default(self, node: LN) -> Iterator[T]:
-        indent = " " * (2 * self.tree_depth)
-        if isinstance(node, Node):
-            _type = type_repr(node.type)
-            out(f"{indent}{_type}", fg="yellow")
-            self.tree_depth += 1
-            for child in node.children:
-                yield from self.visit(child)
-
-            self.tree_depth -= 1
-            out(f"{indent}/{_type}", fg="yellow", bold=False)
-        else:
-            _type = token.tok_name.get(node.type, str(node.type))
-            out(f"{indent}{_type}", fg="blue", nl=False)
-            if node.prefix:
-                # We don't have to handle prefixes for `Node` objects since
-                # that delegates to the first child anyway.
-                out(f" {node.prefix!r}", fg="green", bold=False, nl=False)
-            out(f" {node.value!r}", fg="blue", bold=False)
-
-    @classmethod
-    def show(cls, code: Union[str, Leaf, Node]) -> None:
-        """Pretty-print the lib2to3 AST of a given string of `code`.
-
-        Convenience method for debugging.
-        """
-        v: DebugVisitor[None] = DebugVisitor()
-        if isinstance(code, str):
-            code = lib2to3_parse(code)
-        list(v.visit(code))
-
-
-WHITESPACE: Final = {token.DEDENT, token.INDENT, token.NEWLINE}
-STATEMENT: Final = {
-    syms.if_stmt,
-    syms.while_stmt,
-    syms.for_stmt,
-    syms.try_stmt,
-    syms.except_clause,
-    syms.with_stmt,
-    syms.funcdef,
-    syms.classdef,
-}
-STANDALONE_COMMENT: Final = 153
-token.tok_name[STANDALONE_COMMENT] = "STANDALONE_COMMENT"
-LOGIC_OPERATORS: Final = {"and", "or"}
-COMPARATORS: Final = {
-    token.LESS,
-    token.GREATER,
-    token.EQEQUAL,
-    token.NOTEQUAL,
-    token.LESSEQUAL,
-    token.GREATEREQUAL,
-}
-MATH_OPERATORS: Final = {
-    token.VBAR,
-    token.CIRCUMFLEX,
-    token.AMPER,
-    token.LEFTSHIFT,
-    token.RIGHTSHIFT,
-    token.PLUS,
-    token.MINUS,
-    token.STAR,
-    token.SLASH,
-    token.DOUBLESLASH,
-    token.PERCENT,
-    token.AT,
-    token.TILDE,
-    token.DOUBLESTAR,
-}
-STARS: Final = {token.STAR, token.DOUBLESTAR}
-VARARGS_SPECIALS: Final = STARS | {token.SLASH}
-VARARGS_PARENTS: Final = {
-    syms.arglist,
-    syms.argument,  # double star in arglist
-    syms.trailer,  # single argument to call
-    syms.typedargslist,
-    syms.varargslist,  # lambdas
-}
-UNPACKING_PARENTS: Final = {
-    syms.atom,  # single element of a list or set literal
-    syms.dictsetmaker,
-    syms.listmaker,
-    syms.testlist_gexp,
-    syms.testlist_star_expr,
-}
-TEST_DESCENDANTS: Final = {
-    syms.test,
-    syms.lambdef,
-    syms.or_test,
-    syms.and_test,
-    syms.not_test,
-    syms.comparison,
-    syms.star_expr,
-    syms.expr,
-    syms.xor_expr,
-    syms.and_expr,
-    syms.shift_expr,
-    syms.arith_expr,
-    syms.trailer,
-    syms.term,
-    syms.power,
-}
-ASSIGNMENTS: Final = {
-    "=",
-    "+=",
-    "-=",
-    "*=",
-    "@=",
-    "/=",
-    "%=",
-    "&=",
-    "|=",
-    "^=",
-    "<<=",
-    ">>=",
-    "**=",
-    "//=",
-}
-COMPREHENSION_PRIORITY: Final = 20
-COMMA_PRIORITY: Final = 18
-TERNARY_PRIORITY: Final = 16
-LOGIC_PRIORITY: Final = 14
-STRING_PRIORITY: Final = 12
-COMPARATOR_PRIORITY: Final = 10
-MATH_PRIORITIES: Final = {
-    token.VBAR: 9,
-    token.CIRCUMFLEX: 8,
-    token.AMPER: 7,
-    token.LEFTSHIFT: 6,
-    token.RIGHTSHIFT: 6,
-    token.PLUS: 5,
-    token.MINUS: 5,
-    token.STAR: 4,
-    token.SLASH: 4,
-    token.DOUBLESLASH: 4,
-    token.PERCENT: 4,
-    token.AT: 4,
-    token.TILDE: 3,
-    token.DOUBLESTAR: 2,
-}
-DOT_PRIORITY: Final = 1
-
-
-@dataclass
-class BracketTracker:
-    """Keeps track of brackets on a line."""
-
-    depth: int = 0
-    bracket_match: Dict[Tuple[Depth, NodeType], Leaf] = field(default_factory=dict)
-    delimiters: Dict[LeafID, Priority] = field(default_factory=dict)
-    previous: Optional[Leaf] = None
-    _for_loop_depths: List[int] = field(default_factory=list)
-    _lambda_argument_depths: List[int] = field(default_factory=list)
-    invisible: List[Leaf] = field(default_factory=list)
-
-    def mark(self, leaf: Leaf) -> None:
-        """Mark `leaf` with bracket-related metadata. Keep track of delimiters.
-
-        All leaves receive an int `bracket_depth` field that stores how deep
-        within brackets a given leaf is. 0 means there are no enclosing brackets
-        that started on this line.
-
-        If a leaf is itself a closing bracket, it receives an `opening_bracket`
-        field that it forms a pair with. This is a one-directional link to
-        avoid reference cycles.
-
-        If a leaf is a delimiter (a token on which Black can split the line if
-        needed) and it's on depth 0, its `id()` is stored in the tracker's
-        `delimiters` field.
-        """
-        if leaf.type == token.COMMENT:
-            return
-
-        self.maybe_decrement_after_for_loop_variable(leaf)
-        self.maybe_decrement_after_lambda_arguments(leaf)
-        if leaf.type in CLOSING_BRACKETS:
-            self.depth -= 1
-            try:
-                opening_bracket = self.bracket_match.pop((self.depth, leaf.type))
-            except KeyError as e:
-                raise BracketMatchError(
-                    "Unable to match a closing bracket to the following opening"
-                    f" bracket: {leaf}"
-                ) from e
-            leaf.opening_bracket = opening_bracket
-            if not leaf.value:
-                self.invisible.append(leaf)
-        leaf.bracket_depth = self.depth
-        if self.depth == 0:
-            delim = is_split_before_delimiter(leaf, self.previous)
-            if delim and self.previous is not None:
-                self.delimiters[id(self.previous)] = delim
-            else:
-                delim = is_split_after_delimiter(leaf, self.previous)
-                if delim:
-                    self.delimiters[id(leaf)] = delim
-        if leaf.type in OPENING_BRACKETS:
-            self.bracket_match[self.depth, BRACKET[leaf.type]] = leaf
-            self.depth += 1
-            if not leaf.value:
-                self.invisible.append(leaf)
-        self.previous = leaf
-        self.maybe_increment_lambda_arguments(leaf)
-        self.maybe_increment_for_loop_variable(leaf)
-
-    def any_open_brackets(self) -> bool:
-        """Return True if there is an yet unmatched open bracket on the line."""
-        return bool(self.bracket_match)
-
-    def max_delimiter_priority(self, exclude: Iterable[LeafID] = ()) -> Priority:
-        """Return the highest priority of a delimiter found on the line.
-
-        Values are consistent with what `is_split_*_delimiter()` return.
-        Raises ValueError on no delimiters.
-        """
-        return max(v for k, v in self.delimiters.items() if k not in exclude)
-
-    def delimiter_count_with_priority(self, priority: Priority = 0) -> int:
-        """Return the number of delimiters with the given `priority`.
-
-        If no `priority` is passed, defaults to max priority on the line.
-        """
-        if not self.delimiters:
-            return 0
-
-        priority = priority or self.max_delimiter_priority()
-        return sum(1 for p in self.delimiters.values() if p == priority)
-
-    def maybe_increment_for_loop_variable(self, leaf: Leaf) -> bool:
-        """In a for loop, or comprehension, the variables are often unpacks.
-
-        To avoid splitting on the comma in this situation, increase the depth of
-        tokens between `for` and `in`.
-        """
-        if leaf.type == token.NAME and leaf.value == "for":
-            self.depth += 1
-            self._for_loop_depths.append(self.depth)
-            return True
-
-        return False
-
-    def maybe_decrement_after_for_loop_variable(self, leaf: Leaf) -> bool:
-        """See `maybe_increment_for_loop_variable` above for explanation."""
-        if (
-            self._for_loop_depths
-            and self._for_loop_depths[-1] == self.depth
-            and leaf.type == token.NAME
-            and leaf.value == "in"
-        ):
-            self.depth -= 1
-            self._for_loop_depths.pop()
-            return True
-
-        return False
-
-    def maybe_increment_lambda_arguments(self, leaf: Leaf) -> bool:
-        """In a lambda expression, there might be more than one argument.
-
-        To avoid splitting on the comma in this situation, increase the depth of
-        tokens between `lambda` and `:`.
-        """
-        if leaf.type == token.NAME and leaf.value == "lambda":
-            self.depth += 1
-            self._lambda_argument_depths.append(self.depth)
-            return True
-
-        return False
-
-    def maybe_decrement_after_lambda_arguments(self, leaf: Leaf) -> bool:
-        """See `maybe_increment_lambda_arguments` above for explanation."""
-        if (
-            self._lambda_argument_depths
-            and self._lambda_argument_depths[-1] == self.depth
-            and leaf.type == token.COLON
-        ):
-            self.depth -= 1
-            self._lambda_argument_depths.pop()
-            return True
-
-        return False
-
-    def get_open_lsqb(self) -> Optional[Leaf]:
-        """Return the most recent opening square bracket (if any)."""
-        return self.bracket_match.get((self.depth - 1, token.RSQB))
-
-
-@dataclass
-class Line:
-    """Holds leaves and comments. Can be printed with `str(line)`."""
-
-    mode: Mode
-    depth: int = 0
-    leaves: List[Leaf] = field(default_factory=list)
-    # keys ordered like `leaves`
-    comments: Dict[LeafID, List[Leaf]] = field(default_factory=dict)
-    bracket_tracker: BracketTracker = field(default_factory=BracketTracker)
-    inside_brackets: bool = False
-    should_split_rhs: bool = False
-    magic_trailing_comma: Optional[Leaf] = None
-
-    def append(self, leaf: Leaf, preformatted: bool = False) -> None:
-        """Add a new `leaf` to the end of the line.
-
-        Unless `preformatted` is True, the `leaf` will receive a new consistent
-        whitespace prefix and metadata applied by :class:`BracketTracker`.
-        Trailing commas are maybe removed, unpacked for loop variables are
-        demoted from being delimiters.
-
-        Inline comments are put aside.
-        """
-        has_value = leaf.type in BRACKETS or bool(leaf.value.strip())
-        if not has_value:
-            return
-
-        if token.COLON == leaf.type and self.is_class_paren_empty:
-            del self.leaves[-2:]
-        if self.leaves and not preformatted:
-            # Note: at this point leaf.prefix should be empty except for
-            # imports, for which we only preserve newlines.
-            leaf.prefix += whitespace(
-                leaf, complex_subscript=self.is_complex_subscript(leaf)
-            )
-        if self.inside_brackets or not preformatted:
-            self.bracket_tracker.mark(leaf)
-            if self.mode.magic_trailing_comma:
-                if self.has_magic_trailing_comma(leaf):
-                    self.magic_trailing_comma = leaf
-            elif self.has_magic_trailing_comma(leaf, ensure_removable=True):
-                self.remove_trailing_comma()
-        if not self.append_comment(leaf):
-            self.leaves.append(leaf)
-
-    def append_safe(self, leaf: Leaf, preformatted: bool = False) -> None:
-        """Like :func:`append()` but disallow invalid standalone comment structure.
-
-        Raises ValueError when any `leaf` is appended after a standalone comment
-        or when a standalone comment is not the first leaf on the line.
-        """
-        if self.bracket_tracker.depth == 0:
-            if self.is_comment:
-                raise ValueError("cannot append to standalone comments")
-
-            if self.leaves and leaf.type == STANDALONE_COMMENT:
-                raise ValueError(
-                    "cannot append standalone comments to a populated line"
-                )
-
-        self.append(leaf, preformatted=preformatted)
-
-    @property
-    def is_comment(self) -> bool:
-        """Is this line a standalone comment?"""
-        return len(self.leaves) == 1 and self.leaves[0].type == STANDALONE_COMMENT
-
-    @property
-    def is_decorator(self) -> bool:
-        """Is this line a decorator?"""
-        return bool(self) and self.leaves[0].type == token.AT
-
-    @property
-    def is_import(self) -> bool:
-        """Is this an import line?"""
-        return bool(self) and is_import(self.leaves[0])
-
-    @property
-    def is_class(self) -> bool:
-        """Is this line a class definition?"""
-        return (
-            bool(self)
-            and self.leaves[0].type == token.NAME
-            and self.leaves[0].value == "class"
-        )
-
-    @property
-    def is_stub_class(self) -> bool:
-        """Is this line a class definition with a body consisting only of "..."?"""
-        return self.is_class and self.leaves[-3:] == [
-            Leaf(token.DOT, ".") for _ in range(3)
-        ]
-
-    @property
-    def is_def(self) -> bool:
-        """Is this a function definition? (Also returns True for async defs.)"""
-        try:
-            first_leaf = self.leaves[0]
-        except IndexError:
-            return False
-
-        try:
-            second_leaf: Optional[Leaf] = self.leaves[1]
-        except IndexError:
-            second_leaf = None
-        return (first_leaf.type == token.NAME and first_leaf.value == "def") or (
-            first_leaf.type == token.ASYNC
-            and second_leaf is not None
-            and second_leaf.type == token.NAME
-            and second_leaf.value == "def"
-        )
-
-    @property
-    def is_class_paren_empty(self) -> bool:
-        """Is this a class with no base classes but using parentheses?
-
-        Those are unnecessary and should be removed.
-        """
-        return (
-            bool(self)
-            and len(self.leaves) == 4
-            and self.is_class
-            and self.leaves[2].type == token.LPAR
-            and self.leaves[2].value == "("
-            and self.leaves[3].type == token.RPAR
-            and self.leaves[3].value == ")"
-        )
-
-    @property
-    def is_triple_quoted_string(self) -> bool:
-        """Is the line a triple quoted string?"""
-        return (
-            bool(self)
-            and self.leaves[0].type == token.STRING
-            and self.leaves[0].value.startswith(('"""', "'''"))
-        )
-
-    def contains_standalone_comments(self, depth_limit: int = sys.maxsize) -> bool:
-        """If so, needs to be split before emitting."""
-        for leaf in self.leaves:
-            if leaf.type == STANDALONE_COMMENT and leaf.bracket_depth <= depth_limit:
-                return True
-
-        return False
-
-    def contains_uncollapsable_type_comments(self) -> bool:
-        ignored_ids = set()
-        try:
-            last_leaf = self.leaves[-1]
-            ignored_ids.add(id(last_leaf))
-            if last_leaf.type == token.COMMA or (
-                last_leaf.type == token.RPAR and not last_leaf.value
-            ):
-                # When trailing commas or optional parens are inserted by Black for
-                # consistency, comments after the previous last element are not moved
-                # (they don't have to, rendering will still be correct).  So we ignore
-                # trailing commas and invisible.
-                last_leaf = self.leaves[-2]
-                ignored_ids.add(id(last_leaf))
-        except IndexError:
-            return False
-
-        # A type comment is uncollapsable if it is attached to a leaf
-        # that isn't at the end of the line (since that could cause it
-        # to get associated to a different argument) or if there are
-        # comments before it (since that could cause it to get hidden
-        # behind a comment.
-        comment_seen = False
-        for leaf_id, comments in self.comments.items():
-            for comment in comments:
-                if is_type_comment(comment):
-                    if comment_seen or (
-                        not is_type_comment(comment, " ignore")
-                        and leaf_id not in ignored_ids
-                    ):
-                        return True
-
-                comment_seen = True
-
-        return False
-
-    def contains_unsplittable_type_ignore(self) -> bool:
-        if not self.leaves:
-            return False
-
-        # If a 'type: ignore' is attached to the end of a line, we
-        # can't split the line, because we can't know which of the
-        # subexpressions the ignore was meant to apply to.
-        #
-        # We only want this to apply to actual physical lines from the
-        # original source, though: we don't want the presence of a
-        # 'type: ignore' at the end of a multiline expression to
-        # justify pushing it all onto one line. Thus we
-        # (unfortunately) need to check the actual source lines and
-        # only report an unsplittable 'type: ignore' if this line was
-        # one line in the original code.
-
-        # Grab the first and last line numbers, skipping generated leaves
-        first_line = next((leaf.lineno for leaf in self.leaves if leaf.lineno != 0), 0)
-        last_line = next(
-            (leaf.lineno for leaf in reversed(self.leaves) if leaf.lineno != 0), 0
-        )
-
-        if first_line == last_line:
-            # We look at the last two leaves since a comma or an
-            # invisible paren could have been added at the end of the
-            # line.
-            for node in self.leaves[-2:]:
-                for comment in self.comments.get(id(node), []):
-                    if is_type_comment(comment, " ignore"):
-                        return True
-
-        return False
-
-    def contains_multiline_strings(self) -> bool:
-        return any(is_multiline_string(leaf) for leaf in self.leaves)
-
-    def has_magic_trailing_comma(
-        self, closing: Leaf, ensure_removable: bool = False
-    ) -> bool:
-        """Return True if we have a magic trailing comma, that is when:
-        - there's a trailing comma here
-        - it's not a one-tuple
-        Additionally, if ensure_removable:
-        - it's not from square bracket indexing
-        """
-        if not (
-            closing.type in CLOSING_BRACKETS
-            and self.leaves
-            and self.leaves[-1].type == token.COMMA
-        ):
-            return False
-
-        if closing.type == token.RBRACE:
-            return True
-
-        if closing.type == token.RSQB:
-            if not ensure_removable:
-                return True
-            comma = self.leaves[-1]
-            return bool(comma.parent and comma.parent.type == syms.listmaker)
-
-        if self.is_import:
-            return True
-
-        if not is_one_tuple_between(closing.opening_bracket, closing, self.leaves):
-            return True
-
-        return False
-
-    def append_comment(self, comment: Leaf) -> bool:
-        """Add an inline or standalone comment to the line."""
-        if (
-            comment.type == STANDALONE_COMMENT
-            and self.bracket_tracker.any_open_brackets()
-        ):
-            comment.prefix = ""
-            return False
-
-        if comment.type != token.COMMENT:
-            return False
-
-        if not self.leaves:
-            comment.type = STANDALONE_COMMENT
-            comment.prefix = ""
-            return False
-
-        last_leaf = self.leaves[-1]
-        if (
-            last_leaf.type == token.RPAR
-            and not last_leaf.value
-            and last_leaf.parent
-            and len(list(last_leaf.parent.leaves())) <= 3
-            and not is_type_comment(comment)
-        ):
-            # Comments on an optional parens wrapping a single leaf should belong to
-            # the wrapped node except if it's a type comment. Pinning the comment like
-            # this avoids unstable formatting caused by comment migration.
-            if len(self.leaves) < 2:
-                comment.type = STANDALONE_COMMENT
-                comment.prefix = ""
-                return False
-
-            last_leaf = self.leaves[-2]
-        self.comments.setdefault(id(last_leaf), []).append(comment)
-        return True
-
-    def comments_after(self, leaf: Leaf) -> List[Leaf]:
-        """Generate comments that should appear directly after `leaf`."""
-        return self.comments.get(id(leaf), [])
-
-    def remove_trailing_comma(self) -> None:
-        """Remove the trailing comma and moves the comments attached to it."""
-        trailing_comma = self.leaves.pop()
-        trailing_comma_comments = self.comments.pop(id(trailing_comma), [])
-        self.comments.setdefault(id(self.leaves[-1]), []).extend(
-            trailing_comma_comments
-        )
-
-    def is_complex_subscript(self, leaf: Leaf) -> bool:
-        """Return True iff `leaf` is part of a slice with non-trivial exprs."""
-        open_lsqb = self.bracket_tracker.get_open_lsqb()
-        if open_lsqb is None:
-            return False
-
-        subscript_start = open_lsqb.next_sibling
-
-        if isinstance(subscript_start, Node):
-            if subscript_start.type == syms.listmaker:
-                return False
-
-            if subscript_start.type == syms.subscriptlist:
-                subscript_start = child_towards(subscript_start, leaf)
-        return subscript_start is not None and any(
-            n.type in TEST_DESCENDANTS for n in subscript_start.pre_order()
-        )
-
-    def clone(self) -> "Line":
-        return Line(
-            mode=self.mode,
-            depth=self.depth,
-            inside_brackets=self.inside_brackets,
-            should_split_rhs=self.should_split_rhs,
-            magic_trailing_comma=self.magic_trailing_comma,
-        )
-
-    def __str__(self) -> str:
-        """Render the line."""
-        if not self:
-            return "\n"
-
-        indent = "    " * self.depth
-        leaves = iter(self.leaves)
-        first = next(leaves)
-        res = f"{first.prefix}{indent}{first.value}"
-        for leaf in leaves:
-            res += str(leaf)
-        for comment in itertools.chain.from_iterable(self.comments.values()):
-            res += str(comment)
-
-        return res + "\n"
-
-    def __bool__(self) -> bool:
-        """Return True if the line has leaves or comments."""
-        return bool(self.leaves or self.comments)
-
-
-@dataclass
-class EmptyLineTracker:
-    """Provides a stateful method that returns the number of potential extra
-    empty lines needed before and after the currently processed line.
-
-    Note: this tracker works on lines that haven't been split yet.  It assumes
-    the prefix of the first leaf consists of optional newlines.  Those newlines
-    are consumed by `maybe_empty_lines()` and included in the computation.
-    """
-
-    is_pyi: bool = False
-    previous_line: Optional[Line] = None
-    previous_after: int = 0
-    previous_defs: List[int] = field(default_factory=list)
-
-    def maybe_empty_lines(self, current_line: Line) -> Tuple[int, int]:
-        """Return the number of extra empty lines before and after the `current_line`.
-
-        This is for separating `def`, `async def` and `class` with extra empty
-        lines (two on module-level).
-        """
-        before, after = self._maybe_empty_lines(current_line)
-        before = (
-            # Black should not insert empty lines at the beginning
-            # of the file
-            0
-            if self.previous_line is None
-            else before - self.previous_after
-        )
-        self.previous_after = after
-        self.previous_line = current_line
-        return before, after
-
-    def _maybe_empty_lines(self, current_line: Line) -> Tuple[int, int]:
-        max_allowed = 1
-        if current_line.depth == 0:
-            max_allowed = 1 if self.is_pyi else 2
-        if current_line.leaves:
-            # Consume the first leaf's extra newlines.
-            first_leaf = current_line.leaves[0]
-            before = first_leaf.prefix.count("\n")
-            before = min(before, max_allowed)
-            first_leaf.prefix = ""
-        else:
-            before = 0
-        depth = current_line.depth
-        while self.previous_defs and self.previous_defs[-1] >= depth:
-            self.previous_defs.pop()
-            if self.is_pyi:
-                before = 0 if depth else 1
-            else:
-                before = 1 if depth else 2
-        if current_line.is_decorator or current_line.is_def or current_line.is_class:
-            return self._maybe_empty_lines_for_class_or_def(current_line, before)
-
-        if (
-            self.previous_line
-            and self.previous_line.is_import
-            and not current_line.is_import
-            and depth == self.previous_line.depth
-        ):
-            return (before or 1), 0
-
-        if (
-            self.previous_line
-            and self.previous_line.is_class
-            and current_line.is_triple_quoted_string
-        ):
-            return before, 1
-
-        return before, 0
-
-    def _maybe_empty_lines_for_class_or_def(
-        self, current_line: Line, before: int
-    ) -> Tuple[int, int]:
-        if not current_line.is_decorator:
-            self.previous_defs.append(current_line.depth)
