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permits black to run in AWS Lambda: (#1141)
[etc/vim.git] / black.py
index 46c64a317935952e956a982321afb35d1d3d1dfd..fc1597a44342d751fcd6f165ca3c7a59abddd3ce 100644 (file)
--- a/black.py
+++ b/black.py
@@ -1,23 +1,31 @@
+import ast
 import asyncio
-import pickle
-from asyncio.base_events import BaseEventLoop
+from abc import ABC, abstractmethod
+from collections import defaultdict
 from concurrent.futures import Executor, ProcessPoolExecutor
+from contextlib import contextmanager
+from datetime import datetime
 from enum import Enum
-from functools import partial, wraps
-import keyword
+from functools import lru_cache, partial, wraps
+import io
+import itertools
 import logging
-from multiprocessing import Manager
+from multiprocessing import Manager, freeze_support
 import os
 from pathlib import Path
-import re
-import tokenize
+import pickle
+import regex as re
 import signal
 import sys
+import tempfile
+import tokenize
+import traceback
 from typing import (
     Any,
     Callable,
     Collection,
     Dict,
+    Generator,
     Generic,
     Iterable,
     Iterator,
@@ -31,34 +39,51 @@ from typing import (
     TypeVar,
     Union,
     cast,
+    TYPE_CHECKING,
 )
+from typing_extensions import Final
+from mypy_extensions import mypyc_attr
 
 from appdirs import user_cache_dir
-from attr import dataclass, Factory
+from dataclasses import dataclass, field, replace
 import click
+import toml
+from typed_ast import ast3, ast27
+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 _black_version import version as __version__
+
+if TYPE_CHECKING:
+    import colorama  # noqa: F401
 
-__version__ = "18.5b0"
 DEFAULT_LINE_LENGTH = 88
+DEFAULT_EXCLUDES = r"/(\.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__))
 
+STRING_PREFIX_CHARS: Final = "furbFURB"  # All possible string prefix characters.
+
 
 # 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]
-SplitFunc = Callable[["Line", bool], Iterator["Line"]]
+Transformer = Callable[["Line", Collection["Feature"]], Iterator["Line"]]
 Timestamp = float
 FileSize = int
 CacheInfo = Tuple[Timestamp, FileSize]
@@ -71,49 +96,67 @@ syms = pygram.python_symbols
 
 
 class NothingChanged(UserWarning):
-    """Raised by :func:`format_file` when reformatted code is the same as source."""
+    """Raised when reformatted code is the same as source."""
 
 
-class CannotSplit(Exception):
-    """A readable split that fits the allotted line length is impossible.
+class CannotTransform(Exception):
+    """Base class for errors raised by Transformers."""
 
-    Raised by :func:`left_hand_split`, :func:`right_hand_split`, and
-    :func:`delimiter_split`.
-    """
 
+class CannotSplit(CannotTransform):
+    """A readable split that fits the allotted line length is impossible."""
 
-class FormatError(Exception):
-    """Base exception for `# fmt: on` and `# fmt: off` handling.
 
-    It holds the number of bytes of the prefix consumed before the format
-    control comment appeared.
-    """
+class InvalidInput(ValueError):
+    """Raised when input source code fails all parse attempts."""
 
-    def __init__(self, consumed: int) -> None:
-        super().__init__(consumed)
-        self.consumed = consumed
 
-    def trim_prefix(self, leaf: Leaf) -> None:
-        leaf.prefix = leaf.prefix[self.consumed :]
+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
 
-    def leaf_from_consumed(self, leaf: Leaf) -> Leaf:
-        """Returns a new Leaf from the consumed part of the prefix."""
-        unformatted_prefix = leaf.prefix[: self.consumed]
-        return Leaf(token.NEWLINE, unformatted_prefix)
 
+class Err(Generic[E]):
+    def __init__(self, e: E) -> None:
+        self._e = e
 
-class FormatOn(FormatError):
-    """Found a comment like `# fmt: on` in the file."""
+    def err(self) -> E:
+        return self._e
 