-        if self.previous_line is None:
-            # Don't insert empty lines before the first line in the file.
-            return 0, 0
-
-        if self.previous_line.is_decorator:
-            if self.is_pyi and current_line.is_stub_class:
-                # Insert an empty line after a decorated stub class
-                return 0, 1
-
-            return 0, 0
-
-        if self.previous_line.depth < current_line.depth and (
-            self.previous_line.is_class or self.previous_line.is_def
-        ):
-            return 0, 0
-
-        if (
-            self.previous_line.is_comment
-            and self.previous_line.depth == current_line.depth
-            and before == 0
-        ):
-            return 0, 0
-
-        if self.is_pyi:
-            if self.previous_line.depth > current_line.depth:
-                newlines = 1
-            elif current_line.is_class or self.previous_line.is_class:
-                if current_line.is_stub_class and self.previous_line.is_stub_class:
-                    # No blank line between classes with an empty body
-                    newlines = 0
-                else:
-                    newlines = 1
-            elif (
-                current_line.is_def or current_line.is_decorator
-            ) and not self.previous_line.is_def:
-                # Blank line between a block of functions (maybe with preceding
-                # decorators) and a block of non-functions
-                newlines = 1
-            else:
-                newlines = 0
-        else:
-            newlines = 2
-        if current_line.depth and newlines:
-            newlines -= 1
-        return newlines, 0
-
-
-@dataclass
-class LineGenerator(Visitor[Line]):
-    """Generates reformatted Line objects.  Empty lines are not emitted.
-
-    Note: destroys the tree it's visiting by mutating prefixes of its leaves
-    in ways that will no longer stringify to valid Python code on the tree.
-    """
-
-    mode: Mode
-    remove_u_prefix: bool = False
-    current_line: Line = field(init=False)
-
-    def line(self, indent: int = 0) -> Iterator[Line]:
-        """Generate a line.
-
-        If the line is empty, only emit if it makes sense.
-        If the line is too long, split it first and then generate.
-
-        If any lines were generated, set up a new current_line.
-        """
-        if not self.current_line:
-            self.current_line.depth += indent
-            return  # Line is empty, don't emit. Creating a new one unnecessary.
-
-        complete_line = self.current_line
-        self.current_line = Line(mode=self.mode, depth=complete_line.depth + indent)
-        yield complete_line
-
-    def visit_default(self, node: LN) -> Iterator[Line]:
-        """Default `visit_*()` implementation. Recurses to children of `node`."""
-        if isinstance(node, Leaf):
-            any_open_brackets = self.current_line.bracket_tracker.any_open_brackets()
-            for comment in generate_comments(node):
-                if any_open_brackets:
-                    # any comment within brackets is subject to splitting
-                    self.current_line.append(comment)
-                elif comment.type == token.COMMENT:
-                    # regular trailing comment
-                    self.current_line.append(comment)
-                    yield from self.line()
-
-                else:
-                    # regular standalone comment
-                    yield from self.line()
-
-                    self.current_line.append(comment)
-                    yield from self.line()
-
-            normalize_prefix(node, inside_brackets=any_open_brackets)
-            if self.mode.string_normalization and node.type == token.STRING:
-                normalize_string_prefix(node, remove_u_prefix=self.remove_u_prefix)
-                normalize_string_quotes(node)
-            if node.type == token.NUMBER:
-                normalize_numeric_literal(node)
-            if node.type not in WHITESPACE:
-                self.current_line.append(node)
-        yield from super().visit_default(node)
-
-    def visit_INDENT(self, node: Leaf) -> Iterator[Line]:
-        """Increase indentation level, maybe yield a line."""
-        # In blib2to3 INDENT never holds comments.
-        yield from self.line(+1)
-        yield from self.visit_default(node)
-
-    def visit_DEDENT(self, node: Leaf) -> Iterator[Line]:
-        """Decrease indentation level, maybe yield a line."""
-        # The current line might still wait for trailing comments.  At DEDENT time
-        # there won't be any (they would be prefixes on the preceding NEWLINE).
-        # Emit the line then.
-        yield from self.line()
-
-        # While DEDENT has no value, its prefix may contain standalone comments
-        # that belong to the current indentation level.  Get 'em.
-        yield from self.visit_default(node)
-
-        # Finally, emit the dedent.
-        yield from self.line(-1)
-
-    def visit_stmt(
-        self, node: Node, keywords: Set[str], parens: Set[str]
-    ) -> Iterator[Line]:
-        """Visit a statement.
-
-        This implementation is shared for `if`, `while`, `for`, `try`, `except`,
-        `def`, `with`, `class`, `assert` and assignments.
-
-        The relevant Python language `keywords` for a given statement will be
-        NAME leaves within it. This methods puts those on a separate line.
-
-        `parens` holds a set of string leaf values immediately after which
-        invisible parens should be put.
-        """
-        normalize_invisible_parens(node, parens_after=parens)
-        for child in node.children:
-            if child.type == token.NAME and child.value in keywords:  # type: ignore
-                yield from self.line()
-
-            yield from self.visit(child)
-
-    def visit_suite(self, node: Node) -> Iterator[Line]:
-        """Visit a suite."""
-        if self.mode.is_pyi and is_stub_suite(node):
-            yield from self.visit(node.children[2])
-        else:
-            yield from self.visit_default(node)
-
-    def visit_simple_stmt(self, node: Node) -> Iterator[Line]:
-        """Visit a statement without nested statements."""
-        if first_child_is_arith(node):
-            wrap_in_parentheses(node, node.children[0], visible=False)
-        is_suite_like = node.parent and node.parent.type in STATEMENT
-        if is_suite_like:
-            if self.mode.is_pyi and is_stub_body(node):
-                yield from self.visit_default(node)
-            else:
-                yield from self.line(+1)
-                yield from self.visit_default(node)
-                yield from self.line(-1)
-
-        else:
-            if (
-                not self.mode.is_pyi
-                or not node.parent
-                or not is_stub_suite(node.parent)
-            ):
-                yield from self.line()
-            yield from self.visit_default(node)
-
-    def visit_async_stmt(self, node: Node) -> Iterator[Line]:
-        """Visit `async def`, `async for`, `async with`."""
-        yield from self.line()
-
-        children = iter(node.children)
-        for child in children:
-            yield from self.visit(child)
-
-            if child.type == token.ASYNC:
-                break
-
-        internal_stmt = next(children)
-        for child in internal_stmt.children:
-            yield from self.visit(child)
-
-    def visit_decorators(self, node: Node) -> Iterator[Line]:
-        """Visit decorators."""
-        for child in node.children:
-            yield from self.line()
-            yield from self.visit(child)
-
-    def visit_SEMI(self, leaf: Leaf) -> Iterator[Line]:
-        """Remove a semicolon and put the other statement on a separate line."""
-        yield from self.line()
-
-    def visit_ENDMARKER(self, leaf: Leaf) -> Iterator[Line]:
-        """End of file. Process outstanding comments and end with a newline."""
-        yield from self.visit_default(leaf)
-        yield from self.line()
-
-    def visit_STANDALONE_COMMENT(self, leaf: Leaf) -> Iterator[Line]:
-        if not self.current_line.bracket_tracker.any_open_brackets():
-            yield from self.line()
-        yield from self.visit_default(leaf)
-
-    def visit_factor(self, node: Node) -> Iterator[Line]:
-        """Force parentheses between a unary op and a binary power:
-
-        -2 ** 8 -> -(2 ** 8)
-        """
-        _operator, operand = node.children
-        if (
-            operand.type == syms.power
-            and len(operand.children) == 3
-            and operand.children[1].type == token.DOUBLESTAR
-        ):
-            lpar = Leaf(token.LPAR, "(")
-            rpar = Leaf(token.RPAR, ")")
-            index = operand.remove() or 0
-            node.insert_child(index, Node(syms.atom, [lpar, operand, rpar]))
-        yield from self.visit_default(node)
-
-    def visit_STRING(self, leaf: Leaf) -> Iterator[Line]:
-        if is_docstring(leaf) and "\\\n" not in leaf.value:
-            # We're ignoring docstrings with backslash newline escapes because changing
-            # indentation of those changes the AST representation of the code.
-            prefix = get_string_prefix(leaf.value)
-            lead_len = len(prefix) + 3
-            tail_len = -3
-            indent = " " * 4 * self.current_line.depth
-            docstring = fix_docstring(leaf.value[lead_len:tail_len], indent)
-            if docstring:
-                if leaf.value[lead_len - 1] == docstring[0]:
-                    docstring = " " + docstring
-                if leaf.value[tail_len + 1] == docstring[-1]:
-                    docstring = docstring + " "
-            leaf.value = leaf.value[0:lead_len] + docstring + leaf.value[tail_len:]
-
-        yield from self.visit_default(leaf)
-
-    def __post_init__(self) -> None:
-        """You are in a twisty little maze of passages."""
-        self.current_line = Line(mode=self.mode)
-
-        v = self.visit_stmt
-        Ø: Set[str] = set()
-        self.visit_assert_stmt = partial(v, keywords={"assert"}, parens={"assert", ","})
-        self.visit_if_stmt = partial(
-            v, keywords={"if", "else", "elif"}, parens={"if", "elif"}
-        )
-        self.visit_while_stmt = partial(v, keywords={"while", "else"}, parens={"while"})
-        self.visit_for_stmt = partial(v, keywords={"for", "else"}, parens={"for", "in"})
-        self.visit_try_stmt = partial(
-            v, keywords={"try", "except", "else", "finally"}, parens=Ø
-        )
-        self.visit_except_clause = partial(v, keywords={"except"}, parens=Ø)
-        self.visit_with_stmt = partial(v, keywords={"with"}, parens=Ø)
-        self.visit_funcdef = partial(v, keywords={"def"}, parens=Ø)
-        self.visit_classdef = partial(v, keywords={"class"}, parens=Ø)
-        self.visit_expr_stmt = partial(v, keywords=Ø, parens=ASSIGNMENTS)
-        self.visit_return_stmt = partial(v, keywords={"return"}, parens={"return"})
-        self.visit_import_from = partial(v, keywords=Ø, parens={"import"})
-        self.visit_del_stmt = partial(v, keywords=Ø, parens={"del"})
-        self.visit_async_funcdef = self.visit_async_stmt
-        self.visit_decorated = self.visit_decorators
-
-
-IMPLICIT_TUPLE = {syms.testlist, syms.testlist_star_expr, syms.exprlist}
-BRACKET = {token.LPAR: token.RPAR, token.LSQB: token.RSQB, token.LBRACE: token.RBRACE}
-OPENING_BRACKETS = set(BRACKET.keys())
-CLOSING_BRACKETS = set(BRACKET.values())
-BRACKETS = OPENING_BRACKETS | CLOSING_BRACKETS
-ALWAYS_NO_SPACE = CLOSING_BRACKETS | {token.COMMA, STANDALONE_COMMENT}
-
-
-def whitespace(leaf: Leaf, *, complex_subscript: bool) -> str:  # noqa: C901
-    """Return whitespace prefix if needed for the given `leaf`.
-
-    `complex_subscript` signals whether the given leaf is part of a subscription
-    which has non-trivial arguments, like arithmetic expressions or function calls.
-    """
-    NO = ""
-    SPACE = " "
-    DOUBLESPACE = "  "
-    t = leaf.type
-    p = leaf.parent
-    v = leaf.value
-    if t in ALWAYS_NO_SPACE:
-        return NO
-
-    if t == token.COMMENT:
-        return DOUBLESPACE
-
-    assert p is not None, f"INTERNAL ERROR: hand-made leaf without parent: {leaf!r}"
-    if t == token.COLON and p.type not in {
-        syms.subscript,
-        syms.subscriptlist,
-        syms.sliceop,
-    }:
-        return NO
-
-    prev = leaf.prev_sibling
-    if not prev:
-        prevp = preceding_leaf(p)
-        if not prevp or prevp.type in OPENING_BRACKETS:
-            return NO
-
-        if t == token.COLON:
-            if prevp.type == token.COLON:
-                return NO
-
-            elif prevp.type != token.COMMA and not complex_subscript:
-                return NO
-
-            return SPACE
-
-        if prevp.type == token.EQUAL:
-            if prevp.parent:
-                if prevp.parent.type in {
-                    syms.arglist,
-                    syms.argument,
-                    syms.parameters,
-                    syms.varargslist,
-                }:
-                    return NO
-
-                elif prevp.parent.type == syms.typedargslist:
-                    # A bit hacky: if the equal sign has whitespace, it means we
-                    # previously found it's a typed argument.  So, we're using
-                    # that, too.
-                    return prevp.prefix
-
-        elif prevp.type in VARARGS_SPECIALS:
-            if is_vararg(prevp, within=VARARGS_PARENTS | UNPACKING_PARENTS):
-                return NO
-
-        elif prevp.type == token.COLON:
-            if prevp.parent and prevp.parent.type in {syms.subscript, syms.sliceop}:
-                return SPACE if complex_subscript else NO
-
-        elif (
-            prevp.parent
-            and prevp.parent.type == syms.factor
-            and prevp.type in MATH_OPERATORS
-        ):
-            return NO
-
-        elif (
-            prevp.type == token.RIGHTSHIFT
-            and prevp.parent
-            and prevp.parent.type == syms.shift_expr
-            and prevp.prev_sibling
-            and prevp.prev_sibling.type == token.NAME
-            and prevp.prev_sibling.value == "print"  # type: ignore
-        ):
-            # Python 2 print chevron
-            return NO
-        elif prevp.type == token.AT and p.parent and p.parent.type == syms.decorator:
-            # no space in decorators
-            return NO
-
-    elif prev.type in OPENING_BRACKETS:
-        return NO
-
-    if p.type in {syms.parameters, syms.arglist}:
-        # untyped function signatures or calls
-        if not prev or prev.type != token.COMMA:
-            return NO
-
-    elif p.type == syms.varargslist:
-        # lambdas
-        if prev and prev.type != token.COMMA:
-            return NO
-
-    elif p.type == syms.typedargslist:
-        # typed function signatures
-        if not prev:
-            return NO
-
-        if t == token.EQUAL:
-            if prev.type != syms.tname:
-                return NO
-
-        elif prev.type == token.EQUAL:
-            # A bit hacky: if the equal sign has whitespace, it means we
-            # previously found it's a typed argument.  So, we're using that, too.
-            return prev.prefix
-
-        elif prev.type != token.COMMA:
-            return NO
-
-    elif p.type == syms.tname:
-        # type names
-        if not prev:
-            prevp = preceding_leaf(p)
-            if not prevp or prevp.type != token.COMMA:
-                return NO
-
-    elif p.type == syms.trailer:
-        # attributes and calls
-        if t == token.LPAR or t == token.RPAR:
-            return NO
-
-        if not prev:
-            if t == token.DOT:
-                prevp = preceding_leaf(p)
-                if not prevp or prevp.type != token.NUMBER:
-                    return NO
-
-            elif t == token.LSQB:
-                return NO
-
-        elif prev.type != token.COMMA:
-            return NO
-
-    elif p.type == syms.argument:
-        # single argument
-        if t == token.EQUAL:
-            return NO
-
-        if not prev:
-            prevp = preceding_leaf(p)
-            if not prevp or prevp.type == token.LPAR:
-                return NO
-
-        elif prev.type in {token.EQUAL} | VARARGS_SPECIALS:
-            return NO
-
-    elif p.type == syms.decorator:
-        # decorators
-        return NO
-
-    elif p.type == syms.dotted_name:
-        if prev:
-            return NO
-
-        prevp = preceding_leaf(p)
-        if not prevp or prevp.type == token.AT or prevp.type == token.DOT:
-            return NO
-
-    elif p.type == syms.classdef:
-        if t == token.LPAR:
-            return NO
-
-        if prev and prev.type == token.LPAR:
-            return NO
-
-    elif p.type in {syms.subscript, syms.sliceop}:
-        # indexing
-        if not prev:
-            assert p.parent is not None, "subscripts are always parented"
-            if p.parent.type == syms.subscriptlist:
-                return SPACE
-
-            return NO
-
-        elif not complex_subscript:
-            return NO
-
-    elif p.type == syms.atom:
-        if prev and t == token.DOT:
-            # dots, but not the first one.
-            return NO
-
-    elif p.type == syms.dictsetmaker:
-        # dict unpacking
-        if prev and prev.type == token.DOUBLESTAR:
-            return NO
-
-    elif p.type in {syms.factor, syms.star_expr}:
-        # unary ops
-        if not prev:
-            prevp = preceding_leaf(p)
-            if not prevp or prevp.type in OPENING_BRACKETS:
-                return NO
-
-            prevp_parent = prevp.parent
-            assert prevp_parent is not None
-            if prevp.type == token.COLON and prevp_parent.type in {
-                syms.subscript,
-                syms.sliceop,
-            }:
-                return NO
-
-            elif prevp.type == token.EQUAL and prevp_parent.type == syms.argument:
-                return NO
-
-        elif t in {token.NAME, token.NUMBER, token.STRING}:
-            return NO
-
-    elif p.type == syms.import_from:
-        if t == token.DOT:
-            if prev and prev.type == token.DOT:
-                return NO
-
-        elif t == token.NAME:
-            if v == "import":
-                return SPACE
-
-            if prev and prev.type == token.DOT:
-                return NO
-
-    elif p.type == syms.sliceop:
-        return NO
-
-    return SPACE
-
-
-def preceding_leaf(node: Optional[LN]) -> Optional[Leaf]:
-    """Return the first leaf that precedes `node`, if any."""
-    while node:
-        res = node.prev_sibling
-        if res:
-            if isinstance(res, Leaf):
-                return res
-
-            try:
-                return list(res.leaves())[-1]
-
-            except IndexError:
-                return None
-
-        node = node.parent
-    return None
-
-
-def prev_siblings_are(node: Optional[LN], tokens: List[Optional[NodeType]]) -> bool:
-    """Return if the `node` and its previous siblings match types against the provided
-    list of tokens; the provided `node`has its type matched against the last element in
-    the list.  `None` can be used as the first element to declare that the start of the
-    list is anchored at the start of its parent's children."""
-    if not tokens:
-        return True
-    if tokens[-1] is None:
-        return node is None
-    if not node:
-        return False
-    if node.type != tokens[-1]:
-        return False
-    return prev_siblings_are(node.prev_sibling, tokens[:-1])
-
-
-def child_towards(ancestor: Node, descendant: LN) -> Optional[LN]:
-    """Return the child of `ancestor` that contains `descendant`."""
-    node: Optional[LN] = descendant
-    while node and node.parent != ancestor:
-        node = node.parent
-    return node
-
-
-def container_of(leaf: Leaf) -> LN:
-    """Return `leaf` or one of its ancestors that is the topmost container of it.
-
-    By "container" we mean a node where `leaf` is the very first child.
-    """
-    same_prefix = leaf.prefix
-    container: LN = leaf
-    while container:
-        parent = container.parent
-        if parent is None:
-            break
-
-        if parent.children[0].prefix != same_prefix:
-            break
-
-        if parent.type == syms.file_input:
-            break
-
-        if parent.prev_sibling is not None and parent.prev_sibling.type in BRACKETS:
-            break
-
-        container = parent
-    return container
-
-
-def is_split_after_delimiter(leaf: Leaf, previous: Optional[Leaf] = None) -> Priority:
-    """Return the priority of the `leaf` delimiter, given a line break after it.
-
-    The delimiter priorities returned here are from those delimiters that would
-    cause a line break after themselves.
-
-    Higher numbers are higher priority.
-    """
-    if leaf.type == token.COMMA:
-        return COMMA_PRIORITY
-
-    return 0
-
-
-def is_split_before_delimiter(leaf: Leaf, previous: Optional[Leaf] = None) -> Priority:
-    """Return the priority of the `leaf` delimiter, given a line break before it.
-
-    The delimiter priorities returned here are from those delimiters that would
-    cause a line break before themselves.
-
-    Higher numbers are higher priority.
-    """
-    if is_vararg(leaf, within=VARARGS_PARENTS | UNPACKING_PARENTS):
-        # * and ** might also be MATH_OPERATORS but in this case they are not.
-        # Don't treat them as a delimiter.
-        return 0
-
-    if (
-        leaf.type == token.DOT
-        and leaf.parent
-        and leaf.parent.type not in {syms.import_from, syms.dotted_name}
-        and (previous is None or previous.type in CLOSING_BRACKETS)
-    ):
-        return DOT_PRIORITY
-
-    if (
-        leaf.type in MATH_OPERATORS
-        and leaf.parent
-        and leaf.parent.type not in {syms.factor, syms.star_expr}
-    ):
-        return MATH_PRIORITIES[leaf.type]
-
-    if leaf.type in COMPARATORS:
-        return COMPARATOR_PRIORITY
-
-    if (
-        leaf.type == token.STRING
-        and previous is not None
-        and previous.type == token.STRING
-    ):
-        return STRING_PRIORITY
-
-    if leaf.type not in {token.NAME, token.ASYNC}:
-        return 0
-
-    if (
-        leaf.value == "for"
-        and leaf.parent
-        and leaf.parent.type in {syms.comp_for, syms.old_comp_for}
-        or leaf.type == token.ASYNC
-    ):
-        if (
-            not isinstance(leaf.prev_sibling, Leaf)
-            or leaf.prev_sibling.value != "async"
-        ):
-            return COMPREHENSION_PRIORITY
-
-    if (
-        leaf.value == "if"
-        and leaf.parent
-        and leaf.parent.type in {syms.comp_if, syms.old_comp_if}
-    ):
-        return COMPREHENSION_PRIORITY
-
-    if leaf.value in {"if", "else"} and leaf.parent and leaf.parent.type == syms.test:
-        return TERNARY_PRIORITY
-
-    if leaf.value == "is":
-        return COMPARATOR_PRIORITY
-
-    if (
-        leaf.value == "in"
-        and leaf.parent
-        and leaf.parent.type in {syms.comp_op, syms.comparison}
-        and not (
-            previous is not None
-            and previous.type == token.NAME
-            and previous.value == "not"
-        )
-    ):
-        return COMPARATOR_PRIORITY
-
-    if (
-        leaf.value == "not"
-        and leaf.parent
-        and leaf.parent.type == syms.comp_op
-        and not (
-            previous is not None
-            and previous.type == token.NAME
-            and previous.value == "is"
-        )
-    ):
-        return COMPARATOR_PRIORITY
-
-    if leaf.value in LOGIC_OPERATORS and leaf.parent:
-        return LOGIC_PRIORITY
-
-    return 0
-
-
-FMT_OFF = {"# fmt: off", "# fmt:off", "# yapf: disable"}
-FMT_SKIP = {"# fmt: skip", "# fmt:skip"}
-FMT_PASS = {*FMT_OFF, *FMT_SKIP}
-FMT_ON = {"# fmt: on", "# fmt:on", "# yapf: enable"}
-
-
-def generate_comments(leaf: LN) -> Iterator[Leaf]:
-    """Clean the prefix of the `leaf` and generate comments from it, if any.
-
-    Comments in lib2to3 are shoved into the whitespace prefix.  This happens
-    in `pgen2/driver.py:Driver.parse_tokens()`.  This was a brilliant implementation
-    move because it does away with modifying the grammar to include all the
-    possible places in which comments can be placed.
-
-    The sad consequence for us though is that comments don't "belong" anywhere.
-    This is why this function generates simple parentless Leaf objects for
-    comments.  We simply don't know what the correct parent should be.
-
-    No matter though, we can live without this.  We really only need to
-    differentiate between inline and standalone comments.  The latter don't
-    share the line with any code.
-
-    Inline comments are emitted as regular token.COMMENT leaves.  Standalone
-    are emitted with a fake STANDALONE_COMMENT token identifier.
-    """
-    for pc in list_comments(leaf.prefix, is_endmarker=leaf.type == token.ENDMARKER):
-        yield Leaf(pc.type, pc.value, prefix="\n" * pc.newlines)
-
-
-@dataclass
-class ProtoComment:
-    """Describes a piece of syntax that is a comment.
-
-    It's not a :class:`blib2to3.pytree.Leaf` so that:
+def get_features_used(  # noqa: C901
+    node: Node, *, future_imports: Optional[Set[str]] = None
+) -> Set[Feature]:
+    """Return a set of (relatively) new Python features used in this file.

 
 

-    * it can be cached (`Leaf` objects should not be reused more than once as
-      they store their lineno, column, prefix, and parent information);
-    * `newlines` and `consumed` fields are kept separate from the `value`. This
-      simplifies handling of special marker comments like ``# fmt: off/on``.
+    Currently looking for:
+    - f-strings;
+    - self-documenting expressions in f-strings (f"{x=}");
+    - underscores in numeric literals;
+    - trailing commas after * or ** in function signatures and calls;
+    - positional only arguments in function signatures and lambdas;
+    - assignment expression;
+    - relaxed decorator syntax;
+    - usage of __future__ flags (annotations);
+    - print / exec statements;
+    - parenthesized context managers;
+    - match statements;
+    - except* clause;
+    - variadic generics;

     """
     """

+    features: Set[Feature] = set()
+    if future_imports:
+        features |= {
+            FUTURE_FLAG_TO_FEATURE[future_import]
+            for future_import in future_imports
+            if future_import in FUTURE_FLAG_TO_FEATURE
+        }

 
 

-    type: int  # token.COMMENT or STANDALONE_COMMENT
-    value: str  # content of the comment
-    newlines: int  # how many newlines before the comment
-    consumed: int  # how many characters of the original leaf's prefix did we consume
-
-
-@lru_cache(maxsize=4096)
-def list_comments(prefix: str, *, is_endmarker: bool) -> List[ProtoComment]:
-    """Return a list of :class:`ProtoComment` objects parsed from the given `prefix`."""
-    result: List[ProtoComment] = []
-    if not prefix or "#" not in prefix:
-        return result
-
-    consumed = 0
-    nlines = 0
-    ignored_lines = 0
-    for index, line in enumerate(re.split("\r?\n", prefix)):
-        consumed += len(line) + 1  # adding the length of the split '\n'
-        line = line.lstrip()
-        if not line:
-            nlines += 1
-        if not line.startswith("#"):
-            # Escaped newlines outside of a comment are not really newlines at
-            # all. We treat a single-line comment following an escaped newline
-            # as a simple trailing comment.
-            if line.endswith("\\"):
-                ignored_lines += 1
-            continue
-
-        if index == ignored_lines and not is_endmarker:
-            comment_type = token.COMMENT  # simple trailing comment
-        else:
-            comment_type = STANDALONE_COMMENT
-        comment = make_comment(line)
-        result.append(
-            ProtoComment(
-                type=comment_type, value=comment, newlines=nlines, consumed=consumed
-            )
-        )
-        nlines = 0
-    return result
-
-
-def make_comment(content: str) -> str:
-    """Return a consistently formatted comment from the given `content` string.
-
-    All comments (except for "##", "#!", "#:", '#'", "#%%") should have a single
-    space between the hash sign and the content.
-
-    If `content` didn't start with a hash sign, one is provided.
-    """
-    content = content.rstrip()
-    if not content:
-        return "#"
-
-    if content[0] == "#":
-        content = content[1:]
-    if content and content[0] not in " !:#'%":
-        content = " " + content
-    return "#" + content
-
-
-def transform_line(
-    line: Line, mode: Mode, features: Collection[Feature] = ()
-) -> Iterator[Line]:
-    """Transform a `line`, potentially splitting it into many lines.
-
-    They should fit in the allotted `line_length` but might not be able to.
-
-    `features` are syntactical features that may be used in the output.
-    """
-    if line.is_comment:
-        yield line
-        return
-
-    line_str = line_to_string(line)
-
-    def init_st(ST: Type[StringTransformer]) -> StringTransformer:
-        """Initialize StringTransformer"""
-        return ST(mode.line_length, mode.string_normalization)
-
-    string_merge = init_st(StringMerger)
-    string_paren_strip = init_st(StringParenStripper)
-    string_split = init_st(StringSplitter)
-    string_paren_wrap = init_st(StringParenWrapper)
-
-    transformers: List[Transformer]
-    if (
-        not line.contains_uncollapsable_type_comments()
-        and not line.should_split_rhs
-        and not line.magic_trailing_comma
-        and (
-            is_line_short_enough(line, line_length=mode.line_length, line_str=line_str)
-            or line.contains_unsplittable_type_ignore()
-        )
-        and not (line.inside_brackets and line.contains_standalone_comments())
-    ):
-        # Only apply basic string preprocessing, since lines shouldn't be split here.
-        if mode.experimental_string_processing:
-            transformers = [string_merge, string_paren_strip]
-        else:
-            transformers = []
-    elif line.is_def:
-        transformers = [left_hand_split]
-    else:
-
-        def rhs(line: Line, features: Collection[Feature]) -> Iterator[Line]:
-            """Wraps calls to `right_hand_split`.
-
-            The calls increasingly `omit` right-hand trailers (bracket pairs with
-            content), meaning the trailers get glued together to split on another
-            bracket pair instead.
-            """
-            for omit in generate_trailers_to_omit(line, mode.line_length):
-                lines = list(
-                    right_hand_split(line, mode.line_length, features, omit=omit)
-                )
-                # Note: this check is only able to figure out if the first line of the
-                # *current* transformation fits in the line length.  This is true only
-                # for simple cases.  All others require running more transforms via
-                # `transform_line()`.  This check doesn't know if those would succeed.
-                if is_line_short_enough(lines[0], line_length=mode.line_length):
-                    yield from lines
-                    return
-
-            # All splits failed, best effort split with no omits.
-            # This mostly happens to multiline strings that are by definition
-            # reported as not fitting a single line, as well as lines that contain
-            # trailing commas (those have to be exploded).
-            yield from right_hand_split(
-                line, line_length=mode.line_length, features=features
-            )
-
-        if mode.experimental_string_processing:
-            if line.inside_brackets:
-                transformers = [
-                    string_merge,
-                    string_paren_strip,
-                    string_split,
-                    delimiter_split,
-                    standalone_comment_split,
-                    string_paren_wrap,
-                    rhs,
-                ]
-            else:
-                transformers = [
-                    string_merge,
-                    string_paren_strip,
-                    string_split,
-                    string_paren_wrap,
-                    rhs,
-                ]
-        else:
-            if line.inside_brackets:
-                transformers = [delimiter_split, standalone_comment_split, rhs]
-            else:
-                transformers = [rhs]
-
-    for transform in transformers:
-        # We are accumulating lines in `result` because we might want to abort
-        # mission and return the original line in the end, or attempt a different
-        # split altogether.
-        try:
-            result = run_transformer(line, transform, mode, features, line_str=line_str)
-        except CannotTransform:
-            continue
-        else:
-            yield from result
-            break
-
-    else:
-        yield line
-
-
-@dataclass  # type: ignore
-class StringTransformer(ABC):
-    """
-    An implementation of the Transformer protocol that relies on its
-    subclasses overriding the template methods `do_match(...)` and
-    `do_transform(...)`.
-
-    This Transformer works exclusively on strings (for example, by merging
-    or splitting them).
-
-    The following sections can be found among the docstrings of each concrete
-    StringTransformer subclass.
-
-    Requirements:
-        Which requirements must be met of the given Line for this
-        StringTransformer to be applied?
-
-    Transformations:
-        If the given Line meets all of the above requirements, which string
-        transformations can you expect to be applied to it by this
-        StringTransformer?
-
-    Collaborations:
-        What contractual agreements does this StringTransformer have with other
-        StringTransfomers? Such collaborations should be eliminated/minimized
-        as much as possible.
-    """
-
-    line_length: int
-    normalize_strings: bool
-    __name__ = "StringTransformer"
-
-    @abstractmethod
-    def do_match(self, line: Line) -> TMatchResult:
-        """
-        Returns:
-            * Ok(string_idx) such that `line.leaves[string_idx]` is our target
-            string, if a match was able to be made.
-                OR
-            * Err(CannotTransform), if a match was not able to be made.
-        """
-
-    @abstractmethod
-    def do_transform(self, line: Line, string_idx: int) -> Iterator[TResult[Line]]:
-        """
-        Yields:
-            * Ok(new_line) where new_line is the new transformed line.
-                OR
-            * Err(CannotTransform) if the transformation failed for some reason. The
-            `do_match(...)` template method should usually be used to reject
-            the form of the given Line, but in some cases it is difficult to
-            know whether or not a Line meets the StringTransformer's
-            requirements until the transformation is already midway.
-
-        Side Effects:
-            This method should NOT mutate @line directly, but it MAY mutate the
-            Line's underlying Node structure. (WARNING: If the underlying Node
-            structure IS altered, then this method should NOT be allowed to
-            yield an CannotTransform after that point.)
-        """
-
-    def __call__(self, line: Line, _features: Collection[Feature]) -> Iterator[Line]:
-        """
-        StringTransformer instances have a call signature that mirrors that of
-        the Transformer type.
-
-        Raises:
-            CannotTransform(...) if the concrete StringTransformer class is unable
-            to transform @line.
-        """
-        # Optimization to avoid calling `self.do_match(...)` when the line does
-        # not contain any string.
-        if not any(leaf.type == token.STRING for leaf in line.leaves):
-            raise CannotTransform("There are no strings in this line.")
-
-        match_result = self.do_match(line)
-
-        if isinstance(match_result, Err):
-            cant_transform = match_result.err()
-            raise CannotTransform(
-                f"The string transformer {self.__class__.__name__} does not recognize"
-                " this line as one that it can transform."
-            ) from cant_transform
-
-        string_idx = match_result.ok()
-
-        for line_result in self.do_transform(line, string_idx):
-            if isinstance(line_result, Err):
-                cant_transform = line_result.err()
-                raise CannotTransform(
-                    "StringTransformer failed while attempting to transform string."
-                ) from cant_transform
-            line = line_result.ok()
-            yield line
-
-
-@dataclass
-class CustomSplit:
-    """A custom (i.e. manual) string split.
-
-    A single CustomSplit instance represents a single substring.
-
-    Examples:
-        Consider the following string:
-        ```
-        "Hi there friend."
-        " This is a custom"
-        f" string {split}."
-        ```
-
-        This string will correspond to the following three CustomSplit instances:
-        ```
-        CustomSplit(False, 16)
-        CustomSplit(False, 17)
-        CustomSplit(True, 16)
-        ```
-    """
-
-    has_prefix: bool
-    break_idx: int
-
-
-class CustomSplitMapMixin:
-    """
-    This mixin class is used to map merged strings to a sequence of
-    CustomSplits, which will then be used to re-split the strings iff none of
-    the resultant substrings go over the configured max line length.
-    """
-
-    _Key = Tuple[StringID, str]
-    _CUSTOM_SPLIT_MAP: Dict[_Key, Tuple[CustomSplit, ...]] = defaultdict(tuple)
-
-    @staticmethod
-    def _get_key(string: str) -> "CustomSplitMapMixin._Key":
-        """
-        Returns:
-            A unique identifier that is used internally to map @string to a
-            group of custom splits.
-        """
-        return (id(string), string)
-
-    def add_custom_splits(
-        self, string: str, custom_splits: Iterable[CustomSplit]
-    ) -> None:
-        """Custom Split Map Setter Method
-
-        Side Effects:
-            Adds a mapping from @string to the custom splits @custom_splits.
-        """
-        key = self._get_key(string)
-        self._CUSTOM_SPLIT_MAP[key] = tuple(custom_splits)
-
-    def pop_custom_splits(self, string: str) -> List[CustomSplit]:
-        """Custom Split Map Getter Method
-
-        Returns:
-            * A list of the custom splits that are mapped to @string, if any
-            exist.
-                OR
-            * [], otherwise.
-
-        Side Effects:
-            Deletes the mapping between @string and its associated custom
-            splits (which are returned to the caller).
-        """
-        key = self._get_key(string)
-
-        custom_splits = self._CUSTOM_SPLIT_MAP[key]
-        del self._CUSTOM_SPLIT_MAP[key]
-
-        return list(custom_splits)
-
-    def has_custom_splits(self, string: str) -> bool:
-        """
-        Returns:
-            True iff @string is associated with a set of custom splits.
-        """
-        key = self._get_key(string)
-        return key in self._CUSTOM_SPLIT_MAP
-
-
-class StringMerger(CustomSplitMapMixin, StringTransformer):
-    """StringTransformer that merges strings together.
-
-    Requirements:
-        (A) The line contains adjacent strings such that ALL of the validation checks
-        listed in StringMerger.__validate_msg(...)'s docstring pass.
-            OR
-        (B) The line contains a string which uses line continuation backslashes.
-
-    Transformations:
-        Depending on which of the two requirements above where met, either:
-
-        (A) The string group associated with the target string is merged.
-            OR
-        (B) All line-continuation backslashes are removed from the target string.
-
-    Collaborations:
-        StringMerger provides custom split information to StringSplitter.
-    """
-
-    def do_match(self, line: Line) -> TMatchResult:
-        LL = line.leaves
-
-        is_valid_index = is_valid_index_factory(LL)
-
-        for (i, leaf) in enumerate(LL):
-            if (
-                leaf.type == token.STRING
-                and is_valid_index(i + 1)
-                and LL[i + 1].type == token.STRING
-            ):
-                return Ok(i)
-
-            if leaf.type == token.STRING and "\\\n" in leaf.value:
-                return Ok(i)
-
-        return TErr("This line has no strings that need merging.")
-
-    def do_transform(self, line: Line, string_idx: int) -> Iterator[TResult[Line]]:
-        new_line = line
-        rblc_result = self.__remove_backslash_line_continuation_chars(
-            new_line, string_idx
-        )
-        if isinstance(rblc_result, Ok):
-            new_line = rblc_result.ok()
-
-        msg_result = self.__merge_string_group(new_line, string_idx)
-        if isinstance(msg_result, Ok):
-            new_line = msg_result.ok()
-
-        if isinstance(rblc_result, Err) and isinstance(msg_result, Err):
-            msg_cant_transform = msg_result.err()
-            rblc_cant_transform = rblc_result.err()
-            cant_transform = CannotTransform(
-                "StringMerger failed to merge any strings in this line."
-            )
-
-            # Chain the errors together using `__cause__`.
-            msg_cant_transform.__cause__ = rblc_cant_transform
-            cant_transform.__cause__ = msg_cant_transform
-
-            yield Err(cant_transform)
-        else:
-            yield Ok(new_line)
-
-    @staticmethod
-    def __remove_backslash_line_continuation_chars(
-        line: Line, string_idx: int
-    ) -> TResult[Line]:
-        """
-        Merge strings that were split across multiple lines using
-        line-continuation backslashes.
-
-        Returns:
-            Ok(new_line), if @line contains backslash line-continuation
-            characters.
-                OR
-            Err(CannotTransform), otherwise.
-        """
-        LL = line.leaves
-
-        string_leaf = LL[string_idx]
-        if not (
-            string_leaf.type == token.STRING
-            and "\\\n" in string_leaf.value
-            and not has_triple_quotes(string_leaf.value)
-        ):
-            return TErr(
-                f"String leaf {string_leaf} does not contain any backslash line"
-                " continuation characters."
-            )
-
-        new_line = line.clone()
-        new_line.comments = line.comments.copy()
-        append_leaves(new_line, line, LL)
-
-        new_string_leaf = new_line.leaves[string_idx]
-        new_string_leaf.value = new_string_leaf.value.replace("\\\n", "")
-
-        return Ok(new_line)
-
-    def __merge_string_group(self, line: Line, string_idx: int) -> TResult[Line]:
-        """
-        Merges string group (i.e. set of adjacent strings) where the first
-        string in the group is `line.leaves[string_idx]`.
-
-        Returns:
-            Ok(new_line), if ALL of the validation checks found in
-            __validate_msg(...) pass.
-                OR
-            Err(CannotTransform), otherwise.
-        """
-        LL = line.leaves
-
-        is_valid_index = is_valid_index_factory(LL)
-
-        vresult = self.__validate_msg(line, string_idx)
-        if isinstance(vresult, Err):
-            return vresult
-
-        # If the string group is wrapped inside an Atom node, we must make sure
-        # to later replace that Atom with our new (merged) string leaf.
-        atom_node = LL[string_idx].parent
-
-        # We will place BREAK_MARK in between every two substrings that we
-        # merge. We will then later go through our final result and use the
-        # various instances of BREAK_MARK we find to add the right values to
-        # the custom split map.
-        BREAK_MARK = "@@@@@ BLACK BREAKPOINT MARKER @@@@@"
-
-        QUOTE = LL[string_idx].value[-1]
-
-        def make_naked(string: str, string_prefix: str) -> str:
-            """Strip @string (i.e. make it a "naked" string)
-
-            Pre-conditions:
-                * assert_is_leaf_string(@string)
-
-            Returns:
-                A string that is identical to @string except that
-                @string_prefix has been stripped, the surrounding QUOTE
-                characters have been removed, and any remaining QUOTE
-                characters have been escaped.
-            """
-            assert_is_leaf_string(string)
-
-            RE_EVEN_BACKSLASHES = r"(?:(?<!\\)(?:\\\\)*)"
-            naked_string = string[len(string_prefix) + 1 : -1]
-            naked_string = re.sub(
-                "(" + RE_EVEN_BACKSLASHES + ")" + QUOTE, r"\1\\" + QUOTE, naked_string
-            )
-            return naked_string
-
-        # Holds the CustomSplit objects that will later be added to the custom
-        # split map.
-        custom_splits = []
-
-        # Temporary storage for the 'has_prefix' part of the CustomSplit objects.
-        prefix_tracker = []
-
-        # Sets the 'prefix' variable. This is the prefix that the final merged
-        # string will have.
-        next_str_idx = string_idx
-        prefix = ""
-        while (
-            not prefix
-            and is_valid_index(next_str_idx)
-            and LL[next_str_idx].type == token.STRING
-        ):
-            prefix = get_string_prefix(LL[next_str_idx].value)
-            next_str_idx += 1
-
-        # The next loop merges the string group. The final string will be
-        # contained in 'S'.
-        #
-        # The following convenience variables are used:
-        #
-        #   S: string
-        #   NS: naked string
-        #   SS: next string
-        #   NSS: naked next string
-        S = ""
-        NS = ""
-        num_of_strings = 0
-        next_str_idx = string_idx
-        while is_valid_index(next_str_idx) and LL[next_str_idx].type == token.STRING:
-            num_of_strings += 1
-
-            SS = LL[next_str_idx].value
-            next_prefix = get_string_prefix(SS)
-
-            # If this is an f-string group but this substring is not prefixed
-            # with 'f'...
-            if "f" in prefix and "f" not in next_prefix:
-                # Then we must escape any braces contained in this substring.
-                SS = re.subf(r"(\{|\})", "{1}{1}", SS)
-
-            NSS = make_naked(SS, next_prefix)
-
-            has_prefix = bool(next_prefix)
-            prefix_tracker.append(has_prefix)
-
-            S = prefix + QUOTE + NS + NSS + BREAK_MARK + QUOTE
-            NS = make_naked(S, prefix)
-
-            next_str_idx += 1
-
-        S_leaf = Leaf(token.STRING, S)
-        if self.normalize_strings:
-            normalize_string_quotes(S_leaf)
-
-        # Fill the 'custom_splits' list with the appropriate CustomSplit objects.
-        temp_string = S_leaf.value[len(prefix) + 1 : -1]
-        for has_prefix in prefix_tracker:
-            mark_idx = temp_string.find(BREAK_MARK)
-            assert (
-                mark_idx >= 0
-            ), "Logic error while filling the custom string breakpoint cache."
-
-            temp_string = temp_string[mark_idx + len(BREAK_MARK) :]
-            breakpoint_idx = mark_idx + (len(prefix) if has_prefix else 0) + 1
-            custom_splits.append(CustomSplit(has_prefix, breakpoint_idx))
-
-        string_leaf = Leaf(token.STRING, S_leaf.value.replace(BREAK_MARK, ""))
-
-        if atom_node is not None:
-            replace_child(atom_node, string_leaf)
-
-        # Build the final line ('new_line') that this method will later return.
-        new_line = line.clone()
-        for (i, leaf) in enumerate(LL):
-            if i == string_idx:
-                new_line.append(string_leaf)
-
-            if string_idx <= i < string_idx + num_of_strings:
-                for comment_leaf in line.comments_after(LL[i]):
-                    new_line.append(comment_leaf, preformatted=True)
-                continue
-
-            append_leaves(new_line, line, [leaf])
-
-        self.add_custom_splits(string_leaf.value, custom_splits)
-        return Ok(new_line)
-
-    @staticmethod
-    def __validate_msg(line: Line, string_idx: int) -> TResult[None]:
-        """Validate (M)erge (S)tring (G)roup
-
-        Transform-time string validation logic for __merge_string_group(...).
-
-        Returns:
-            * Ok(None), if ALL validation checks (listed below) pass.
-                OR
-            * Err(CannotTransform), if any of the following are true:
-                - The target string group does not contain ANY stand-alone comments.
-                - The target string is not in a string group (i.e. it has no
-                  adjacent strings).
-                - The string group has more than one inline comment.
-                - The string group has an inline comment that appears to be a pragma.
-                - The set of all string prefixes in the string group is of
-                  length greater than one and is not equal to {"", "f"}.
-                - The string group consists of raw strings.
-        """
-        # We first check for "inner" stand-alone comments (i.e. stand-alone
-        # comments that have a string leaf before them AND after them).
-        for inc in [1, -1]:
-            i = string_idx
-            found_sa_comment = False
-            is_valid_index = is_valid_index_factory(line.leaves)
-            while is_valid_index(i) and line.leaves[i].type in [
-                token.STRING,
-                STANDALONE_COMMENT,
-            ]:
-                if line.leaves[i].type == STANDALONE_COMMENT:
-                    found_sa_comment = True
-                elif found_sa_comment:
-                    return TErr(
-                        "StringMerger does NOT merge string groups which contain "
-                        "stand-alone comments."
-                    )
-
-                i += inc
-
-        num_of_inline_string_comments = 0
-        set_of_prefixes = set()
-        num_of_strings = 0
-        for leaf in line.leaves[string_idx:]:
-            if leaf.type != token.STRING:
-                # If the string group is trailed by a comma, we count the
-                # comments trailing the comma to be one of the string group's
-                # comments.
-                if leaf.type == token.COMMA and id(leaf) in line.comments:
-                    num_of_inline_string_comments += 1
-                break
-
-            if has_triple_quotes(leaf.value):
-                return TErr("StringMerger does NOT merge multiline strings.")
-
-            num_of_strings += 1
-            prefix = get_string_prefix(leaf.value)
-            if "r" in prefix:
-                return TErr("StringMerger does NOT merge raw strings.")
-
-            set_of_prefixes.add(prefix)
-
-            if id(leaf) in line.comments:
-                num_of_inline_string_comments += 1
-                if contains_pragma_comment(line.comments[id(leaf)]):
-                    return TErr("Cannot merge strings which have pragma comments.")
-
-        if num_of_strings < 2:
-            return TErr(
-                f"Not enough strings to merge (num_of_strings={num_of_strings})."
-            )
-
-        if num_of_inline_string_comments > 1:
-            return TErr(
-                f"Too many inline string comments ({num_of_inline_string_comments})."
-            )
-
-        if len(set_of_prefixes) > 1 and set_of_prefixes != {"", "f"}:
-            return TErr(f"Too many different prefixes ({set_of_prefixes}).")
-
-        return Ok(None)
-
-
-class StringParenStripper(StringTransformer):
-    """StringTransformer that strips surrounding parentheses from strings.
-
-    Requirements:
-        The line contains a string which is surrounded by parentheses and:
-            - The target string is NOT the only argument to a function call.
-            - The target string is NOT a "pointless" string.
-            - If the target string contains a PERCENT, the brackets are not
-              preceeded or followed by an operator with higher precedence than
-              PERCENT.
-
-    Transformations:
-        The parentheses mentioned in the 'Requirements' section are stripped.
-
-    Collaborations:
-        StringParenStripper has its own inherent usefulness, but it is also
-        relied on to clean up the parentheses created by StringParenWrapper (in
-        the event that they are no longer needed).
-    """
-
-    def do_match(self, line: Line) -> TMatchResult:
-        LL = line.leaves
-
-        is_valid_index = is_valid_index_factory(LL)
-
-        for (idx, leaf) in enumerate(LL):
-            # Should be a string...
-            if leaf.type != token.STRING:
-                continue
-
-            # If this is a "pointless" string...
-            if (
-                leaf.parent
-                and leaf.parent.parent
-                and leaf.parent.parent.type == syms.simple_stmt
-            ):
-                continue
-
-            # Should be preceded by a non-empty LPAR...
-            if (
-                not is_valid_index(idx - 1)
-                or LL[idx - 1].type != token.LPAR
-                or is_empty_lpar(LL[idx - 1])
-            ):
-                continue
-
-            # That LPAR should NOT be preceded by a function name or a closing
-            # bracket (which could be a function which returns a function or a
-            # list/dictionary that contains a function)...
-            if is_valid_index(idx - 2) and (
-                LL[idx - 2].type == token.NAME or LL[idx - 2].type in CLOSING_BRACKETS
-            ):
-                continue
-
-            string_idx = idx
-
-            # Skip the string trailer, if one exists.
-            string_parser = StringParser()
-            next_idx = string_parser.parse(LL, string_idx)
-
-            # if the leaves in the parsed string include a PERCENT, we need to
-            # make sure the initial LPAR is NOT preceded by an operator with
-            # higher or equal precedence to PERCENT
-            if is_valid_index(idx - 2):
-                # mypy can't quite follow unless we name this
-                before_lpar = LL[idx - 2]
-                if token.PERCENT in {leaf.type for leaf in LL[idx - 1 : next_idx]} and (
-                    (
-                        before_lpar.type
-                        in {
-                            token.STAR,
-                            token.AT,
-                            token.SLASH,
-                            token.DOUBLESLASH,
-                            token.PERCENT,
-                            token.TILDE,
-                            token.DOUBLESTAR,
-                            token.AWAIT,
-                            token.LSQB,
-                            token.LPAR,
-                        }
-                    )
-                    or (
-                        # only unary PLUS/MINUS
-                        before_lpar.parent
-                        and before_lpar.parent.type == syms.factor
-                        and (before_lpar.type in {token.PLUS, token.MINUS})
-                    )
-                ):
-                    continue
-
-            # Should be followed by a non-empty RPAR...
-            if (
-                is_valid_index(next_idx)
-                and LL[next_idx].type == token.RPAR
-                and not is_empty_rpar(LL[next_idx])
-            ):
-                # That RPAR should NOT be followed by anything with higher
-                # precedence than PERCENT
-                if is_valid_index(next_idx + 1) and LL[next_idx + 1].type in {
-                    token.DOUBLESTAR,
-                    token.LSQB,
-                    token.LPAR,
-                    token.DOT,
-                }:
-                    continue
-
-                return Ok(string_idx)
-
-        return TErr("This line has no strings wrapped in parens.")
-
-    def do_transform(self, line: Line, string_idx: int) -> Iterator[TResult[Line]]:
-        LL = line.leaves
-
-        string_parser = StringParser()
-        rpar_idx = string_parser.parse(LL, string_idx)
-
-        for leaf in (LL[string_idx - 1], LL[rpar_idx]):
-            if line.comments_after(leaf):
-                yield TErr(
-                    "Will not strip parentheses which have comments attached to them."
-                )
-                return
-
-        new_line = line.clone()
-        new_line.comments = line.comments.copy()
-        try:
-            append_leaves(new_line, line, LL[: string_idx - 1])
-        except BracketMatchError:
-            # HACK: I believe there is currently a bug somewhere in
-            # right_hand_split() that is causing brackets to not be tracked
-            # properly by a shared BracketTracker.
-            append_leaves(new_line, line, LL[: string_idx - 1], preformatted=True)
-
-        string_leaf = Leaf(token.STRING, LL[string_idx].value)
-        LL[string_idx - 1].remove()
-        replace_child(LL[string_idx], string_leaf)
-        new_line.append(string_leaf)
-
-        append_leaves(
-            new_line, line, LL[string_idx + 1 : rpar_idx] + LL[rpar_idx + 1 :]
-        )
-
-        LL[rpar_idx].remove()
-
-        yield Ok(new_line)
-
-
-class BaseStringSplitter(StringTransformer):
-    """
-    Abstract class for StringTransformers which transform a Line's strings by splitting
-    them or placing them on their own lines where necessary to avoid going over
-    the configured line length.
-
-    Requirements:
-        * The target string value is responsible for the line going over the
-        line length limit. It follows that after all of black's other line
-        split methods have been exhausted, this line (or one of the resulting
-        lines after all line splits are performed) would still be over the
-        line_length limit unless we split this string.
-            AND
-        * The target string is NOT a "pointless" string (i.e. a string that has
-        no parent or siblings).
-            AND
-        * The target string is not followed by an inline comment that appears
-        to be a pragma.
-            AND
-        * The target string is not a multiline (i.e. triple-quote) string.
-    """
-
-    @abstractmethod
-    def do_splitter_match(self, line: Line) -> TMatchResult:
-        """
-        BaseStringSplitter asks its clients to override this method instead of
-        `StringTransformer.do_match(...)`.
-
-        Follows the same protocol as `StringTransformer.do_match(...)`.
-
-        Refer to `help(StringTransformer.do_match)` for more information.
-        """
-
-    def do_match(self, line: Line) -> TMatchResult:
-        match_result = self.do_splitter_match(line)
-        if isinstance(match_result, Err):
-            return match_result
-
-        string_idx = match_result.ok()
-        vresult = self.__validate(line, string_idx)
-        if isinstance(vresult, Err):
-            return vresult
-
-        return match_result
-
-    def __validate(self, line: Line, string_idx: int) -> TResult[None]:
-        """
-        Checks that @line meets all of the requirements listed in this classes'
-        docstring. Refer to `help(BaseStringSplitter)` for a detailed
-        description of those requirements.
-
-        Returns:
-            * Ok(None), if ALL of the requirements are met.
-                OR
-            * Err(CannotTransform), if ANY of the requirements are NOT met.
-        """
-        LL = line.leaves
-
-        string_leaf = LL[string_idx]
-
-        max_string_length = self.__get_max_string_length(line, string_idx)
-        if len(string_leaf.value) <= max_string_length:
-            return TErr(
-                "The string itself is not what is causing this line to be too long."
-            )
-
-        if not string_leaf.parent or [L.type for L in string_leaf.parent.children] == [
-            token.STRING,
-            token.NEWLINE,
-        ]:
-            return TErr(
-                f"This string ({string_leaf.value}) appears to be pointless (i.e. has"
-                " no parent)."
-            )
-
-        if id(line.leaves[string_idx]) in line.comments and contains_pragma_comment(
-            line.comments[id(line.leaves[string_idx])]
-        ):
-            return TErr(
-                "Line appears to end with an inline pragma comment. Splitting the line"
-                " could modify the pragma's behavior."
-            )
-
-        if has_triple_quotes(string_leaf.value):
-            return TErr("We cannot split multiline strings.")
-
-        return Ok(None)
-
-    def __get_max_string_length(self, line: Line, string_idx: int) -> int:
-        """
-        Calculates the max string length used when attempting to determine
-        whether or not the target string is responsible for causing the line to
-        go over the line length limit.
-
-        WARNING: This method is tightly coupled to both StringSplitter and
-        (especially) StringParenWrapper. There is probably a better way to
-        accomplish what is being done here.
-
-        Returns:
-            max_string_length: such that `line.leaves[string_idx].value >
-            max_string_length` implies that the target string IS responsible
-            for causing this line to exceed the line length limit.
-        """
-        LL = line.leaves
-
-        is_valid_index = is_valid_index_factory(LL)
-
-        # We use the shorthand "WMA4" in comments to abbreviate "We must
-        # account for". When giving examples, we use STRING to mean some/any
-        # valid string.
-        #
-        # Finally, we use the following convenience variables:
-        #
-        #   P:  The leaf that is before the target string leaf.
-        #   N:  The leaf that is after the target string leaf.
-        #   NN: The leaf that is after N.
-
-        # WMA4 the whitespace at the beginning of the line.
-        offset = line.depth * 4
-
-        if is_valid_index(string_idx - 1):
-            p_idx = string_idx - 1
-            if (
-                LL[string_idx - 1].type == token.LPAR
-                and LL[string_idx - 1].value == ""
-                and string_idx >= 2
-            ):
-                # If the previous leaf is an empty LPAR placeholder, we should skip it.
-                p_idx -= 1
-
-            P = LL[p_idx]
-            if P.type == token.PLUS:
-                # WMA4 a space and a '+' character (e.g. `+ STRING`).
-                offset += 2
-
-            if P.type == token.COMMA:
-                # WMA4 a space, a comma, and a closing bracket [e.g. `), STRING`].
-                offset += 3
-
-            if P.type in [token.COLON, token.EQUAL, token.NAME]:
-                # This conditional branch is meant to handle dictionary keys,
-                # variable assignments, 'return STRING' statement lines, and
-                # 'else STRING' ternary expression lines.
-
-                # WMA4 a single space.
-                offset += 1
-
-                # WMA4 the lengths of any leaves that came before that space,
-                # but after any closing bracket before that space.
-                for leaf in reversed(LL[: p_idx + 1]):
-                    offset += len(str(leaf))
-                    if leaf.type in CLOSING_BRACKETS:
-                        break
-
-        if is_valid_index(string_idx + 1):
-            N = LL[string_idx + 1]
-            if N.type == token.RPAR and N.value == "" and len(LL) > string_idx + 2:
-                # If the next leaf is an empty RPAR placeholder, we should skip it.
-                N = LL[string_idx + 2]
-
-            if N.type == token.COMMA:
-                # WMA4 a single comma at the end of the string (e.g `STRING,`).
-                offset += 1
-
-            if is_valid_index(string_idx + 2):
-                NN = LL[string_idx + 2]
-
-                if N.type == token.DOT and NN.type == token.NAME:
-                    # This conditional branch is meant to handle method calls invoked
-                    # off of a string literal up to and including the LPAR character.
-
-                    # WMA4 the '.' character.
-                    offset += 1
-
-                    if (
-                        is_valid_index(string_idx + 3)
-                        and LL[string_idx + 3].type == token.LPAR
-                    ):
-                        # WMA4 the left parenthesis character.
-                        offset += 1
-
-                    # WMA4 the length of the method's name.
-                    offset += len(NN.value)
-
-        has_comments = False
-        for comment_leaf in line.comments_after(LL[string_idx]):
-            if not has_comments:
-                has_comments = True
-                # WMA4 two spaces before the '#' character.
-                offset += 2
-
-            # WMA4 the length of the inline comment.
-            offset += len(comment_leaf.value)
-
-        max_string_length = self.line_length - offset
-        return max_string_length
-
-
-class StringSplitter(CustomSplitMapMixin, BaseStringSplitter):
-    """
-    StringTransformer that splits "atom" strings (i.e. strings which exist on
-    lines by themselves).
-
-    Requirements:
-        * The line consists ONLY of a single string (with the exception of a
-        '+' symbol which MAY exist at the start of the line), MAYBE a string
-        trailer, and MAYBE a trailing comma.
-            AND
-        * All of the requirements listed in BaseStringSplitter's docstring.
-
-    Transformations:
-        The string mentioned in the 'Requirements' section is split into as
-        many substrings as necessary to adhere to the configured line length.
-
-        In the final set of substrings, no substring should be smaller than
-        MIN_SUBSTR_SIZE characters.
-
-        The string will ONLY be split on spaces (i.e. each new substring should
-        start with a space). Note that the string will NOT be split on a space
-        which is escaped with a backslash.
-
-        If the string is an f-string, it will NOT be split in the middle of an
-        f-expression (e.g. in f"FooBar: {foo() if x else bar()}", {foo() if x
-        else bar()} is an f-expression).
-
-        If the string that is being split has an associated set of custom split
-        records and those custom splits will NOT result in any line going over
-        the configured line length, those custom splits are used. Otherwise the
-        string is split as late as possible (from left-to-right) while still
-        adhering to the transformation rules listed above.
-
-    Collaborations:
-        StringSplitter relies on StringMerger to construct the appropriate
-        CustomSplit objects and add them to the custom split map.
-    """
-
-    MIN_SUBSTR_SIZE = 6
-    # Matches an "f-expression" (e.g. {var}) that might be found in an f-string.
-    RE_FEXPR = r"""
-    (?<!\{) (?:\{\{)* \{ (?!\{)
-        (?:
-            [^\{\}]
-            | \{\{
-            | \}\}
-            | (?R)
-        )+?
-    (?<!\}) \} (?:\}\})* (?!\})
-    """
-
-    def do_splitter_match(self, line: Line) -> TMatchResult:
-        LL = line.leaves
-
-        is_valid_index = is_valid_index_factory(LL)
-
-        idx = 0
-
-        # The first leaf MAY be a '+' symbol...
-        if is_valid_index(idx) and LL[idx].type == token.PLUS:
-            idx += 1
-
-        # The next/first leaf MAY be an empty LPAR...
-        if is_valid_index(idx) and is_empty_lpar(LL[idx]):
-            idx += 1
-
-        # The next/first leaf MUST be a string...
-        if not is_valid_index(idx) or LL[idx].type != token.STRING:
-            return TErr("Line does not start with a string.")
-
-        string_idx = idx
-
-        # Skip the string trailer, if one exists.
-        string_parser = StringParser()
-        idx = string_parser.parse(LL, string_idx)
-
-        # That string MAY be followed by an empty RPAR...
-        if is_valid_index(idx) and is_empty_rpar(LL[idx]):
-            idx += 1
-
-        # That string / empty RPAR leaf MAY be followed by a comma...
-        if is_valid_index(idx) and LL[idx].type == token.COMMA:
-            idx += 1
-
-        # But no more leaves are allowed...
-        if is_valid_index(idx):
-            return TErr("This line does not end with a string.")
-
-        return Ok(string_idx)
-
-    def do_transform(self, line: Line, string_idx: int) -> Iterator[TResult[Line]]:
-        LL = line.leaves
-
-        QUOTE = LL[string_idx].value[-1]
-
-        is_valid_index = is_valid_index_factory(LL)
-        insert_str_child = insert_str_child_factory(LL[string_idx])
-
-        prefix = get_string_prefix(LL[string_idx].value)
-
-        # We MAY choose to drop the 'f' prefix from substrings that don't
-        # contain any f-expressions, but ONLY if the original f-string
-        # contains at least one f-expression. Otherwise, we will alter the AST
-        # of the program.
-        drop_pointless_f_prefix = ("f" in prefix) and re.search(
-            self.RE_FEXPR, LL[string_idx].value, re.VERBOSE
-        )
-
-        first_string_line = True
-        starts_with_plus = LL[0].type == token.PLUS
-
-        def line_needs_plus() -> bool:
-            return first_string_line and starts_with_plus
-
-        def maybe_append_plus(new_line: Line) -> None:
-            """
-            Side Effects:
-                If @line starts with a plus and this is the first line we are
-                constructing, this function appends a PLUS leaf to @new_line
-                and replaces the old PLUS leaf in the node structure. Otherwise
-                this function does nothing.
-            """
-            if line_needs_plus():
-                plus_leaf = Leaf(token.PLUS, "+")
-                replace_child(LL[0], plus_leaf)
-                new_line.append(plus_leaf)
-
-        ends_with_comma = (
-            is_valid_index(string_idx + 1) and LL[string_idx + 1].type == token.COMMA
-        )
-
-        def max_last_string() -> int:
-            """
-            Returns:
-                The max allowed length of the string value used for the last
-                line we will construct.
-            """
-            result = self.line_length
-            result -= line.depth * 4
-            result -= 1 if ends_with_comma else 0
-            result -= 2 if line_needs_plus() else 0
-            return result
-
-        # --- Calculate Max Break Index (for string value)
-        # We start with the line length limit
-        max_break_idx = self.line_length
-        # The last index of a string of length N is N-1.
-        max_break_idx -= 1
-        # Leading whitespace is not present in the string value (e.g. Leaf.value).
-        max_break_idx -= line.depth * 4
-        if max_break_idx < 0:
-            yield TErr(
-                f"Unable to split {LL[string_idx].value} at such high of a line depth:"
-                f" {line.depth}"
-            )
-            return
-
-        # Check if StringMerger registered any custom splits.
-        custom_splits = self.pop_custom_splits(LL[string_idx].value)
-        # We use them ONLY if none of them would produce lines that exceed the
-        # line limit.
-        use_custom_breakpoints = bool(
-            custom_splits
-            and all(csplit.break_idx <= max_break_idx for csplit in custom_splits)
-        )
-
-        # Temporary storage for the remaining chunk of the string line that
-        # can't fit onto the line currently being constructed.
-        rest_value = LL[string_idx].value
-
-        def more_splits_should_be_made() -> bool:
-            """
-            Returns:
-                True iff `rest_value` (the remaining string value from the last
-                split), should be split again.
-            """
-            if use_custom_breakpoints:
-                return len(custom_splits) > 1
-            else:
-                return len(rest_value) > max_last_string()
-
-        string_line_results: List[Ok[Line]] = []
-        while more_splits_should_be_made():
-            if use_custom_breakpoints:
-                # Custom User Split (manual)
-                csplit = custom_splits.pop(0)
-                break_idx = csplit.break_idx
-            else:
-                # Algorithmic Split (automatic)
-                max_bidx = max_break_idx - 2 if line_needs_plus() else max_break_idx
-                maybe_break_idx = self.__get_break_idx(rest_value, max_bidx)
-                if maybe_break_idx is None:
-                    # If we are unable to algorithmically determine a good split
-                    # and this string has custom splits registered to it, we
-                    # fall back to using them--which means we have to start
-                    # over from the beginning.
-                    if custom_splits:
-                        rest_value = LL[string_idx].value
-                        string_line_results = []
-                        first_string_line = True
-                        use_custom_breakpoints = True
-                        continue
-
-                    # Otherwise, we stop splitting here.
-                    break
-
-                break_idx = maybe_break_idx
-
-            # --- Construct `next_value`
-            next_value = rest_value[:break_idx] + QUOTE
-            if (
-                # Are we allowed to try to drop a pointless 'f' prefix?
-                drop_pointless_f_prefix
-                # If we are, will we be successful?
-                and next_value != self.__normalize_f_string(next_value, prefix)
-            ):
-                # If the current custom split did NOT originally use a prefix,
-                # then `csplit.break_idx` will be off by one after removing
-                # the 'f' prefix.
-                break_idx = (
-                    break_idx + 1
-                    if use_custom_breakpoints and not csplit.has_prefix
-                    else break_idx
-                )
-                next_value = rest_value[:break_idx] + QUOTE
-                next_value = self.__normalize_f_string(next_value, prefix)
-
-            # --- Construct `next_leaf`
-            next_leaf = Leaf(token.STRING, next_value)
-            insert_str_child(next_leaf)
-            self.__maybe_normalize_string_quotes(next_leaf)
-
-            # --- Construct `next_line`
-            next_line = line.clone()
-            maybe_append_plus(next_line)
-            next_line.append(next_leaf)
-            string_line_results.append(Ok(next_line))
-
-            rest_value = prefix + QUOTE + rest_value[break_idx:]
-            first_string_line = False
-
-        yield from string_line_results
-
-        if drop_pointless_f_prefix:
-            rest_value = self.__normalize_f_string(rest_value, prefix)
-
-        rest_leaf = Leaf(token.STRING, rest_value)
-        insert_str_child(rest_leaf)
-
-        # NOTE: I could not find a test case that verifies that the following
-        # line is actually necessary, but it seems to be. Otherwise we risk
-        # not normalizing the last substring, right?
-        self.__maybe_normalize_string_quotes(rest_leaf)
-
-        last_line = line.clone()
-        maybe_append_plus(last_line)
-
-        # If there are any leaves to the right of the target string...
-        if is_valid_index(string_idx + 1):
-            # We use `temp_value` here to determine how long the last line
-            # would be if we were to append all the leaves to the right of the
-            # target string to the last string line.
-            temp_value = rest_value
-            for leaf in LL[string_idx + 1 :]:
-                temp_value += str(leaf)
-                if leaf.type == token.LPAR:
-                    break
-
-            # Try to fit them all on the same line with the last substring...
-            if (
-                len(temp_value) <= max_last_string()
-                or LL[string_idx + 1].type == token.COMMA
-            ):
-                last_line.append(rest_leaf)
-                append_leaves(last_line, line, LL[string_idx + 1 :])
-                yield Ok(last_line)
-            # Otherwise, place the last substring on one line and everything
-            # else on a line below that...
-            else:
-                last_line.append(rest_leaf)
-                yield Ok(last_line)
-
-                non_string_line = line.clone()
-                append_leaves(non_string_line, line, LL[string_idx + 1 :])
-                yield Ok(non_string_line)
-        # Else the target string was the last leaf...
-        else:
-            last_line.append(rest_leaf)
-            last_line.comments = line.comments.copy()
-            yield Ok(last_line)
-
-    def __get_break_idx(self, string: str, max_break_idx: int) -> Optional[int]:
-        """
-        This method contains the algorithm that StringSplitter uses to
-        determine which character to split each string at.
-
-        Args:
-            @string: The substring that we are attempting to split.
-            @max_break_idx: The ideal break index. We will return this value if it
-            meets all the necessary conditions. In the likely event that it
-            doesn't we will try to find the closest index BELOW @max_break_idx
-            that does. If that fails, we will expand our search by also
-            considering all valid indices ABOVE @max_break_idx.
-
-        Pre-Conditions:
-            * assert_is_leaf_string(@string)
-            * 0 <= @max_break_idx < len(@string)
-
-        Returns:
-            break_idx, if an index is able to be found that meets all of the
-            conditions listed in the 'Transformations' section of this classes'
-            docstring.
-                OR
-            None, otherwise.
-        """
-        is_valid_index = is_valid_index_factory(string)
-
-        assert is_valid_index(max_break_idx)
-        assert_is_leaf_string(string)
-
-        _fexpr_slices: Optional[List[Tuple[Index, Index]]] = None
-
-        def fexpr_slices() -> Iterator[Tuple[Index, Index]]:
-            """
-            Yields:
-                All ranges of @string which, if @string were to be split there,
-                would result in the splitting of an f-expression (which is NOT
-                allowed).
-            """
-            nonlocal _fexpr_slices
-
-            if _fexpr_slices is None:
-                _fexpr_slices = []
-                for match in re.finditer(self.RE_FEXPR, string, re.VERBOSE):
-                    _fexpr_slices.append(match.span())
-
-            yield from _fexpr_slices
-
-        is_fstring = "f" in get_string_prefix(string)
-
-        def breaks_fstring_expression(i: Index) -> bool:
-            """
-            Returns:
-                True iff returning @i would result in the splitting of an
-                f-expression (which is NOT allowed).
-            """
-            if not is_fstring:
-                return False
-
-            for (start, end) in fexpr_slices():
-                if start <= i < end:
-                    return True
-
-            return False
-
-        def passes_all_checks(i: Index) -> bool:
-            """
-            Returns:
-                True iff ALL of the conditions listed in the 'Transformations'
-                section of this classes' docstring would be be met by returning @i.
-            """
-            is_space = string[i] == " "
-
-            is_not_escaped = True
-            j = i - 1
-            while is_valid_index(j) and string[j] == "\\":
-                is_not_escaped = not is_not_escaped
-                j -= 1
-
-            is_big_enough = (
-                len(string[i:]) >= self.MIN_SUBSTR_SIZE
-                and len(string[:i]) >= self.MIN_SUBSTR_SIZE
-            )
-            return (
-                is_space
-                and is_not_escaped
-                and is_big_enough
-                and not breaks_fstring_expression(i)
-            )
-
-        # First, we check all indices BELOW @max_break_idx.
-        break_idx = max_break_idx
-        while is_valid_index(break_idx - 1) and not passes_all_checks(break_idx):
-            break_idx -= 1
-
-        if not passes_all_checks(break_idx):
-            # If that fails, we check all indices ABOVE @max_break_idx.
-            #
-            # If we are able to find a valid index here, the next line is going
-            # to be longer than the specified line length, but it's probably
-            # better than doing nothing at all.
-            break_idx = max_break_idx + 1
-            while is_valid_index(break_idx + 1) and not passes_all_checks(break_idx):
-                break_idx += 1
-
-            if not is_valid_index(break_idx) or not passes_all_checks(break_idx):
-                return None
-
-        return break_idx
-
-    def __maybe_normalize_string_quotes(self, leaf: Leaf) -> None:
-        if self.normalize_strings:
-            normalize_string_quotes(leaf)
-
-    def __normalize_f_string(self, string: str, prefix: str) -> str:
-        """
-        Pre-Conditions:
-            * assert_is_leaf_string(@string)
-
-        Returns:
-            * If @string is an f-string that contains no f-expressions, we
-            return a string identical to @string except that the 'f' prefix
-            has been stripped and all double braces (i.e. '{{' or '}}') have
-            been normalized (i.e. turned into '{' or '}').
-                OR
-            * Otherwise, we return @string.
-        """
-        assert_is_leaf_string(string)
-
-        if "f" in prefix and not re.search(self.RE_FEXPR, string, re.VERBOSE):
-            new_prefix = prefix.replace("f", "")
-
-            temp = string[len(prefix) :]
-            temp = re.sub(r"\{\{", "{", temp)
-            temp = re.sub(r"\}\}", "}", temp)
-            new_string = temp
-
-            return f"{new_prefix}{new_string}"
-        else:
-            return string
-
-
-class StringParenWrapper(CustomSplitMapMixin, BaseStringSplitter):
-    """
-    StringTransformer that splits non-"atom" strings (i.e. strings that do not
-    exist on lines by themselves).
-
-    Requirements:
-        All of the requirements listed in BaseStringSplitter's docstring in
-        addition to the requirements listed below:
-
-        * The line is a return/yield statement, which returns/yields a string.
-            OR
-        * The line is part of a ternary expression (e.g. `x = y if cond else
-        z`) such that the line starts with `else <string>`, where <string> is
-        some string.
-            OR
-        * The line is an assert statement, which ends with a string.
-            OR
-        * The line is an assignment statement (e.g. `x = <string>` or `x +=
-        <string>`) such that the variable is being assigned the value of some
-        string.
-            OR
-        * The line is a dictionary key assignment where some valid key is being
-        assigned the value of some string.
-
-    Transformations:
-        The chosen string is wrapped in parentheses and then split at the LPAR.
-
-        We then have one line which ends with an LPAR and another line that
-        starts with the chosen string. The latter line is then split again at
-        the RPAR. This results in the RPAR (and possibly a trailing comma)
-        being placed on its own line.
-
-        NOTE: If any leaves exist to the right of the chosen string (except
-        for a trailing comma, which would be placed after the RPAR), those
-        leaves are placed inside the parentheses.  In effect, the chosen
-        string is not necessarily being "wrapped" by parentheses. We can,
-        however, count on the LPAR being placed directly before the chosen
-        string.
-
-        In other words, StringParenWrapper creates "atom" strings. These
-        can then be split again by StringSplitter, if necessary.
-
-    Collaborations:
-        In the event that a string line split by StringParenWrapper is
-        changed such that it no longer needs to be given its own line,
-        StringParenWrapper relies on StringParenStripper to clean up the
-        parentheses it created.
-    """
-
-    def do_splitter_match(self, line: Line) -> TMatchResult:
-        LL = line.leaves
-
-        string_idx = (
-            self._return_match(LL)
-            or self._else_match(LL)
-            or self._assert_match(LL)
-            or self._assign_match(LL)
-            or self._dict_match(LL)
-        )
-
-        if string_idx is not None:
-            string_value = line.leaves[string_idx].value
-            # If the string has no spaces...
-            if " " not in string_value:
-                # And will still violate the line length limit when split...
-                max_string_length = self.line_length - ((line.depth + 1) * 4)
-                if len(string_value) > max_string_length:
-                    # And has no associated custom splits...
-                    if not self.has_custom_splits(string_value):
-                        # Then we should NOT put this string on its own line.
-                        return TErr(
-                            "We do not wrap long strings in parentheses when the"
-                            " resultant line would still be over the specified line"
-                            " length and can't be split further by StringSplitter."
-                        )
-            return Ok(string_idx)
-
-        return TErr("This line does not contain any non-atomic strings.")
-
-    @staticmethod
-    def _return_match(LL: List[Leaf]) -> Optional[int]:
-        """
-        Returns:
-            string_idx such that @LL[string_idx] is equal to our target (i.e.
-            matched) string, if this line matches the return/yield statement
-            requirements listed in the 'Requirements' section of this classes'
-            docstring.
-                OR
-            None, otherwise.
-        """
-        # If this line is apart of a return/yield statement and the first leaf
-        # contains either the "return" or "yield" keywords...
-        if parent_type(LL[0]) in [syms.return_stmt, syms.yield_expr] and LL[
-            0
-        ].value in ["return", "yield"]:
-            is_valid_index = is_valid_index_factory(LL)
-
-            idx = 2 if is_valid_index(1) and is_empty_par(LL[1]) else 1
-            # The next visible leaf MUST contain a string...
-            if is_valid_index(idx) and LL[idx].type == token.STRING:
-                return idx
-
-        return None
-
-    @staticmethod
-    def _else_match(LL: List[Leaf]) -> Optional[int]:
-        """
-        Returns:
-            string_idx such that @LL[string_idx] is equal to our target (i.e.
-            matched) string, if this line matches the ternary expression
-            requirements listed in the 'Requirements' section of this classes'
-            docstring.
-                OR
-            None, otherwise.
-        """
-        # If this line is apart of a ternary expression and the first leaf
-        # contains the "else" keyword...
-        if (
-            parent_type(LL[0]) == syms.test
-            and LL[0].type == token.NAME
-            and LL[0].value == "else"
-        ):
-            is_valid_index = is_valid_index_factory(LL)
-
-            idx = 2 if is_valid_index(1) and is_empty_par(LL[1]) else 1
-            # The next visible leaf MUST contain a string...
-            if is_valid_index(idx) and LL[idx].type == token.STRING:
-                return idx
-
-        return None
-
-    @staticmethod
-    def _assert_match(LL: List[Leaf]) -> Optional[int]:
-        """
-        Returns:
-            string_idx such that @LL[string_idx] is equal to our target (i.e.
-            matched) string, if this line matches the assert statement
-            requirements listed in the 'Requirements' section of this classes'
-            docstring.
-                OR
-            None, otherwise.
-        """
-        # If this line is apart of an assert statement and the first leaf
-        # contains the "assert" keyword...
-        if parent_type(LL[0]) == syms.assert_stmt and LL[0].value == "assert":
-            is_valid_index = is_valid_index_factory(LL)
-
-            for (i, leaf) in enumerate(LL):
-                # We MUST find a comma...
-                if leaf.type == token.COMMA:
-                    idx = i + 2 if is_empty_par(LL[i + 1]) else i + 1
-
-                    # That comma MUST be followed by a string...
-                    if is_valid_index(idx) and LL[idx].type == token.STRING:
-                        string_idx = idx
-
-                        # Skip the string trailer, if one exists.
-                        string_parser = StringParser()
-                        idx = string_parser.parse(LL, string_idx)
-
-                        # But no more leaves are allowed...
-                        if not is_valid_index(idx):
-                            return string_idx
-
-        return None
-
-    @staticmethod
-    def _assign_match(LL: List[Leaf]) -> Optional[int]:
-        """
-        Returns:
-            string_idx such that @LL[string_idx] is equal to our target (i.e.
-            matched) string, if this line matches the assignment statement
-            requirements listed in the 'Requirements' section of this classes'
-            docstring.
-                OR
-            None, otherwise.
-        """
-        # If this line is apart of an expression statement or is a function
-        # argument AND the first leaf contains a variable name...
-        if (
-            parent_type(LL[0]) in [syms.expr_stmt, syms.argument, syms.power]
-            and LL[0].type == token.NAME
-        ):
-            is_valid_index = is_valid_index_factory(LL)
-
-            for (i, leaf) in enumerate(LL):
-                # We MUST find either an '=' or '+=' symbol...
-                if leaf.type in [token.EQUAL, token.PLUSEQUAL]:
-                    idx = i + 2 if is_empty_par(LL[i + 1]) else i + 1
-
-                    # That symbol MUST be followed by a string...
-                    if is_valid_index(idx) and LL[idx].type == token.STRING:
-                        string_idx = idx
-
-                        # Skip the string trailer, if one exists.
-                        string_parser = StringParser()
-                        idx = string_parser.parse(LL, string_idx)
-
-                        # The next leaf MAY be a comma iff this line is apart
-                        # of a function argument...
-                        if (
-                            parent_type(LL[0]) == syms.argument
-                            and is_valid_index(idx)
-                            and LL[idx].type == token.COMMA
-                        ):
-                            idx += 1
-
-                        # But no more leaves are allowed...
-                        if not is_valid_index(idx):
-                            return string_idx
-
-        return None
-
-    @staticmethod
-    def _dict_match(LL: List[Leaf]) -> Optional[int]:
-        """
-        Returns:
-            string_idx such that @LL[string_idx] is equal to our target (i.e.
-            matched) string, if this line matches the dictionary key assignment
-            statement requirements listed in the 'Requirements' section of this
-            classes' docstring.
-                OR
-            None, otherwise.
-        """
-        # If this line is apart of a dictionary key assignment...
-        if syms.dictsetmaker in [parent_type(LL[0]), parent_type(LL[0].parent)]:
-            is_valid_index = is_valid_index_factory(LL)
-
-            for (i, leaf) in enumerate(LL):
-                # We MUST find a colon...
-                if leaf.type == token.COLON:
-                    idx = i + 2 if is_empty_par(LL[i + 1]) else i + 1
-
-                    # That colon MUST be followed by a string...
-                    if is_valid_index(idx) and LL[idx].type == token.STRING:
-                        string_idx = idx
-
-                        # Skip the string trailer, if one exists.
-                        string_parser = StringParser()
-                        idx = string_parser.parse(LL, string_idx)
-
-                        # That string MAY be followed by a comma...
-                        if is_valid_index(idx) and LL[idx].type == token.COMMA:
-                            idx += 1
-
-                        # But no more leaves are allowed...
-                        if not is_valid_index(idx):
-                            return string_idx
-
-        return None
-
-    def do_transform(self, line: Line, string_idx: int) -> Iterator[TResult[Line]]:
-        LL = line.leaves
-
-        is_valid_index = is_valid_index_factory(LL)
-        insert_str_child = insert_str_child_factory(LL[string_idx])
-
-        comma_idx = -1
-        ends_with_comma = False
-        if LL[comma_idx].type == token.COMMA:
-            ends_with_comma = True
-
-        leaves_to_steal_comments_from = [LL[string_idx]]
-        if ends_with_comma:
-            leaves_to_steal_comments_from.append(LL[comma_idx])
-
-        # --- First Line
-        first_line = line.clone()
-        left_leaves = LL[:string_idx]
-
-        # We have to remember to account for (possibly invisible) LPAR and RPAR
-        # leaves that already wrapped the target string. If these leaves do
-        # exist, we will replace them with our own LPAR and RPAR leaves.
-        old_parens_exist = False
-        if left_leaves and left_leaves[-1].type == token.LPAR:
-            old_parens_exist = True
-            leaves_to_steal_comments_from.append(left_leaves[-1])
-            left_leaves.pop()
-
-        append_leaves(first_line, line, left_leaves)
-
-        lpar_leaf = Leaf(token.LPAR, "(")
-        if old_parens_exist:
-            replace_child(LL[string_idx - 1], lpar_leaf)
-        else:
-            insert_str_child(lpar_leaf)
-        first_line.append(lpar_leaf)
-
-        # We throw inline comments that were originally to the right of the
-        # target string to the top line. They will now be shown to the right of
-        # the LPAR.
-        for leaf in leaves_to_steal_comments_from:
-            for comment_leaf in line.comments_after(leaf):
-                first_line.append(comment_leaf, preformatted=True)
-
-        yield Ok(first_line)
-
-        # --- Middle (String) Line
-        # We only need to yield one (possibly too long) string line, since the
-        # `StringSplitter` will break it down further if necessary.
-        string_value = LL[string_idx].value
-        string_line = Line(
-            mode=line.mode,
-            depth=line.depth + 1,
-            inside_brackets=True,
-            should_split_rhs=line.should_split_rhs,
-            magic_trailing_comma=line.magic_trailing_comma,
-        )
-        string_leaf = Leaf(token.STRING, string_value)
-        insert_str_child(string_leaf)
-        string_line.append(string_leaf)
-
-        old_rpar_leaf = None
-        if is_valid_index(string_idx + 1):
-            right_leaves = LL[string_idx + 1 :]
-            if ends_with_comma:
-                right_leaves.pop()
-
-            if old_parens_exist:
-                assert (
-                    right_leaves and right_leaves[-1].type == token.RPAR
-                ), "Apparently, old parentheses do NOT exist?!"
-                old_rpar_leaf = right_leaves.pop()
-
-            append_leaves(string_line, line, right_leaves)
-
-        yield Ok(string_line)
-
-        # --- Last Line
-        last_line = line.clone()
-        last_line.bracket_tracker = first_line.bracket_tracker
-
-        new_rpar_leaf = Leaf(token.RPAR, ")")
-        if old_rpar_leaf is not None:
-            replace_child(old_rpar_leaf, new_rpar_leaf)
-        else:
-            insert_str_child(new_rpar_leaf)
-        last_line.append(new_rpar_leaf)
-
-        # If the target string ended with a comma, we place this comma to the
-        # right of the RPAR on the last line.
-        if ends_with_comma:
-            comma_leaf = Leaf(token.COMMA, ",")
-            replace_child(LL[comma_idx], comma_leaf)
-            last_line.append(comma_leaf)
-
-        yield Ok(last_line)
-
-
-class StringParser:
-    """
-    A state machine that aids in parsing a string's "trailer", which can be
-    either non-existent, an old-style formatting sequence (e.g. `% varX` or `%
-    (varX, varY)`), or a method-call / attribute access (e.g. `.format(varX,
-    varY)`).
-
-    NOTE: A new StringParser object MUST be instantiated for each string
-    trailer we need to parse.
-
-    Examples:
-        We shall assume that `line` equals the `Line` object that corresponds
-        to the following line of python code:
-        ```
-        x = "Some {}.".format("String") + some_other_string
-        ```
-
-        Furthermore, we will assume that `string_idx` is some index such that:
-        ```
-        assert line.leaves[string_idx].value == "Some {}."
-        ```
-
-        The following code snippet then holds:
-        ```
-        string_parser = StringParser()
-        idx = string_parser.parse(line.leaves, string_idx)
-        assert line.leaves[idx].type == token.PLUS
-        ```
-    """
-
-    DEFAULT_TOKEN = -1
-
-    # String Parser States
-    START = 1
-    DOT = 2
-    NAME = 3
-    PERCENT = 4
-    SINGLE_FMT_ARG = 5
-    LPAR = 6
-    RPAR = 7
-    DONE = 8
-
-    # Lookup Table for Next State
-    _goto: Dict[Tuple[ParserState, NodeType], ParserState] = {
-        # A string trailer may start with '.' OR '%'.
-        (START, token.DOT): DOT,
-        (START, token.PERCENT): PERCENT,
-        (START, DEFAULT_TOKEN): DONE,
-        # A '.' MUST be followed by an attribute or method name.
-        (DOT, token.NAME): NAME,
-        # A method name MUST be followed by an '(', whereas an attribute name
-        # is the last symbol in the string trailer.
-        (NAME, token.LPAR): LPAR,
-        (NAME, DEFAULT_TOKEN): DONE,
-        # A '%' symbol can be followed by an '(' or a single argument (e.g. a
-        # string or variable name).
-        (PERCENT, token.LPAR): LPAR,
-        (PERCENT, DEFAULT_TOKEN): SINGLE_FMT_ARG,
-        # If a '%' symbol is followed by a single argument, that argument is
-        # the last leaf in the string trailer.
-        (SINGLE_FMT_ARG, DEFAULT_TOKEN): DONE,
-        # If present, a ')' symbol is the last symbol in a string trailer.
-        # (NOTE: LPARS and nested RPARS are not included in this lookup table,
-        # since they are treated as a special case by the parsing logic in this
-        # classes' implementation.)
-        (RPAR, DEFAULT_TOKEN): DONE,
-    }
-
-    def __init__(self) -> None:
-        self._state = self.START
-        self._unmatched_lpars = 0
-
-    def parse(self, leaves: List[Leaf], string_idx: int) -> int:
-        """
-        Pre-conditions:
-            * @leaves[@string_idx].type == token.STRING
-
-        Returns:
-            The index directly after the last leaf which is apart of the string
-            trailer, if a "trailer" exists.
-                OR
-            @string_idx + 1, if no string "trailer" exists.
-        """
-        assert leaves[string_idx].type == token.STRING
-
-        idx = string_idx + 1
-        while idx < len(leaves) and self._next_state(leaves[idx]):
-            idx += 1
-        return idx
-
-    def _next_state(self, leaf: Leaf) -> bool:
-        """
-        Pre-conditions:
-            * On the first call to this function, @leaf MUST be the leaf that
-            was directly after the string leaf in question (e.g. if our target
-            string is `line.leaves[i]` then the first call to this method must
-            be `line.leaves[i + 1]`).
-            * On the next call to this function, the leaf parameter passed in
-            MUST be the leaf directly following @leaf.
-
-        Returns:
-            True iff @leaf is apart of the string's trailer.
-        """
-        # We ignore empty LPAR or RPAR leaves.
-        if is_empty_par(leaf):
-            return True
-
-        next_token = leaf.type
-        if next_token == token.LPAR:
-            self._unmatched_lpars += 1
-
-        current_state = self._state
-
-        # The LPAR parser state is a special case. We will return True until we
-        # find the matching RPAR token.
-        if current_state == self.LPAR:
-            if next_token == token.RPAR:
-                self._unmatched_lpars -= 1
-                if self._unmatched_lpars == 0:
-                    self._state = self.RPAR
-        # Otherwise, we use a lookup table to determine the next state.
-        else:
-            # If the lookup table matches the current state to the next
-            # token, we use the lookup table.
-            if (current_state, next_token) in self._goto:
-                self._state = self._goto[current_state, next_token]
-            else:
-                # Otherwise, we check if a the current state was assigned a
-                # default.
-                if (current_state, self.DEFAULT_TOKEN) in self._goto:
-                    self._state = self._goto[current_state, self.DEFAULT_TOKEN]
-                # If no default has been assigned, then this parser has a logic
-                # error.
-                else:
-                    raise RuntimeError(f"{self.__class__.__name__} LOGIC ERROR!")
-
-            if self._state == self.DONE:
-                return False
-
-        return True
-
-
-def TErr(err_msg: str) -> Err[CannotTransform]:
-    """(T)ransform Err
-
-    Convenience function used when working with the TResult type.
-    """
-    cant_transform = CannotTransform(err_msg)
-    return Err(cant_transform)
-
-
-def contains_pragma_comment(comment_list: List[Leaf]) -> bool:
-    """
-    Returns:
-        True iff one of the comments in @comment_list is a pragma used by one
-        of the more common static analysis tools for python (e.g. mypy, flake8,
-        pylint).
-    """
-    for comment in comment_list:
-        if comment.value.startswith(("# type:", "# noqa", "# pylint:")):
-            return True
-
-    return False
-
-
-def insert_str_child_factory(string_leaf: Leaf) -> Callable[[LN], None]:
-    """
-    Factory for a convenience function that is used to orphan @string_leaf
-    and then insert multiple new leaves into the same part of the node
-    structure that @string_leaf had originally occupied.
-
-    Examples:
-        Let `string_leaf = Leaf(token.STRING, '"foo"')` and `N =
-        string_leaf.parent`. Assume the node `N` has the following
-        original structure:
-
-        Node(
-            expr_stmt, [
-                Leaf(NAME, 'x'),
-                Leaf(EQUAL, '='),
-                Leaf(STRING, '"foo"'),
-            ]
-        )
-
-        We then run the code snippet shown below.
-        ```
-        insert_str_child = insert_str_child_factory(string_leaf)
-
-        lpar = Leaf(token.LPAR, '(')
-        insert_str_child(lpar)
-
-        bar = Leaf(token.STRING, '"bar"')
-        insert_str_child(bar)
-
-        rpar = Leaf(token.RPAR, ')')
-        insert_str_child(rpar)
-        ```
-
-        After which point, it follows that `string_leaf.parent is None` and
-        the node `N` now has the following structure:
-
-        Node(
-            expr_stmt, [
-                Leaf(NAME, 'x'),
-                Leaf(EQUAL, '='),
-                Leaf(LPAR, '('),
-                Leaf(STRING, '"bar"'),
-                Leaf(RPAR, ')'),
-            ]
-        )
-    """
-    string_parent = string_leaf.parent
-    string_child_idx = string_leaf.remove()
-
-    def insert_str_child(child: LN) -> None:
-        nonlocal string_child_idx
-
-        assert string_parent is not None
-        assert string_child_idx is not None
-
-        string_parent.insert_child(string_child_idx, child)
-        string_child_idx += 1
-
-    return insert_str_child
-
-
-def has_triple_quotes(string: str) -> bool:
-    """
-    Returns:
-        True iff @string starts with three quotation characters.
-    """
-    raw_string = string.lstrip(STRING_PREFIX_CHARS)
-    return raw_string[:3] in {'"""', "'''"}
-
-
-def parent_type(node: Optional[LN]) -> Optional[NodeType]:
-    """
-    Returns:
-        @node.parent.type, if @node is not None and has a parent.
-            OR
-        None, otherwise.
-    """
-    if node is None or node.parent is None:
-        return None
-
-    return node.parent.type
-
-
-def is_empty_par(leaf: Leaf) -> bool:
-    return is_empty_lpar(leaf) or is_empty_rpar(leaf)
-
-
-def is_empty_lpar(leaf: Leaf) -> bool:
-    return leaf.type == token.LPAR and leaf.value == ""
-
-
-def is_empty_rpar(leaf: Leaf) -> bool:
-    return leaf.type == token.RPAR and leaf.value == ""
-
-
-def is_valid_index_factory(seq: Sequence[Any]) -> Callable[[int], bool]:
-    """
-    Examples:
-        ```
-        my_list = [1, 2, 3]
-
-        is_valid_index = is_valid_index_factory(my_list)
-
-        assert is_valid_index(0)
-        assert is_valid_index(2)
-
-        assert not is_valid_index(3)
-        assert not is_valid_index(-1)
-        ```
-    """
-
-    def is_valid_index(idx: int) -> bool:
-        """
-        Returns:
-            True iff @idx is positive AND seq[@idx] does NOT raise an
-            IndexError.
-        """
-        return 0 <= idx < len(seq)
-
-    return is_valid_index
-
-
-def line_to_string(line: Line) -> str:
-    """Returns the string representation of @line.
-
-    WARNING: This is known to be computationally expensive.
-    """
-    return str(line).strip("\n")
-
-
-def append_leaves(
-    new_line: Line, old_line: Line, leaves: List[Leaf], preformatted: bool = False
-) -> None:
-    """
-    Append leaves (taken from @old_line) to @new_line, making sure to fix the
-    underlying Node structure where appropriate.
-
-    All of the leaves in @leaves are duplicated. The duplicates are then
-    appended to @new_line and used to replace their originals in the underlying
-    Node structure. Any comments attached to the old leaves are reattached to
-    the new leaves.
-
-    Pre-conditions:
-        set(@leaves) is a subset of set(@old_line.leaves).
-    """
-    for old_leaf in leaves:
-        new_leaf = Leaf(old_leaf.type, old_leaf.value)
-        replace_child(old_leaf, new_leaf)
-        new_line.append(new_leaf, preformatted=preformatted)
-
-        for comment_leaf in old_line.comments_after(old_leaf):
-            new_line.append(comment_leaf, preformatted=True)
-
-
-def replace_child(old_child: LN, new_child: LN) -> None:
-    """
-    Side Effects:
-        * If @old_child.parent is set, replace @old_child with @new_child in
-        @old_child's underlying Node structure.
-            OR
-        * Otherwise, this function does nothing.
-    """
-    parent = old_child.parent
-    if not parent:
-        return
-
-    child_idx = old_child.remove()
-    if child_idx is not None:
-        parent.insert_child(child_idx, new_child)
-
-
-def get_string_prefix(string: str) -> str:
-    """
-    Pre-conditions:
-        * assert_is_leaf_string(@string)
-
-    Returns:
-        @string's prefix (e.g. '', 'r', 'f', or 'rf').
-    """
-    assert_is_leaf_string(string)
-
-    prefix = ""
-    prefix_idx = 0
-    while string[prefix_idx] in STRING_PREFIX_CHARS:
-        prefix += string[prefix_idx].lower()
-        prefix_idx += 1
-
-    return prefix
-
-
-def assert_is_leaf_string(string: str) -> None:
-    """
-    Checks the pre-condition that @string has the format that you would expect
-    of `leaf.value` where `leaf` is some Leaf such that `leaf.type ==
-    token.STRING`. A more precise description of the pre-conditions that are
-    checked are listed below.
-
-    Pre-conditions:
-        * @string starts with either ', ", <prefix>', or <prefix>" where
-        `set(<prefix>)` is some subset of `set(STRING_PREFIX_CHARS)`.
-        * @string ends with a quote character (' or ").
-
-    Raises:
-        AssertionError(...) if the pre-conditions listed above are not
-        satisfied.
-    """
-    dquote_idx = string.find('"')
-    squote_idx = string.find("'")
-    if -1 in [dquote_idx, squote_idx]:
-        quote_idx = max(dquote_idx, squote_idx)
-    else:
-        quote_idx = min(squote_idx, dquote_idx)
-
-    assert (
-        0 <= quote_idx < len(string) - 1
-    ), f"{string!r} is missing a starting quote character (' or \")."
-    assert string[-1] in (
-        "'",
-        '"',
-    ), f"{string!r} is missing an ending quote character (' or \")."
-    assert set(string[:quote_idx]).issubset(
-        set(STRING_PREFIX_CHARS)
-    ), f"{set(string[:quote_idx])} is NOT a subset of {set(STRING_PREFIX_CHARS)}."
-
-
-def left_hand_split(line: Line, _features: Collection[Feature] = ()) -> Iterator[Line]:
-    """Split line into many lines, starting with the first matching bracket pair.
-
-    Note: this usually looks weird, only use this for function definitions.
-    Prefer RHS otherwise.  This is why this function is not symmetrical with
-    :func:`right_hand_split` which also handles optional parentheses.
-    """
-    tail_leaves: List[Leaf] = []
-    body_leaves: List[Leaf] = []
-    head_leaves: List[Leaf] = []
-    current_leaves = head_leaves
-    matching_bracket: Optional[Leaf] = None
-    for leaf in line.leaves:
-        if (
-            current_leaves is body_leaves
-            and leaf.type in CLOSING_BRACKETS
-            and leaf.opening_bracket is matching_bracket
-        ):
-            current_leaves = tail_leaves if body_leaves else head_leaves
-        current_leaves.append(leaf)
-        if current_leaves is head_leaves:
-            if leaf.type in OPENING_BRACKETS:
-                matching_bracket = leaf
-                current_leaves = body_leaves
-    if not matching_bracket:
-        raise CannotSplit("No brackets found")
-
-    head = bracket_split_build_line(head_leaves, line, matching_bracket)
-    body = bracket_split_build_line(body_leaves, line, matching_bracket, is_body=True)
-    tail = bracket_split_build_line(tail_leaves, line, matching_bracket)
-    bracket_split_succeeded_or_raise(head, body, tail)
-    for result in (head, body, tail):
-        if result:
-            yield result
-
-
-def right_hand_split(
-    line: Line,
-    line_length: int,
-    features: Collection[Feature] = (),
-    omit: Collection[LeafID] = (),
-) -> Iterator[Line]:
-    """Split line into many lines, starting with the last matching bracket pair.
-
-    If the split was by optional parentheses, attempt splitting without them, too.
-    `omit` is a collection of closing bracket IDs that shouldn't be considered for
-    this split.
-
-    Note: running this function modifies `bracket_depth` on the leaves of `line`.
-    """
-    tail_leaves: List[Leaf] = []
-    body_leaves: List[Leaf] = []
-    head_leaves: List[Leaf] = []
-    current_leaves = tail_leaves
-    opening_bracket: Optional[Leaf] = None
-    closing_bracket: Optional[Leaf] = None
-    for leaf in reversed(line.leaves):
-        if current_leaves is body_leaves:
-            if leaf is opening_bracket:
-                current_leaves = head_leaves if body_leaves else tail_leaves
-        current_leaves.append(leaf)
-        if current_leaves is tail_leaves:
-            if leaf.type in CLOSING_BRACKETS and id(leaf) not in omit:
-                opening_bracket = leaf.opening_bracket
-                closing_bracket = leaf
-                current_leaves = body_leaves
-    if not (opening_bracket and closing_bracket and head_leaves):
-        # If there is no opening or closing_bracket that means the split failed and
-        # all content is in the tail.  Otherwise, if `head_leaves` are empty, it means
-        # the matching `opening_bracket` wasn't available on `line` anymore.
-        raise CannotSplit("No brackets found")
-
-    tail_leaves.reverse()
-    body_leaves.reverse()
-    head_leaves.reverse()
-    head = bracket_split_build_line(head_leaves, line, opening_bracket)
-    body = bracket_split_build_line(body_leaves, line, opening_bracket, is_body=True)
-    tail = bracket_split_build_line(tail_leaves, line, opening_bracket)
-    bracket_split_succeeded_or_raise(head, body, tail)
-    if (
-        Feature.FORCE_OPTIONAL_PARENTHESES not in features
-        # the opening bracket is an optional paren
-        and opening_bracket.type == token.LPAR
-        and not opening_bracket.value
-        # the closing bracket is an optional paren
-        and closing_bracket.type == token.RPAR
-        and not closing_bracket.value
-        # it's not an import (optional parens are the only thing we can split on
-        # in this case; attempting a split without them is a waste of time)
-        and not line.is_import
-        # there are no standalone comments in the body
-        and not body.contains_standalone_comments(0)
-        # and we can actually remove the parens
-        and can_omit_invisible_parens(body, line_length, omit_on_explode=omit)
-    ):
-        omit = {id(closing_bracket), *omit}
-        try:
-            yield from right_hand_split(line, line_length, features=features, omit=omit)
-            return
-
-        except CannotSplit:
-            if not (
-                can_be_split(body)
-                or is_line_short_enough(body, line_length=line_length)
-            ):
-                raise CannotSplit(
-                    "Splitting failed, body is still too long and can't be split."
-                )
-
-            elif head.contains_multiline_strings() or tail.contains_multiline_strings():
-                raise CannotSplit(
-                    "The current optional pair of parentheses is bound to fail to"
-                    " satisfy the splitting algorithm because the head or the tail"
-                    " contains multiline strings which by definition never fit one"
-                    " line."
-                )
-
-    ensure_visible(opening_bracket)
-    ensure_visible(closing_bracket)
-    for result in (head, body, tail):
-        if result:
-            yield result
-
-
-def bracket_split_succeeded_or_raise(head: Line, body: Line, tail: Line) -> None:
-    """Raise :exc:`CannotSplit` if the last left- or right-hand split failed.
-
-    Do nothing otherwise.
-
-    A left- or right-hand split is based on a pair of brackets. Content before
-    (and including) the opening bracket is left on one line, content inside the
-    brackets is put on a separate line, and finally content starting with and
-    following the closing bracket is put on a separate line.
-
-    Those are called `head`, `body`, and `tail`, respectively. If the split
-    produced the same line (all content in `head`) or ended up with an empty `body`
-    and the `tail` is just the closing bracket, then it's considered failed.
-    """
-    tail_len = len(str(tail).strip())
-    if not body:
-        if tail_len == 0:
-            raise CannotSplit("Splitting brackets produced the same line")
-
-        elif tail_len < 3:
-            raise CannotSplit(
-                f"Splitting brackets on an empty body to save {tail_len} characters is"
-                " not worth it"
-            )
-
-
-def bracket_split_build_line(
-    leaves: List[Leaf], original: Line, opening_bracket: Leaf, *, is_body: bool = False
-) -> Line:
-    """Return a new line with given `leaves` and respective comments from `original`.
-
-    If `is_body` is True, the result line is one-indented inside brackets and as such
-    has its first leaf's prefix normalized and a trailing comma added when expected.
-    """
-    result = Line(mode=original.mode, depth=original.depth)
-    if is_body:
-        result.inside_brackets = True
-        result.depth += 1
-        if leaves:
-            # Since body is a new indent level, remove spurious leading whitespace.
-            normalize_prefix(leaves[0], inside_brackets=True)
-            # Ensure a trailing comma for imports and standalone function arguments, but
-            # be careful not to add one after any comments or within type annotations.
-            no_commas = (
-                original.is_def
-                and opening_bracket.value == "("
-                and not any(leaf.type == token.COMMA for leaf in leaves)
-            )
-
-            if original.is_import or no_commas:
-                for i in range(len(leaves) - 1, -1, -1):
-                    if leaves[i].type == STANDALONE_COMMENT:
-                        continue
-
-                    if leaves[i].type != token.COMMA:
-                        new_comma = Leaf(token.COMMA, ",")
-                        leaves.insert(i + 1, new_comma)
-                    break
-
-    # Populate the line
-    for leaf in leaves:
-        result.append(leaf, preformatted=True)
-        for comment_after in original.comments_after(leaf):
-            result.append(comment_after, preformatted=True)
-    if is_body and should_split_line(result, opening_bracket):
-        result.should_split_rhs = True
-    return result
-
-
-def dont_increase_indentation(split_func: Transformer) -> Transformer:
-    """Normalize prefix of the first leaf in every line returned by `split_func`.
-
-    This is a decorator over relevant split functions.
-    """
-
-    @wraps(split_func)
-    def split_wrapper(line: Line, features: Collection[Feature] = ()) -> Iterator[Line]:
-        for line in split_func(line, features):
-            normalize_prefix(line.leaves[0], inside_brackets=True)
-            yield line
-
-    return split_wrapper
-
-
-@dont_increase_indentation
-def delimiter_split(line: Line, features: Collection[Feature] = ()) -> Iterator[Line]:
-    """Split according to delimiters of the highest priority.
-
-    If the appropriate Features are given, the split will add trailing commas
-    also in function signatures and calls that contain `*` and `**`.
-    """
-    try:
-        last_leaf = line.leaves[-1]
-    except IndexError:
-        raise CannotSplit("Line empty")
-
-    bt = line.bracket_tracker
-    try:
-        delimiter_priority = bt.max_delimiter_priority(exclude={id(last_leaf)})
-    except ValueError:
-        raise CannotSplit("No delimiters found")
-
-    if delimiter_priority == DOT_PRIORITY:
-        if bt.delimiter_count_with_priority(delimiter_priority) == 1:
-            raise CannotSplit("Splitting a single attribute from its owner looks wrong")
-
-    current_line = Line(
-        mode=line.mode, depth=line.depth, inside_brackets=line.inside_brackets
-    )
-    lowest_depth = sys.maxsize
-    trailing_comma_safe = True
-
-    def append_to_line(leaf: Leaf) -> Iterator[Line]:
-        """Append `leaf` to current line or to new line if appending impossible."""
-        nonlocal current_line
-        try:
-            current_line.append_safe(leaf, preformatted=True)
-        except ValueError:
-            yield current_line
-
-            current_line = Line(
-                mode=line.mode, depth=line.depth, inside_brackets=line.inside_brackets
-            )
-            current_line.append(leaf)
-
-    for leaf in line.leaves:
-        yield from append_to_line(leaf)
-
-        for comment_after in line.comments_after(leaf):
-            yield from append_to_line(comment_after)
-
-        lowest_depth = min(lowest_depth, leaf.bracket_depth)
-        if leaf.bracket_depth == lowest_depth:
-            if is_vararg(leaf, within={syms.typedargslist}):
-                trailing_comma_safe = (
-                    trailing_comma_safe and Feature.TRAILING_COMMA_IN_DEF in features
-                )
-            elif is_vararg(leaf, within={syms.arglist, syms.argument}):
-                trailing_comma_safe = (
-                    trailing_comma_safe and Feature.TRAILING_COMMA_IN_CALL in features
-                )
-
-        leaf_priority = bt.delimiters.get(id(leaf))
-        if leaf_priority == delimiter_priority:
-            yield current_line
-
-            current_line = Line(
-                mode=line.mode, depth=line.depth, inside_brackets=line.inside_brackets
-            )
-    if current_line:
-        if (
-            trailing_comma_safe
-            and delimiter_priority == COMMA_PRIORITY
-            and current_line.leaves[-1].type != token.COMMA
-            and current_line.leaves[-1].type != STANDALONE_COMMENT
-        ):
-            new_comma = Leaf(token.COMMA, ",")
-            current_line.append(new_comma)
-        yield current_line
-
-
-@dont_increase_indentation
-def standalone_comment_split(
-    line: Line, features: Collection[Feature] = ()
-) -> Iterator[Line]:
-    """Split standalone comments from the rest of the line."""
-    if not line.contains_standalone_comments(0):
-        raise CannotSplit("Line does not have any standalone comments")
-
-    current_line = Line(
-        mode=line.mode, depth=line.depth, inside_brackets=line.inside_brackets
-    )
-
-    def append_to_line(leaf: Leaf) -> Iterator[Line]:
-        """Append `leaf` to current line or to new line if appending impossible."""
-        nonlocal current_line
-        try:
-            current_line.append_safe(leaf, preformatted=True)
-        except ValueError:
-            yield current_line
-
-            current_line = Line(
-                line.mode, depth=line.depth, inside_brackets=line.inside_brackets
-            )
-            current_line.append(leaf)
-
-    for leaf in line.leaves:
-        yield from append_to_line(leaf)
-
-        for comment_after in line.comments_after(leaf):
-            yield from append_to_line(comment_after)
-
-    if current_line:
-        yield current_line
-
-
-def is_import(leaf: Leaf) -> bool:
-    """Return True if the given leaf starts an import statement."""
-    p = leaf.parent
-    t = leaf.type
-    v = leaf.value
-    return bool(
-        t == token.NAME
-        and (
-            (v == "import" and p and p.type == syms.import_name)
-            or (v == "from" and p and p.type == syms.import_from)
-        )
-    )
-
-
-def is_type_comment(leaf: Leaf, suffix: str = "") -> bool:
-    """Return True if the given leaf is a special comment.
-    Only returns true for type comments for now."""
-    t = leaf.type
-    v = leaf.value
-    return t in {token.COMMENT, STANDALONE_COMMENT} and v.startswith("# type:" + suffix)
-
-
-def normalize_prefix(leaf: Leaf, *, inside_brackets: bool) -> None:
-    """Leave existing extra newlines if not `inside_brackets`. Remove everything
-    else.
-
-    Note: don't use backslashes for formatting or you'll lose your voting rights.
-    """
-    if not inside_brackets:
-        spl = leaf.prefix.split("#")
-        if "\\" not in spl[0]:
-            nl_count = spl[-1].count("\n")
-            if len(spl) > 1:
-                nl_count -= 1
-            leaf.prefix = "\n" * nl_count
-            return
-
-    leaf.prefix = ""
-
-
-def normalize_string_prefix(leaf: Leaf, remove_u_prefix: bool = False) -> None:
-    """Make all string prefixes lowercase.
-
-    If remove_u_prefix is given, also removes any u prefix from the string.
-
-    Note: Mutates its argument.
-    """
-    match = re.match(r"^([" + STRING_PREFIX_CHARS + r"]*)(.*)$", leaf.value, re.DOTALL)
-    assert match is not None, f"failed to match string {leaf.value!r}"
-    orig_prefix = match.group(1)
-    new_prefix = orig_prefix.replace("F", "f").replace("B", "b").replace("U", "u")
-    if remove_u_prefix:
-        new_prefix = new_prefix.replace("u", "")
-    leaf.value = f"{new_prefix}{match.group(2)}"
-
-
-def normalize_string_quotes(leaf: Leaf) -> None:
-    """Prefer double quotes but only if it doesn't cause more escaping.
-
-    Adds or removes backslashes as appropriate. Doesn't parse and fix
-    strings nested in f-strings (yet).
-
-    Note: Mutates its argument.
-    """
-    value = leaf.value.lstrip(STRING_PREFIX_CHARS)
-    if value[:3] == '"""':
-        return
-
-    elif value[:3] == "'''":
-        orig_quote = "'''"
-        new_quote = '"""'
-    elif value[0] == '"':
-        orig_quote = '"'
-        new_quote = "'"
-    else:
-        orig_quote = "'"
-        new_quote = '"'
-    first_quote_pos = leaf.value.find(orig_quote)
-    if first_quote_pos == -1:
-        return  # There's an internal error
-
-    prefix = leaf.value[:first_quote_pos]
-    unescaped_new_quote = re.compile(rf"(([^\\]|^)(\\\\)*){new_quote}")
-    escaped_new_quote = re.compile(rf"([^\\]|^)\\((?:\\\\)*){new_quote}")
-    escaped_orig_quote = re.compile(rf"([^\\]|^)\\((?:\\\\)*){orig_quote}")
-    body = leaf.value[first_quote_pos + len(orig_quote) : -len(orig_quote)]
-    if "r" in prefix.casefold():
-        if unescaped_new_quote.search(body):
-            # There's at least one unescaped new_quote in this raw string
-            # so converting is impossible
-            return
-
-        # Do not introduce or remove backslashes in raw strings
-        new_body = body
-    else:
-        # remove unnecessary escapes
-        new_body = sub_twice(escaped_new_quote, rf"\1\2{new_quote}", body)
-        if body != new_body:
-            # Consider the string without unnecessary escapes as the original
-            body = new_body
-            leaf.value = f"{prefix}{orig_quote}{body}{orig_quote}"
-        new_body = sub_twice(escaped_orig_quote, rf"\1\2{orig_quote}", new_body)
-        new_body = sub_twice(unescaped_new_quote, rf"\1\\{new_quote}", new_body)
-    if "f" in prefix.casefold():
-        matches = re.findall(
-            r"""
-            (?:[^{]|^)\{  # start of the string or a non-{ followed by a single {
-                ([^{].*?)  # contents of the brackets except if begins with {{
-            \}(?:[^}]|$)  # A } followed by end of the string or a non-}
-            """,
-            new_body,
-            re.VERBOSE,
-        )
-        for m in matches:
-            if "\\" in str(m):
-                # Do not introduce backslashes in interpolated expressions
-                return
-
-    if new_quote == '"""' and new_body[-1:] == '"':
-        # edge case:
-        new_body = new_body[:-1] + '\\"'
-    orig_escape_count = body.count("\\")
-    new_escape_count = new_body.count("\\")
-    if new_escape_count > orig_escape_count:
-        return  # Do not introduce more escaping
-
-    if new_escape_count == orig_escape_count and orig_quote == '"':
-        return  # Prefer double quotes
-
-    leaf.value = f"{prefix}{new_quote}{new_body}{new_quote}"
-
-
-def normalize_numeric_literal(leaf: Leaf) -> None:
-    """Normalizes numeric (float, int, and complex) literals.
-
-    All letters used in the representation are normalized to lowercase (except
-    in Python 2 long literals).
-    """
-    text = leaf.value.lower()
-    if text.startswith(("0o", "0b")):
-        # Leave octal and binary literals alone.
-        pass
-    elif text.startswith("0x"):
-        text = format_hex(text)
-    elif "e" in text:
-        text = format_scientific_notation(text)
-    elif text.endswith(("j", "l")):
-        text = format_long_or_complex_number(text)
-    else:
-        text = format_float_or_int_string(text)
-    leaf.value = text
-
-
-def format_hex(text: str) -> str:
-    """
-    Formats a hexadecimal string like "0x12b3"
-
-    Uses lowercase because of similarity between "B" and "8", which
-    can cause security issues.
-    see: https://github.com/psf/black/issues/1692
-    """
-
-    before, after = text[:2], text[2:]
-    return f"{before}{after.lower()}"
-
-
-def format_scientific_notation(text: str) -> str:
-    """Formats a numeric string utilizing scentific notation"""
-    before, after = text.split("e")
-    sign = ""
-    if after.startswith("-"):
-        after = after[1:]
-        sign = "-"
-    elif after.startswith("+"):
-        after = after[1:]
-    before = format_float_or_int_string(before)
-    return f"{before}e{sign}{after}"
-
-
-def format_long_or_complex_number(text: str) -> str:
-    """Formats a long or complex string like `10L` or `10j`"""
-    number = text[:-1]
-    suffix = text[-1]
-    # Capitalize in "2L" because "l" looks too similar to "1".
-    if suffix == "l":
-        suffix = "L"
-    return f"{format_float_or_int_string(number)}{suffix}"
-
-
-def format_float_or_int_string(text: str) -> str:
-    """Formats a float string like "1.0"."""
-    if "." not in text:
-        return text
-
-    before, after = text.split(".")
-    return f"{before or 0}.{after or 0}"
-
-
-def normalize_invisible_parens(node: Node, parens_after: Set[str]) -> None:
-    """Make existing optional parentheses invisible or create new ones.
-
-    `parens_after` is a set of string leaf values immediately after which parens
-    should be put.
-
-    Standardizes on visible parentheses for single-element tuples, and keeps
-    existing visible parentheses for other tuples and generator expressions.
-    """
-    for pc in list_comments(node.prefix, is_endmarker=False):
-        if pc.value in FMT_OFF:
-            # This `node` has a prefix with `# fmt: off`, don't mess with parens.
-            return
-    check_lpar = False
-    for index, child in enumerate(list(node.children)):
-        # Fixes a bug where invisible parens are not properly stripped from
-        # assignment statements that contain type annotations.
-        if isinstance(child, Node) and child.type == syms.annassign:
-            normalize_invisible_parens(child, parens_after=parens_after)
-
-        # Add parentheses around long tuple unpacking in assignments.
-        if (
-            index == 0
-            and isinstance(child, Node)
-            and child.type == syms.testlist_star_expr
-        ):
-            check_lpar = True
-
-        if check_lpar:
-            if child.type == syms.atom:
-                if maybe_make_parens_invisible_in_atom(child, parent=node):
-                    wrap_in_parentheses(node, child, visible=False)
-            elif is_one_tuple(child):
-                wrap_in_parentheses(node, child, visible=True)
-            elif node.type == syms.import_from:
-                # "import from" nodes store parentheses directly as part of
-                # the statement
-                if child.type == token.LPAR:
-                    # make parentheses invisible
-                    child.value = ""  # type: ignore
-                    node.children[-1].value = ""  # type: ignore
-                elif child.type != token.STAR:
-                    # insert invisible parentheses
-                    node.insert_child(index, Leaf(token.LPAR, ""))
-                    node.append_child(Leaf(token.RPAR, ""))
-                break
-
-            elif not (isinstance(child, Leaf) and is_multiline_string(child)):
-                wrap_in_parentheses(node, child, visible=False)
-
-        check_lpar = isinstance(child, Leaf) and child.value in parens_after
-
-
-def normalize_fmt_off(node: Node) -> None:
-    """Convert content between `# fmt: off`/`# fmt: on` into standalone comments."""
-    try_again = True
-    while try_again:
-        try_again = convert_one_fmt_off_pair(node)
-
-
-def convert_one_fmt_off_pair(node: Node) -> bool:
-    """Convert content of a single `# fmt: off`/`# fmt: on` into a standalone comment.
-
-    Returns True if a pair was converted.
-    """
-    for leaf in node.leaves():
-        previous_consumed = 0
-        for comment in list_comments(leaf.prefix, is_endmarker=False):
-            if comment.value not in FMT_PASS:
-                previous_consumed = comment.consumed
-                continue
-            # We only want standalone comments. If there's no previous leaf or
-            # the previous leaf is indentation, it's a standalone comment in
-            # disguise.
-            if comment.value in FMT_PASS and comment.type != STANDALONE_COMMENT:
-                prev = preceding_leaf(leaf)
-                if prev:
-                    if comment.value in FMT_OFF and prev.type not in WHITESPACE:
-                        continue
-                    if comment.value in FMT_SKIP and prev.type in WHITESPACE:
-                        continue
-
-            ignored_nodes = list(generate_ignored_nodes(leaf, comment))
-            if not ignored_nodes:
-                continue
-
-            first = ignored_nodes[0]  # Can be a container node with the `leaf`.
-            parent = first.parent
-            prefix = first.prefix
-            first.prefix = prefix[comment.consumed :]
-            hidden_value = "".join(str(n) for n in ignored_nodes)
-            if comment.value in FMT_OFF:
-                hidden_value = comment.value + "\n" + hidden_value
-            if comment.value in FMT_SKIP:
-                hidden_value += "  " + comment.value
-            if hidden_value.endswith("\n"):
-                # That happens when one of the `ignored_nodes` ended with a NEWLINE
-                # leaf (possibly followed by a DEDENT).
-                hidden_value = hidden_value[:-1]
-            first_idx: Optional[int] = None
-            for ignored in ignored_nodes:
-                index = ignored.remove()
-                if first_idx is None:
-                    first_idx = index
-            assert parent is not None, "INTERNAL ERROR: fmt: on/off handling (1)"
-            assert first_idx is not None, "INTERNAL ERROR: fmt: on/off handling (2)"
-            parent.insert_child(
-                first_idx,
-                Leaf(
-                    STANDALONE_COMMENT,
-                    hidden_value,
-                    prefix=prefix[:previous_consumed] + "\n" * comment.newlines,
-                ),
-            )
-            return True
-
-    return False
-
-
-def generate_ignored_nodes(leaf: Leaf, comment: ProtoComment) -> Iterator[LN]:
-    """Starting from the container of `leaf`, generate all leaves until `# fmt: on`.
-
-    If comment is skip, returns leaf only.
-    Stops at the end of the block.
-    """
-    container: Optional[LN] = container_of(leaf)
-    if comment.value in FMT_SKIP:
-        prev_sibling = leaf.prev_sibling
-        if comment.value in leaf.prefix and prev_sibling is not None:
-            leaf.prefix = leaf.prefix.replace(comment.value, "")
-            siblings = [prev_sibling]
-            while (
-                "\n" not in prev_sibling.prefix
-                and prev_sibling.prev_sibling is not None
-            ):
-                prev_sibling = prev_sibling.prev_sibling
-                siblings.insert(0, prev_sibling)
-            for sibling in siblings:
-                yield sibling
-        elif leaf.parent is not None:
-            yield leaf.parent
-        return
-    while container is not None and container.type != token.ENDMARKER:
-        if is_fmt_on(container):
-            return
-
-        # fix for fmt: on in children
-        if contains_fmt_on_at_column(container, leaf.column):
-            for child in container.children:
-                if contains_fmt_on_at_column(child, leaf.column):
-                    return
-                yield child
-        else:
-            yield container
-            container = container.next_sibling
-
-
-def is_fmt_on(container: LN) -> bool:
-    """Determine whether formatting is switched on within a container.
-    Determined by whether the last `# fmt:` comment is `on` or `off`.
-    """
-    fmt_on = False
-    for comment in list_comments(container.prefix, is_endmarker=False):
-        if comment.value in FMT_ON:
-            fmt_on = True
-        elif comment.value in FMT_OFF:
-            fmt_on = False
-    return fmt_on
-
-
-def contains_fmt_on_at_column(container: LN, column: int) -> bool:
-    """Determine if children at a given column have formatting switched on."""
-    for child in container.children:
-        if (
-            isinstance(child, Node)
-            and first_leaf_column(child) == column
-            or isinstance(child, Leaf)
-            and child.column == column
-        ):
-            if is_fmt_on(child):
-                return True
-
-    return False
-
-
-def first_leaf_column(node: Node) -> Optional[int]:
-    """Returns the column of the first leaf child of a node."""
-    for child in node.children:
-        if isinstance(child, Leaf):
-            return child.column
-    return None
-
-
-def maybe_make_parens_invisible_in_atom(node: LN, parent: LN) -> bool:
-    """If it's safe, make the parens in the atom `node` invisible, recursively.
-    Additionally, remove repeated, adjacent invisible parens from the atom `node`
-    as they are redundant.
-
-    Returns whether the node should itself be wrapped in invisible parentheses.
-
-    """
-
-    if (
-        node.type != syms.atom
-        or is_empty_tuple(node)
-        or is_one_tuple(node)
-        or (is_yield(node) and parent.type != syms.expr_stmt)
-        or max_delimiter_priority_in_atom(node) >= COMMA_PRIORITY
-    ):
-        return False
-
-    if is_walrus_assignment(node):
-        if parent.type in [syms.annassign, syms.expr_stmt]:
-            return False
-
-    first = node.children[0]
-    last = node.children[-1]
-    if first.type == token.LPAR and last.type == token.RPAR:
-        middle = node.children[1]
-        # make parentheses invisible
-        first.value = ""  # type: ignore
-        last.value = ""  # type: ignore
-        maybe_make_parens_invisible_in_atom(middle, parent=parent)
-
-        if is_atom_with_invisible_parens(middle):
-            # Strip the invisible parens from `middle` by replacing
-            # it with the child in-between the invisible parens
-            middle.replace(middle.children[1])
-
-        return False
-
-    return True
-
-
-def is_atom_with_invisible_parens(node: LN) -> bool:
-    """Given a `LN`, determines whether it's an atom `node` with invisible
-    parens. Useful in dedupe-ing and normalizing parens.
-    """
-    if isinstance(node, Leaf) or node.type != syms.atom:
-        return False
-
-    first, last = node.children[0], node.children[-1]
-    return (
-        isinstance(first, Leaf)
-        and first.type == token.LPAR
-        and first.value == ""
-        and isinstance(last, Leaf)
-        and last.type == token.RPAR
-        and last.value == ""
-    )
-
-
-def is_empty_tuple(node: LN) -> bool:
-    """Return True if `node` holds an empty tuple."""
-    return (
-        node.type == syms.atom
-        and len(node.children) == 2
-        and node.children[0].type == token.LPAR
-        and node.children[1].type == token.RPAR
-    )
-
-
-def unwrap_singleton_parenthesis(node: LN) -> Optional[LN]:
-    """Returns `wrapped` if `node` is of the shape ( wrapped ).
-
-    Parenthesis can be optional. Returns None otherwise"""
-    if len(node.children) != 3:
-        return None
-
-    lpar, wrapped, rpar = node.children
-    if not (lpar.type == token.LPAR and rpar.type == token.RPAR):
-        return None
-
-    return wrapped
-
-
-def first_child_is_arith(node: Node) -> bool:
-    """Whether first child is an arithmetic or a binary arithmetic expression"""
-    expr_types = {
-        syms.arith_expr,
-        syms.shift_expr,
-        syms.xor_expr,
-        syms.and_expr,
-    }
-    return bool(node.children and node.children[0].type in expr_types)
-
-
-def wrap_in_parentheses(parent: Node, child: LN, *, visible: bool = True) -> None:
-    """Wrap `child` in parentheses.
-
-    This replaces `child` with an atom holding the parentheses and the old
-    child.  That requires moving the prefix.
-
-    If `visible` is False, the leaves will be valueless (and thus invisible).
-    """
-    lpar = Leaf(token.LPAR, "(" if visible else "")
-    rpar = Leaf(token.RPAR, ")" if visible else "")
-    prefix = child.prefix
-    child.prefix = ""
-    index = child.remove() or 0
-    new_child = Node(syms.atom, [lpar, child, rpar])
-    new_child.prefix = prefix
-    parent.insert_child(index, new_child)
-
-
-def is_one_tuple(node: LN) -> bool:
-    """Return True if `node` holds a tuple with one element, with or without parens."""
-    if node.type == syms.atom:
-        gexp = unwrap_singleton_parenthesis(node)
-        if gexp is None or gexp.type != syms.testlist_gexp:
-            return False
-
-        return len(gexp.children) == 2 and gexp.children[1].type == token.COMMA
-
-    return (
-        node.type in IMPLICIT_TUPLE
-        and len(node.children) == 2
-        and node.children[1].type == token.COMMA
-    )
-
-
-def is_walrus_assignment(node: LN) -> bool:
-    """Return True iff `node` is of the shape ( test := test )"""
-    inner = unwrap_singleton_parenthesis(node)
-    return inner is not None and inner.type == syms.namedexpr_test
-
-
-def is_simple_decorator_trailer(node: LN, last: bool = False) -> bool:
-    """Return True iff `node` is a trailer valid in a simple decorator"""
-    return node.type == syms.trailer and (
-        (
-            len(node.children) == 2
-            and node.children[0].type == token.DOT
-            and node.children[1].type == token.NAME
-        )
-        # last trailer can be arguments
-        or (
-            last
-            and len(node.children) == 3
-            and node.children[0].type == token.LPAR
-            # and node.children[1].type == syms.argument
-            and node.children[2].type == token.RPAR
-        )
-    )
-
-
-def is_simple_decorator_expression(node: LN) -> bool:
-    """Return True iff `node` could be a 'dotted name' decorator
-
-    This function takes the node of the 'namedexpr_test' of the new decorator
-    grammar and test if it would be valid under the old decorator grammar.
-
-    The old grammar was: decorator: @ dotted_name [arguments] NEWLINE
-    The new grammar is : decorator: @ namedexpr_test NEWLINE
-    """
-    if node.type == token.NAME:
-        return True
-    if node.type == syms.power:
-        if node.children:
-            return (
-                node.children[0].type == token.NAME
-                and all(map(is_simple_decorator_trailer, node.children[1:-1]))
-                and (
-                    len(node.children) < 2
-                    or is_simple_decorator_trailer(node.children[-1], last=True)
-                )
-            )
-    return False
-
-
-def is_yield(node: LN) -> bool:
-    """Return True if `node` holds a `yield` or `yield from` expression."""
-    if node.type == syms.yield_expr:
-        return True
-
-    if node.type == token.NAME and node.value == "yield":  # type: ignore
-        return True
-
-    if node.type != syms.atom:
-        return False
-
-    if len(node.children) != 3:
-        return False
-
-    lpar, expr, rpar = node.children
-    if lpar.type == token.LPAR and rpar.type == token.RPAR:
-        return is_yield(expr)
-
-    return False
-
-
-def is_vararg(leaf: Leaf, within: Set[NodeType]) -> bool:
-    """Return True if `leaf` is a star or double star in a vararg or kwarg.
-
-    If `within` includes VARARGS_PARENTS, this applies to function signatures.
-    If `within` includes UNPACKING_PARENTS, it applies to right hand-side
-    extended iterable unpacking (PEP 3132) and additional unpacking
-    generalizations (PEP 448).
-    """
-    if leaf.type not in VARARGS_SPECIALS or not leaf.parent:
-        return False
-
-    p = leaf.parent
-    if p.type == syms.star_expr:
-        # Star expressions are also used as assignment targets in extended
-        # iterable unpacking (PEP 3132).  See what its parent is instead.
-        if not p.parent:
-            return False
-
-        p = p.parent
-
-    return p.type in within
-
-
-def is_multiline_string(leaf: Leaf) -> bool:
-    """Return True if `leaf` is a multiline string that actually spans many lines."""
-    return has_triple_quotes(leaf.value) and "\n" in leaf.value
-
-
-def is_stub_suite(node: Node) -> bool:
-    """Return True if `node` is a suite with a stub body."""
-    if (
-        len(node.children) != 4
-        or node.children[0].type != token.NEWLINE
-        or node.children[1].type != token.INDENT
-        or node.children[3].type != token.DEDENT
-    ):
-        return False
-
-    return is_stub_body(node.children[2])
-
-
-def is_stub_body(node: LN) -> bool:
-    """Return True if `node` is a simple statement containing an ellipsis."""
-    if not isinstance(node, Node) or node.type != syms.simple_stmt:
-        return False
-
-    if len(node.children) != 2:
-        return False
-
-    child = node.children[0]
-    return (
-        child.type == syms.atom
-        and len(child.children) == 3
-        and all(leaf == Leaf(token.DOT, ".") for leaf in child.children)
-    )
-
-
-def max_delimiter_priority_in_atom(node: LN) -> Priority:
-    """Return maximum delimiter priority inside `node`.
-
-    This is specific to atoms with contents contained in a pair of parentheses.
-    If `node` isn't an atom or there are no enclosing parentheses, returns 0.
-    """
-    if node.type != syms.atom:
-        return 0
-
-    first = node.children[0]
-    last = node.children[-1]
-    if not (first.type == token.LPAR and last.type == token.RPAR):
-        return 0
-
-    bt = BracketTracker()
-    for c in node.children[1:-1]:
-        if isinstance(c, Leaf):
-            bt.mark(c)
-        else:
-            for leaf in c.leaves():
-                bt.mark(leaf)
-    try:
-        return bt.max_delimiter_priority()
-
-    except ValueError:
-        return 0
-
-
-def ensure_visible(leaf: Leaf) -> None:
-    """Make sure parentheses are visible.
-
-    They could be invisible as part of some statements (see
-    :func:`normalize_invisible_parens` and :func:`visit_import_from`).
-    """
-    if leaf.type == token.LPAR:
-        leaf.value = "("
-    elif leaf.type == token.RPAR:
-        leaf.value = ")"
-
-
-def should_split_line(line: Line, opening_bracket: Leaf) -> bool:
-    """Should `line` be immediately split with `delimiter_split()` after RHS?"""
-
-    if not (opening_bracket.parent and opening_bracket.value in "[{("):
-        return False
-
-    # We're essentially checking if the body is delimited by commas and there's more
-    # than one of them (we're excluding the trailing comma and if the delimiter priority
-    # is still commas, that means there's more).
-    exclude = set()
-    trailing_comma = False
-    try:
-        last_leaf = line.leaves[-1]
-        if last_leaf.type == token.COMMA:
-            trailing_comma = True
-            exclude.add(id(last_leaf))
-        max_priority = line.bracket_tracker.max_delimiter_priority(exclude=exclude)
-    except (IndexError, ValueError):
-        return False
-
-    return max_priority == COMMA_PRIORITY and (
-        (line.mode.magic_trailing_comma and trailing_comma)
-        # always explode imports
-        or opening_bracket.parent.type in {syms.atom, syms.import_from}
-    )
-
-
-def is_one_tuple_between(opening: Leaf, closing: Leaf, leaves: List[Leaf]) -> bool:
-    """Return True if content between `opening` and `closing` looks like a one-tuple."""
-    if opening.type != token.LPAR and closing.type != token.RPAR:
-        return False
-
-    depth = closing.bracket_depth + 1
-    for _opening_index, leaf in enumerate(leaves):
-        if leaf is opening:
-            break
-
-    else:
-        raise LookupError("Opening paren not found in `leaves`")
-
-    commas = 0
-    _opening_index += 1
-    for leaf in leaves[_opening_index:]:
-        if leaf is closing:
-            break
-
-        bracket_depth = leaf.bracket_depth
-        if bracket_depth == depth and leaf.type == token.COMMA:
-            commas += 1
-            if leaf.parent and leaf.parent.type in {
-                syms.arglist,
-                syms.typedargslist,
-            }:
-                commas += 1
-                break
-
-    return commas < 2
-
-
-def get_features_used(node: Node) -> Set[Feature]:
-    """Return a set of (relatively) new Python features used in this file.
-
-    Currently looking for:
-    - f-strings;
-    - underscores in numeric literals;
-    - trailing commas after * or ** in function signatures and calls;
-    - positional only arguments in function signatures and lambdas;
-    - assignment expression;
-    - relaxed decorator syntax;
-    """
-    features: Set[Feature] = set()
-    for n in node.pre_order():
-        if n.type == token.STRING:
-            value_head = n.value[:2]  # type: ignore
-            if value_head in {'f"', 'F"', "f'", "F'", "rf", "fr", "RF", "FR"}:
-                features.add(Feature.F_STRINGS)
-
-        elif n.type == token.NUMBER:
-            if "_" in n.value:  # type: ignore
-                features.add(Feature.NUMERIC_UNDERSCORES)
+    for n in node.pre_order():
+        if is_string_token(n):
+            value_head = n.value[:2]
+            if value_head in {'f"', 'F"', "f'", "F'", "rf", "fr", "RF", "FR"}:
+                features.add(Feature.F_STRINGS)
+                if Feature.DEBUG_F_STRINGS not in features:
+                    for span_beg, span_end in iter_fexpr_spans(n.value):
+                        if n.value[span_beg : span_end - 1].rstrip().endswith("="):
+                            features.add(Feature.DEBUG_F_STRINGS)
+                            break
+
+        elif is_number_token(n):
+            if "_" in n.value:
+                features.add(Feature.NUMERIC_UNDERSCORES)

 
         elif n.type == token.SLASH:
 
         elif n.type == token.SLASH:

-            if n.parent and n.parent.type in {syms.typedargslist, syms.arglist}:
+            if n.parent and n.parent.type in {
+                syms.typedargslist,
+                syms.arglist,
+                syms.varargslist,
+            }:

                 features.add(Feature.POS_ONLY_ARGUMENTS)
 
         elif n.type == token.COLONEQUAL:
                 features.add(Feature.POS_ONLY_ARGUMENTS)
 
         elif n.type == token.COLONEQUAL:


@@ -5969,89 +1226,71 @@ def get_features_used(node: Node) -> Set[Feature]:
                         if argch.type in STARS:
                             features.add(feature)
 
                         if argch.type in STARS:
                             features.add(feature)
 

-    return features
-
-
-def detect_target_versions(node: Node) -> Set[TargetVersion]:
-    """Detect the version to target based on the nodes used."""
-    features = get_features_used(node)
-    return {
-        version for version in TargetVersion if features <= VERSION_TO_FEATURES[version]
-    }
+        elif (
+            n.type in {syms.return_stmt, syms.yield_expr}
+            and len(n.children) >= 2
+            and n.children[1].type == syms.testlist_star_expr
+            and any(child.type == syms.star_expr for child in n.children[1].children)
+        ):
+            features.add(Feature.UNPACKING_ON_FLOW)

 
 

+        elif (
+            n.type == syms.annassign
+            and len(n.children) >= 4
+            and n.children[3].type == syms.testlist_star_expr
+        ):
+            features.add(Feature.ANN_ASSIGN_EXTENDED_RHS)

 
 

-def generate_trailers_to_omit(line: Line, line_length: int) -> Iterator[Set[LeafID]]:
-    """Generate sets of closing bracket IDs that should be omitted in a RHS.
+        elif (
+            n.type == syms.with_stmt
+            and len(n.children) > 2
+            and n.children[1].type == syms.atom
+        ):
+            atom_children = n.children[1].children
+            if (
+                len(atom_children) == 3
+                and atom_children[0].type == token.LPAR
+                and atom_children[1].type == syms.testlist_gexp
+                and atom_children[2].type == token.RPAR
+            ):
+                features.add(Feature.PARENTHESIZED_CONTEXT_MANAGERS)

 
 

-    Brackets can be omitted if the entire trailer up to and including
-    a preceding closing bracket fits in one line.
+        elif n.type == syms.match_stmt:
+            features.add(Feature.PATTERN_MATCHING)

 
 

-    Yielded sets are cumulative (contain results of previous yields, too).  First
-    set is empty, unless the line should explode, in which case bracket pairs until
-    the one that needs to explode are omitted.
-    """
+        elif (
+            n.type == syms.except_clause
+            and len(n.children) >= 2
+            and n.children[1].type == token.STAR
+        ):
+            features.add(Feature.EXCEPT_STAR)

 
 

-    omit: Set[LeafID] = set()
-    if not line.magic_trailing_comma:
-        yield omit
-
-    length = 4 * line.depth
-    opening_bracket: Optional[Leaf] = None
-    closing_bracket: Optional[Leaf] = None
-    inner_brackets: Set[LeafID] = set()
-    for index, leaf, leaf_length in enumerate_with_length(line, reversed=True):
-        length += leaf_length
-        if length > line_length:
-            break
+        elif n.type in {syms.subscriptlist, syms.trailer} and any(
+            child.type == syms.star_expr for child in n.children
+        ):
+            features.add(Feature.VARIADIC_GENERICS)

 
 

-        has_inline_comment = leaf_length > len(leaf.value) + len(leaf.prefix)
-        if leaf.type == STANDALONE_COMMENT or has_inline_comment:
-            break
+        elif (
+            n.type == syms.tname_star
+            and len(n.children) == 3
+            and n.children[2].type == syms.star_expr
+        ):
+            features.add(Feature.VARIADIC_GENERICS)

 
 

-        if opening_bracket:
-            if leaf is opening_bracket:
-                opening_bracket = None
-            elif leaf.type in CLOSING_BRACKETS:
-                prev = line.leaves[index - 1] if index > 0 else None
-                if (
-                    prev
-                    and prev.type == token.COMMA
-                    and not is_one_tuple_between(
-                        leaf.opening_bracket, leaf, line.leaves
-                    )
-                ):
-                    # Never omit bracket pairs with trailing commas.
-                    # We need to explode on those.
-                    break
-
-                inner_brackets.add(id(leaf))
-        elif leaf.type in CLOSING_BRACKETS:
-            prev = line.leaves[index - 1] if index > 0 else None
-            if prev and prev.type in OPENING_BRACKETS:
-                # Empty brackets would fail a split so treat them as "inner"
-                # brackets (e.g. only add them to the `omit` set if another
-                # pair of brackets was good enough.
-                inner_brackets.add(id(leaf))
-                continue
+        elif n.type in (syms.type_stmt, syms.typeparams):
+            features.add(Feature.TYPE_PARAMS)

 
 

-            if closing_bracket:
-                omit.add(id(closing_bracket))
-                omit.update(inner_brackets)
-                inner_brackets.clear()
-                yield omit
+    return features

 
 

-            if (
-                prev
-                and prev.type == token.COMMA
-                and not is_one_tuple_between(leaf.opening_bracket, leaf, line.leaves)
-            ):
-                # Never omit bracket pairs with trailing commas.
-                # We need to explode on those.
-                break

 
 

-            if leaf.value:
-                opening_bracket = leaf.opening_bracket
-                closing_bracket = leaf
+def detect_target_versions(
+    node: Node, *, future_imports: Optional[Set[str]] = None
+) -> Set[TargetVersion]:
+    """Detect the version to target based on the nodes used."""
+    features = get_features_used(node, future_imports=future_imports)
+    return {
+        version for version in TargetVersion if features <= VERSION_TO_FEATURES[version]
+    }

 
 
 def get_future_imports(node: Node) -> Set[str]:
 
 
 def get_future_imports(node: Node) -> Set[str]:


@@ -6104,389 +1343,45 @@ def get_future_imports(node: Node) -> Set[str]:
     return imports
 
 
     return imports
 
 

-@lru_cache()
-def get_gitignore(root: Path) -> PathSpec:
-    """ Return a PathSpec matching gitignore content if present."""
-    gitignore = root / ".gitignore"
-    lines: List[str] = []
-    if gitignore.is_file():
-        with gitignore.open() as gf:
-            lines = gf.readlines()
-    return PathSpec.from_lines("gitwildmatch", lines)
-
-
-def normalize_path_maybe_ignore(
-    path: Path, root: Path, report: "Report"
-) -> Optional[str]:
-    """Normalize `path`. May return `None` if `path` was ignored.
-
-    `report` is where "path ignored" output goes.
-    """
-    try:
-        abspath = path if path.is_absolute() else Path.cwd() / path
-        normalized_path = abspath.resolve().relative_to(root).as_posix()
-    except OSError as e:
-        report.path_ignored(path, f"cannot be read because {e}")
-        return None
-
-    except ValueError:
-        if path.is_symlink():
-            report.path_ignored(path, f"is a symbolic link that points outside {root}")
-            return None
-
-        raise
-
-    return normalized_path
-
-
-def path_is_excluded(
-    normalized_path: str,
-    pattern: Optional[Pattern[str]],
-) -> bool:
-    match = pattern.search(normalized_path) if pattern else None
-    return bool(match and match.group(0))
-
-
-def gen_python_files(
-    paths: Iterable[Path],
-    root: Path,
-    include: Optional[Pattern[str]],
-    exclude: Pattern[str],
-    extend_exclude: Optional[Pattern[str]],
-    force_exclude: Optional[Pattern[str]],
-    report: "Report",
-    gitignore: PathSpec,
-) -> Iterator[Path]:
-    """Generate all files under `path` whose paths are not excluded by the
-    `exclude_regex`, `extend_exclude`, or `force_exclude` regexes,
-    but are included by the `include` regex.
-
-    Symbolic links pointing outside of the `root` directory are ignored.
-
-    `report` is where output about exclusions goes.
-    """
-    assert root.is_absolute(), f"INTERNAL ERROR: `root` must be absolute but is {root}"
-    for child in paths:
-        normalized_path = normalize_path_maybe_ignore(child, root, report)
-        if normalized_path is None:
-            continue
-
-        # First ignore files matching .gitignore
-        if gitignore.match_file(normalized_path):
-            report.path_ignored(child, "matches the .gitignore file content")
-            continue
-
-        # Then ignore with `--exclude` `--extend-exclude` and `--force-exclude` options.
-        normalized_path = "/" + normalized_path
-        if child.is_dir():
-            normalized_path += "/"
-
-        if path_is_excluded(normalized_path, exclude):
-            report.path_ignored(child, "matches the --exclude regular expression")
-            continue
-
-        if path_is_excluded(normalized_path, extend_exclude):
-            report.path_ignored(
-                child, "matches the --extend-exclude regular expression"
-            )
-            continue
-
-        if path_is_excluded(normalized_path, force_exclude):
-            report.path_ignored(child, "matches the --force-exclude regular expression")
-            continue
-
-        if child.is_dir():
-            yield from gen_python_files(
-                child.iterdir(),
-                root,
-                include,
-                exclude,
-                extend_exclude,
-                force_exclude,
-                report,
-                gitignore,
-            )
-
-        elif child.is_file():
-            include_match = include.search(normalized_path) if include else True
-            if include_match:
-                yield child
-
-
-@lru_cache()
-def find_project_root(srcs: Iterable[str]) -> Path:
-    """Return a directory containing .git, .hg, or pyproject.toml.
-
-    That directory will be a common parent of all files and directories
-    passed in `srcs`.
-
-    If no directory in the tree contains a marker that would specify it's the
-    project root, the root of the file system is returned.
-    """
-    if not srcs:
-        return Path("/").resolve()
-
-    path_srcs = [Path(Path.cwd(), src).resolve() for src in srcs]
-
-    # A list of lists of parents for each 'src'. 'src' is included as a
-    # "parent" of itself if it is a directory
-    src_parents = [
-        list(path.parents) + ([path] if path.is_dir() else []) for path in path_srcs
-    ]
-
-    common_base = max(
-        set.intersection(*(set(parents) for parents in src_parents)),
-        key=lambda path: path.parts,
-    )
-
-    for directory in (common_base, *common_base.parents):
-        if (directory / ".git").exists():
-            return directory
-
-        if (directory / ".hg").is_dir():
-            return directory
-
-        if (directory / "pyproject.toml").is_file():
-            return directory
-
-    return directory
-
-
-@lru_cache()
-def find_user_pyproject_toml() -> Path:
-    r"""Return the path to the top-level user configuration for black.
-
-    This looks for ~\.black on Windows and ~/.config/black on Linux and other
-    Unix systems.
-    """
-    if sys.platform == "win32":
-        # Windows
-        user_config_path = Path.home() / ".black"
-    else:
-        config_root = os.environ.get("XDG_CONFIG_HOME", "~/.config")
-        user_config_path = Path(config_root).expanduser() / "black"
-    return user_config_path.resolve()
-
-
-@dataclass
-class Report:
-    """Provides a reformatting counter. Can be rendered with `str(report)`."""
-
-    check: bool = False
-    diff: bool = False
-    quiet: bool = False
-    verbose: bool = False
-    change_count: int = 0
-    same_count: int = 0
-    failure_count: int = 0
-
-    def done(self, src: Path, changed: Changed) -> None:
-        """Increment the counter for successful reformatting. Write out a message."""
-        if changed is Changed.YES:
-            reformatted = "would reformat" if self.check or self.diff else "reformatted"
-            if self.verbose or not self.quiet:
-                out(f"{reformatted} {src}")
-            self.change_count += 1
-        else:
-            if self.verbose:
-                if changed is Changed.NO:
-                    msg = f"{src} already well formatted, good job."
-                else:
-                    msg = f"{src} wasn't modified on disk since last run."
-                out(msg, bold=False)
-            self.same_count += 1
-
-    def failed(self, src: Path, message: str) -> None:
-        """Increment the counter for failed reformatting. Write out a message."""
-        err(f"error: cannot format {src}: {message}")
-        self.failure_count += 1
-
-    def path_ignored(self, path: Path, message: str) -> None:
-        if self.verbose:
-            out(f"{path} ignored: {message}", bold=False)
-
-    @property
-    def return_code(self) -> int:
-        """Return the exit code that the app should use.
-
-        This considers the current state of changed files and failures:
-        - if there were any failures, return 123;
-        - if any files were changed and --check is being used, return 1;
-        - otherwise return 0.
-        """
-        # According to http://tldp.org/LDP/abs/html/exitcodes.html starting with
-        # 126 we have special return codes reserved by the shell.
-        if self.failure_count:
-            return 123
-
-        elif self.change_count and self.check:
-            return 1
-
-        return 0
-
-    def __str__(self) -> str:
-        """Render a color report of the current state.
-
-        Use `click.unstyle` to remove colors.
-        """
-        if self.check or self.diff:
-            reformatted = "would be reformatted"
-            unchanged = "would be left unchanged"
-            failed = "would fail to reformat"
-        else:
-            reformatted = "reformatted"
-            unchanged = "left unchanged"
-            failed = "failed to reformat"
-        report = []
-        if self.change_count:
-            s = "s" if self.change_count > 1 else ""
-            report.append(
-                click.style(f"{self.change_count} file{s} {reformatted}", bold=True)
-            )
-        if self.same_count:
-            s = "s" if self.same_count > 1 else ""
-            report.append(f"{self.same_count} file{s} {unchanged}")
-        if self.failure_count:
-            s = "s" if self.failure_count > 1 else ""
-            report.append(
-                click.style(f"{self.failure_count} file{s} {failed}", fg="red")
-            )
-        return ", ".join(report) + "."
-
-
-def parse_ast(src: str) -> Union[ast.AST, ast3.AST, ast27.AST]:
-    filename = "<unknown>"
-    if sys.version_info >= (3, 8):
-        # TODO: support Python 4+ ;)
-        for minor_version in range(sys.version_info[1], 4, -1):
-            try:
-                return ast.parse(src, filename, feature_version=(3, minor_version))
-            except SyntaxError:
-                continue
-    else:
-        for feature_version in (7, 6):
-            try:
-                return ast3.parse(src, filename, feature_version=feature_version)
-            except SyntaxError:
-                continue
-    if ast27.__name__ == "ast":
-        raise SyntaxError(
-            "The requested source code has invalid Python 3 syntax.\n"
-            "If you are trying to format Python 2 files please reinstall Black"
-            " with the 'python2' extra: `python3 -m pip install black[python2]`."
-        )
-    return ast27.parse(src)
-
-
-def _fixup_ast_constants(
-    node: Union[ast.AST, ast3.AST, ast27.AST]
-) -> Union[ast.AST, ast3.AST, ast27.AST]:
-    """Map ast nodes deprecated in 3.8 to Constant."""
-    if isinstance(node, (ast.Str, ast3.Str, ast27.Str, ast.Bytes, ast3.Bytes)):
-        return ast.Constant(value=node.s)
-
-    if isinstance(node, (ast.Num, ast3.Num, ast27.Num)):
-        return ast.Constant(value=node.n)
-
-    if isinstance(node, (ast.NameConstant, ast3.NameConstant)):
-        return ast.Constant(value=node.value)
-
-    return node
-
-
-def _stringify_ast(
-    node: Union[ast.AST, ast3.AST, ast27.AST], depth: int = 0
-) -> Iterator[str]:
-    """Simple visitor generating strings to compare ASTs by content."""
-
-    node = _fixup_ast_constants(node)
-
-    yield f"{'  ' * depth}{node.__class__.__name__}("
-
-    for field in sorted(node._fields):  # noqa: F402
-        # TypeIgnore has only one field 'lineno' which breaks this comparison
-        type_ignore_classes = (ast3.TypeIgnore, ast27.TypeIgnore)
-        if sys.version_info >= (3, 8):
-            type_ignore_classes += (ast.TypeIgnore,)
-        if isinstance(node, type_ignore_classes):
-            break
-
-        try:
-            value = getattr(node, field)
-        except AttributeError:
-            continue
-
-        yield f"{'  ' * (depth+1)}{field}="
-
-        if isinstance(value, list):
-            for item in value:
-                # Ignore nested tuples within del statements, because we may insert
-                # parentheses and they change the AST.
-                if (
-                    field == "targets"
-                    and isinstance(node, (ast.Delete, ast3.Delete, ast27.Delete))
-                    and isinstance(item, (ast.Tuple, ast3.Tuple, ast27.Tuple))
-                ):
-                    for item in item.elts:
-                        yield from _stringify_ast(item, depth + 2)
-
-                elif isinstance(item, (ast.AST, ast3.AST, ast27.AST)):
-                    yield from _stringify_ast(item, depth + 2)
-
-        elif isinstance(value, (ast.AST, ast3.AST, ast27.AST)):
-            yield from _stringify_ast(value, depth + 2)
-
-        else:
-            # Constant strings may be indented across newlines, if they are
-            # docstrings; fold spaces after newlines when comparing. Similarly,
-            # trailing and leading space may be removed.
-            if (
-                isinstance(node, ast.Constant)
-                and field == "value"
-                and isinstance(value, str)
-            ):
-                normalized = re.sub(r" *\n[ \t]*", "\n", value).strip()
-            else:
-                normalized = value
-            yield f"{'  ' * (depth+2)}{normalized!r},  # {value.__class__.__name__}"
-
-    yield f"{'  ' * depth})  # /{node.__class__.__name__}"
-
-

 def assert_equivalent(src: str, dst: str) -> None:
     """Raise AssertionError if `src` and `dst` aren't equivalent."""
     try:
         src_ast = parse_ast(src)
     except Exception as exc:
         raise AssertionError(
 def assert_equivalent(src: str, dst: str) -> None:
     """Raise AssertionError if `src` and `dst` aren't equivalent."""
     try:
         src_ast = parse_ast(src)
     except Exception as exc:
         raise AssertionError(

-            "cannot use --safe with this file; failed to parse source file.  AST"
-            f" error message: {exc}"
-        )
+            "cannot use --safe with this file; failed to parse source file AST: "
+            f"{exc}\n"
+            "This could be caused by running Black with an older Python version "
+            "that does not support new syntax used in your source file."
+        ) from exc

 
     try:
         dst_ast = parse_ast(dst)
     except Exception as exc:
         log = dump_to_file("".join(traceback.format_tb(exc.__traceback__)), dst)
         raise AssertionError(
 
     try:
         dst_ast = parse_ast(dst)
     except Exception as exc:
         log = dump_to_file("".join(traceback.format_tb(exc.__traceback__)), dst)
         raise AssertionError(

-            f"INTERNAL ERROR: Black produced invalid code: {exc}. Please report a bug"
-            " on https://github.com/psf/black/issues.  This invalid output might be"
-            f" helpful: {log}"
+            f"INTERNAL ERROR: Black produced invalid code: {exc}. "
+            "Please report a bug on https://github.com/psf/black/issues.  "
+            f"This invalid output might be helpful: {log}"

         ) from None
 
         ) from None
 

-    src_ast_str = "\n".join(_stringify_ast(src_ast))
-    dst_ast_str = "\n".join(_stringify_ast(dst_ast))
+    src_ast_str = "\n".join(stringify_ast(src_ast))
+    dst_ast_str = "\n".join(stringify_ast(dst_ast))

     if src_ast_str != dst_ast_str:
         log = dump_to_file(diff(src_ast_str, dst_ast_str, "src", "dst"))
         raise AssertionError(
             "INTERNAL ERROR: Black produced code that is not equivalent to the"
     if src_ast_str != dst_ast_str:
         log = dump_to_file(diff(src_ast_str, dst_ast_str, "src", "dst"))
         raise AssertionError(
             "INTERNAL ERROR: Black produced code that is not equivalent to the"

-            " source.  Please report a bug on https://github.com/psf/black/issues. "
-            f" This diff might be helpful: {log}"
+            " source.  Please report a bug on "
+            f"https://github.com/psf/black/issues.  This diff might be helpful: {log}"

         ) from None
 
 
 def assert_stable(src: str, dst: str, mode: Mode) -> None:
     """Raise AssertionError if `dst` reformats differently the second time."""
         ) from None
 
 
 def assert_stable(src: str, dst: str, mode: Mode) -> None:
     """Raise AssertionError if `dst` reformats differently the second time."""

-    newdst = format_str(dst, mode=mode)
+    # We shouldn't call format_str() here, because that formats the string
+    # twice and may hide a bug where we bounce back and forth between two
+    # versions.
+    newdst = _format_str_once(dst, mode=mode)

     if dst != newdst:
         log = dump_to_file(
             str(mode),
     if dst != newdst:
         log = dump_to_file(
             str(mode),


@@ -6500,19 +1395,6 @@ def assert_stable(src: str, dst: str, mode: Mode) -> None:
         ) from None
 
 
         ) from None
 
 

-@mypyc_attr(patchable=True)
-def dump_to_file(*output: str, ensure_final_newline: bool = True) -> str:
-    """Dump `output` to a temporary file. Return path to the file."""
-    with tempfile.NamedTemporaryFile(
-        mode="w", prefix="blk_", suffix=".log", delete=False, encoding="utf8"
-    ) as f:
-        for lines in output:
-            f.write(lines)
-            if ensure_final_newline and lines and lines[-1] != "\n":
-                f.write("\n")
-    return f.name
-
-

 @contextmanager
 def nullcontext() -> Iterator[None]:
     """Return an empty context manager.
 @contextmanager
 def nullcontext() -> Iterator[None]:
     """Return an empty context manager.


@@ -6522,493 +1404,15 @@ def nullcontext() -> Iterator[None]:
     yield
 
 
     yield
 
 

-def diff(a: str, b: str, a_name: str, b_name: str) -> str:
-    """Return a unified diff string between strings `a` and `b`."""
-    import difflib
-
-    a_lines = [line for line in a.splitlines(keepends=True)]
-    b_lines = [line for line in b.splitlines(keepends=True)]
-    diff_lines = []
-    for line in difflib.unified_diff(
-        a_lines, b_lines, fromfile=a_name, tofile=b_name, n=5
-    ):
-        # Work around https://bugs.python.org/issue2142
-        # See https://www.gnu.org/software/diffutils/manual/html_node/Incomplete-Lines.html
-        if line[-1] == "\n":
-            diff_lines.append(line)
-        else:
-            diff_lines.append(line + "\n")
-            diff_lines.append("\\ No newline at end of file\n")
-    return "".join(diff_lines)
-
-
-def cancel(tasks: Iterable["asyncio.Task[Any]"]) -> None:
-    """asyncio signal handler that cancels all `tasks` and reports to stderr."""
-    err("Aborted!")
-    for task in tasks:
-        task.cancel()
-
-
-def shutdown(loop: asyncio.AbstractEventLoop) -> None:
-    """Cancel all pending tasks on `loop`, wait for them, and close the loop."""
-    try:
-        if sys.version_info[:2] >= (3, 7):
-            all_tasks = asyncio.all_tasks
-        else:
-            all_tasks = asyncio.Task.all_tasks
-        # This part is borrowed from asyncio/runners.py in Python 3.7b2.
-        to_cancel = [task for task in all_tasks(loop) if not task.done()]
-        if not to_cancel:
-            return
-
-        for task in to_cancel:
-            task.cancel()
-        loop.run_until_complete(
-            asyncio.gather(*to_cancel, loop=loop, return_exceptions=True)
-        )
-    finally:
-        # `concurrent.futures.Future` objects cannot be cancelled once they
-        # are already running. There might be some when the `shutdown()` happened.
-        # Silence their logger's spew about the event loop being closed.
-        cf_logger = logging.getLogger("concurrent.futures")
-        cf_logger.setLevel(logging.CRITICAL)
-        loop.close()
-
-
-def sub_twice(regex: Pattern[str], replacement: str, original: str) -> str:
-    """Replace `regex` with `replacement` twice on `original`.
-
-    This is used by string normalization to perform replaces on
-    overlapping matches.
-    """
-    return regex.sub(replacement, regex.sub(replacement, original))
-
-
-def re_compile_maybe_verbose(regex: str) -> Pattern[str]:
-    """Compile a regular expression string in `regex`.
-
-    If it contains newlines, use verbose mode.
-    """
-    if "\n" in regex:
-        regex = "(?x)" + regex
-    compiled: Pattern[str] = re.compile(regex)
-    return compiled
-
-
-def enumerate_reversed(sequence: Sequence[T]) -> Iterator[Tuple[Index, T]]:
-    """Like `reversed(enumerate(sequence))` if that were possible."""
-    index = len(sequence) - 1
-    for element in reversed(sequence):
-        yield (index, element)
-        index -= 1
-
-
-def enumerate_with_length(
-    line: Line, reversed: bool = False
-) -> Iterator[Tuple[Index, Leaf, int]]:
-    """Return an enumeration of leaves with their length.
-
-    Stops prematurely on multiline strings and standalone comments.
-    """
-    op = cast(
-        Callable[[Sequence[Leaf]], Iterator[Tuple[Index, Leaf]]],
-        enumerate_reversed if reversed else enumerate,
-    )
-    for index, leaf in op(line.leaves):
-        length = len(leaf.prefix) + len(leaf.value)
-        if "\n" in leaf.value:
-            return  # Multiline strings, we can't continue.
-
-        for comment in line.comments_after(leaf):
-            length += len(comment.value)
-
-        yield index, leaf, length
-
-
-def is_line_short_enough(line: Line, *, line_length: int, line_str: str = "") -> bool:
-    """Return True if `line` is no longer than `line_length`.
-
-    Uses the provided `line_str` rendering, if any, otherwise computes a new one.
-    """
-    if not line_str:
-        line_str = line_to_string(line)
-    return (
-        len(line_str) <= line_length
-        and "\n" not in line_str  # multiline strings
-        and not line.contains_standalone_comments()
-    )
-
-
-def can_be_split(line: Line) -> bool:
-    """Return False if the line cannot be split *for sure*.
-
-    This is not an exhaustive search but a cheap heuristic that we can use to
-    avoid some unfortunate formattings (mostly around wrapping unsplittable code
-    in unnecessary parentheses).
-    """
-    leaves = line.leaves
-    if len(leaves) < 2:
-        return False
-
-    if leaves[0].type == token.STRING and leaves[1].type == token.DOT:
-        call_count = 0
-        dot_count = 0
-        next = leaves[-1]
-        for leaf in leaves[-2::-1]:
-            if leaf.type in OPENING_BRACKETS:
-                if next.type not in CLOSING_BRACKETS:
-                    return False
-
-                call_count += 1
-            elif leaf.type == token.DOT:
-                dot_count += 1
-            elif leaf.type == token.NAME:
-                if not (next.type == token.DOT or next.type in OPENING_BRACKETS):
-                    return False
-
-            elif leaf.type not in CLOSING_BRACKETS:
-                return False
-
-            if dot_count > 1 and call_count > 1:
-                return False
-
-    return True
-
-
-def can_omit_invisible_parens(
-    line: Line,
-    line_length: int,
-    omit_on_explode: Collection[LeafID] = (),
-) -> bool:
-    """Does `line` have a shape safe to reformat without optional parens around it?
-
-    Returns True for only a subset of potentially nice looking formattings but
-    the point is to not return false positives that end up producing lines that
-    are too long.
-    """
-    bt = line.bracket_tracker
-    if not bt.delimiters:
-        # Without delimiters the optional parentheses are useless.
-        return True
-
-    max_priority = bt.max_delimiter_priority()
-    if bt.delimiter_count_with_priority(max_priority) > 1:
-        # With more than one delimiter of a kind the optional parentheses read better.
-        return False
-
-    if max_priority == DOT_PRIORITY:
-        # A single stranded method call doesn't require optional parentheses.
-        return True
-
-    assert len(line.leaves) >= 2, "Stranded delimiter"
-
-    # With a single delimiter, omit if the expression starts or ends with
-    # a bracket.
-    first = line.leaves[0]
-    second = line.leaves[1]
-    if first.type in OPENING_BRACKETS and second.type not in CLOSING_BRACKETS:
-        if _can_omit_opening_paren(line, first=first, line_length=line_length):
-            return True
-
-        # Note: we are not returning False here because a line might have *both*
-        # a leading opening bracket and a trailing closing bracket.  If the
-        # opening bracket doesn't match our rule, maybe the closing will.
-
-    penultimate = line.leaves[-2]
-    last = line.leaves[-1]
-    if line.magic_trailing_comma:
-        try:
-            penultimate, last = last_two_except(line.leaves, omit=omit_on_explode)
-        except LookupError:
-            # Turns out we'd omit everything.  We cannot skip the optional parentheses.
-            return False
-
-    if (
-        last.type == token.RPAR
-        or last.type == token.RBRACE
-        or (
-            # don't use indexing for omitting optional parentheses;
-            # it looks weird
-            last.type == token.RSQB
-            and last.parent
-            and last.parent.type != syms.trailer
-        )
-    ):
-        if penultimate.type in OPENING_BRACKETS:
-            # Empty brackets don't help.
-            return False
-
-        if is_multiline_string(first):
-            # Additional wrapping of a multiline string in this situation is
-            # unnecessary.
-            return True
-
-        if line.magic_trailing_comma and penultimate.type == token.COMMA:
-            # The rightmost non-omitted bracket pair is the one we want to explode on.
-            return True
-
-        if _can_omit_closing_paren(line, last=last, line_length=line_length):
-            return True
-
-    return False
-
-
-def _can_omit_opening_paren(line: Line, *, first: Leaf, line_length: int) -> bool:
-    """See `can_omit_invisible_parens`."""
-    remainder = False
-    length = 4 * line.depth
-    _index = -1
-    for _index, leaf, leaf_length in enumerate_with_length(line):
-        if leaf.type in CLOSING_BRACKETS and leaf.opening_bracket is first:
-            remainder = True
-        if remainder:
-            length += leaf_length
-            if length > line_length:
-                break
-
-            if leaf.type in OPENING_BRACKETS:
-                # There are brackets we can further split on.
-                remainder = False
-
-    else:
-        # checked the entire string and line length wasn't exceeded
-        if len(line.leaves) == _index + 1:
-            return True
-
-    return False
-
-
-def _can_omit_closing_paren(line: Line, *, last: Leaf, line_length: int) -> bool:
-    """See `can_omit_invisible_parens`."""
-    length = 4 * line.depth
-    seen_other_brackets = False
-    for _index, leaf, leaf_length in enumerate_with_length(line):
-        length += leaf_length
-        if leaf is last.opening_bracket:
-            if seen_other_brackets or length <= line_length:
-                return True
-
-        elif leaf.type in OPENING_BRACKETS:
-            # There are brackets we can further split on.
-            seen_other_brackets = True
-
-    return False
-
-
-def last_two_except(leaves: List[Leaf], omit: Collection[LeafID]) -> Tuple[Leaf, Leaf]:
-    """Return (penultimate, last) leaves skipping brackets in `omit` and contents."""
-    stop_after = None
-    last = None
-    for leaf in reversed(leaves):
-        if stop_after:
-            if leaf is stop_after:
-                stop_after = None
-            continue
-
-        if last:
-            return leaf, last
-
-        if id(leaf) in omit:
-            stop_after = leaf.opening_bracket
-        else:
-            last = leaf
-    else:
-        raise LookupError("Last two leaves were also skipped")
-
-
-def run_transformer(
-    line: Line,
-    transform: Transformer,
-    mode: Mode,
-    features: Collection[Feature],
-    *,
-    line_str: str = "",
-) -> List[Line]:
-    if not line_str:
-        line_str = line_to_string(line)
-    result: List[Line] = []
-    for transformed_line in transform(line, features):
-        if str(transformed_line).strip("\n") == line_str:
-            raise CannotTransform("Line transformer returned an unchanged result")
-
-        result.extend(transform_line(transformed_line, mode=mode, features=features))
-
-    if not (
-        transform.__name__ == "rhs"
-        and line.bracket_tracker.invisible
-        and not any(bracket.value for bracket in line.bracket_tracker.invisible)
-        and not line.contains_multiline_strings()
-        and not result[0].contains_uncollapsable_type_comments()
-        and not result[0].contains_unsplittable_type_ignore()
-        and not is_line_short_enough(result[0], line_length=mode.line_length)
-    ):
-        return result
-
-    line_copy = line.clone()
-    append_leaves(line_copy, line, line.leaves)
-    features_fop = set(features) | {Feature.FORCE_OPTIONAL_PARENTHESES}
-    second_opinion = run_transformer(
-        line_copy, transform, mode, features_fop, line_str=line_str
-    )
-    if all(
-        is_line_short_enough(ln, line_length=mode.line_length) for ln in second_opinion
-    ):
-        result = second_opinion
-    return result
-
-
-def get_cache_file(mode: Mode) -> Path:
-    return CACHE_DIR / f"cache.{mode.get_cache_key()}.pickle"
-
-
-def read_cache(mode: Mode) -> Cache:
-    """Read the cache if it exists and is well formed.
-
-    If it is not well formed, the call to write_cache later should resolve the issue.
-    """
-    cache_file = get_cache_file(mode)
-    if not cache_file.exists():
-        return {}
-
-    with cache_file.open("rb") as fobj:
-        try:
-            cache: Cache = pickle.load(fobj)
-        except (pickle.UnpicklingError, ValueError):
-            return {}
-
-    return cache
-
-
-def get_cache_info(path: Path) -> CacheInfo:
-    """Return the information used to check if a file is already formatted or not."""
-    stat = path.stat()
-    return stat.st_mtime, stat.st_size
-
-
-def filter_cached(cache: Cache, sources: Iterable[Path]) -> Tuple[Set[Path], Set[Path]]:
-    """Split an iterable of paths in `sources` into two sets.
-
-    The first contains paths of files that modified on disk or are not in the
-    cache. The other contains paths to non-modified files.
-    """
-    todo, done = set(), set()
-    for src in sources:
-        res_src = src.resolve()
-        if cache.get(str(res_src)) != get_cache_info(res_src):
-            todo.add(src)
-        else:
-            done.add(src)
-    return todo, done
-
-
-def write_cache(cache: Cache, sources: Iterable[Path], mode: Mode) -> None:
-    """Update the cache file."""
-    cache_file = get_cache_file(mode)
-    try:
-        CACHE_DIR.mkdir(parents=True, exist_ok=True)
-        new_cache = {
-            **cache,
-            **{str(src.resolve()): get_cache_info(src) for src in sources},
-        }
-        with tempfile.NamedTemporaryFile(dir=str(cache_file.parent), delete=False) as f:
-            pickle.dump(new_cache, f, protocol=4)
-        os.replace(f.name, cache_file)
-    except OSError:
-        pass
-
-
-def patch_click() -> None:
-    """Make Click not crash.
-
-    On certain misconfigured environments, Python 3 selects the ASCII encoding as the
-    default which restricts paths that it can access during the lifetime of the
-    application.  Click refuses to work in this scenario by raising a RuntimeError.
-
-    In case of Black the likelihood that non-ASCII characters are going to be used in
-    file paths is minimal since it's Python source code.  Moreover, this crash was
-    spurious on Python 3.7 thanks to PEP 538 and PEP 540.
-    """
-    try:
-        from click import core
-        from click import _unicodefun  # type: ignore
-    except ModuleNotFoundError:
-        return
-
-    for module in (core, _unicodefun):
-        if hasattr(module, "_verify_python3_env"):
-            module._verify_python3_env = lambda: None
-
-

 def patched_main() -> None:
 def patched_main() -> None:

-    freeze_support()
-    patch_click()
-    main()
-
-
-def is_docstring(leaf: Leaf) -> bool:
-    if not is_multiline_string(leaf):
-        # For the purposes of docstring re-indentation, we don't need to do anything
-        # with single-line docstrings.
-        return False
-
-    if prev_siblings_are(
-        leaf.parent, [None, token.NEWLINE, token.INDENT, syms.simple_stmt]
-    ):
-        return True
-
-    # Multiline docstring on the same line as the `def`.
-    if prev_siblings_are(leaf.parent, [syms.parameters, token.COLON, syms.simple_stmt]):
-        # `syms.parameters` is only used in funcdefs and async_funcdefs in the Python
-        # grammar. We're safe to return True without further checks.
-        return True
-
-    return False
-
+    # PyInstaller patches multiprocessing to need freeze_support() even in non-Windows
+    # environments so just assume we always need to call it if frozen.
+    if getattr(sys, "frozen", False):
+        from multiprocessing import freeze_support

 
 

-def lines_with_leading_tabs_expanded(s: str) -> List[str]:
-    """
-    Splits string into lines and expands only leading tabs (following the normal
-    Python rules)
-    """
-    lines = []
-    for line in s.splitlines():
-        # Find the index of the first non-whitespace character after a string of
-        # whitespace that includes at least one tab
-        match = re.match(r"\s*\t+\s*(\S)", line)
-        if match:
-            first_non_whitespace_idx = match.start(1)
-
-            lines.append(
-                line[:first_non_whitespace_idx].expandtabs()
-                + line[first_non_whitespace_idx:]
-            )
-        else:
-            lines.append(line)
-    return lines
+        freeze_support()

 
 

-
-def fix_docstring(docstring: str, prefix: str) -> str:
-    # https://www.python.org/dev/peps/pep-0257/#handling-docstring-indentation
-    if not docstring:
-        return ""
-    lines = lines_with_leading_tabs_expanded(docstring)
-    # Determine minimum indentation (first line doesn't count):
-    indent = sys.maxsize
-    for line in lines[1:]:
-        stripped = line.lstrip()
-        if stripped:
-            indent = min(indent, len(line) - len(stripped))
-    # Remove indentation (first line is special):
-    trimmed = [lines[0].strip()]
-    if indent < sys.maxsize:
-        last_line_idx = len(lines) - 2
-        for i, line in enumerate(lines[1:]):
-            stripped_line = line[indent:].rstrip()
-            if stripped_line or i == last_line_idx:
-                trimmed.append(prefix + stripped_line)
-            else:
-                trimmed.append("")
-    return "\n".join(trimmed)
+    main()

 
 
 if __name__ == "__main__":
 
 
 if __name__ == "__main__":