 
-class FormatOff(FormatError):
-    """Found a comment like `# fmt: off` in the file."""
+# 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):
     NO = 0
     YES = 1
     DIFF = 2
+    CHECK = 3
+    COLOR_DIFF = 4
+
+    @classmethod
+    def from_configuration(
+        cls, *, check: bool, diff: bool, color: bool = False
+    ) -> "WriteBack":
+        if check and not diff:
+            return cls.CHECK
+
+        if diff and color:
+            return cls.COLOR_DIFF
+
+        return cls.DIFF if diff else cls.YES
 
 
 class Changed(Enum):
@@ -122,22 +165,224 @@ class Changed(Enum):
     YES = 2
 
 
-@click.command()
+class TargetVersion(Enum):
+    PY27 = 2
+    PY33 = 3
+    PY34 = 4
+    PY35 = 5
+    PY36 = 6
+    PY37 = 7
+    PY38 = 8
+
+    def is_python2(self) -> bool:
+        return self is TargetVersion.PY27
+
+
+PY36_VERSIONS = {TargetVersion.PY36, TargetVersion.PY37, TargetVersion.PY38}
+
+
+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
+
+
+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,
+    },
+}
+
+
+@dataclass
+class Mode:
+    target_versions: Set[TargetVersion] = field(default_factory=set)
+    line_length: int = DEFAULT_LINE_LENGTH
+    string_normalization: bool = True
+    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
+
+
+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: 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"
+    return str(path_pyproject_toml) if path_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]:
+    """Inject Black configuration from "pyproject.toml" into defaults in `ctx`.
+
+    Returns the path to a successfully found and read configuration file, None
+    otherwise.
+    """
+    if not value:
+        value = find_pyproject_toml(ctx.params.get("src", ()))
+        if value is None:
+            return None
+
+    try:
+        config = parse_pyproject_toml(value)
+    except (toml.TomlDecodeError, OSError) as e:
+        raise click.FileError(
+            filename=value, hint=f"Error reading configuration file: {e}"
+        )
+
+    if not config:
+        return None
+
+    target_version = config.get("target_version")
+    if target_version is not None and not isinstance(target_version, list):
+        raise click.BadOptionUsage(
+            "target-version", f"Config key target-version must be a list"
+        )
+
+    default_map: Dict[str, Any] = {}
+    if ctx.default_map:
+        default_map.update(ctx.default_map)
+    default_map.update(config)
+
+    ctx.default_map = default_map
+    return value
+
+
+def target_version_option_callback(
+    c: click.Context, p: Union[click.Option, click.Parameter], v: Tuple[str, ...]
+) -> List[TargetVersion]:
+    """Compute the target versions from a --target-version flag.
+
+    This is its own function because mypy couldn't infer the type correctly
+    when it was a lambda, causing mypyc trouble.
+    """
+    return [TargetVersion[val.upper()] for val in v]
+
+
+@click.command(context_settings=dict(help_option_names=["-h", "--help"]))
+@click.option("-c", "--code", type=str, help="Format the code passed in as a string.")
 @click.option(
     "-l",
     "--line-length",
     type=int,
     default=DEFAULT_LINE_LENGTH,
-    help="How many character per line to allow.",
+    help="How many characters per line to allow.",
     show_default=True,
 )
+@click.option(
+    "-t",
+    "--target-version",
+    type=click.Choice([v.name.lower() for v in TargetVersion]),
+    callback=target_version_option_callback,
+    multiple=True,
+    help=(
+        "Python versions that should be supported by Black's output. [default: per-file"
+        " auto-detection]"
+    ),
+)
+@click.option(
+    "--py36",
+    is_flag=True,
+    help=(
+        "Allow using Python 3.6-only syntax on all input files.  This will put trailing"
+        " commas in function signatures and calls also after *args and **kwargs."
+        " Deprecated; use --target-version instead. [default: per-file auto-detection]"
+    ),
+)
+@click.option(
+    "--pyi",
+    is_flag=True,
+    help=(
+        "Format all input files like typing stubs regardless of file extension (useful"
+        " when piping source on standard input)."
+    ),
+)
+@click.option(
+    "-S",
+    "--skip-string-normalization",
+    is_flag=True,
+    help="Don't normalize string quotes or prefixes.",
+)
 @click.option(
     "--check",
     is_flag=True,
     help=(
-        "Don't write the files back, just return the status.  Return code 0 "
-        "means nothing would change.  Return code 1 means some files would be "
-        "reformatted.  Return code 123 means there was an internal error."
+        "Don't write the files back, just return the status.  Return code 0 means"
+        " nothing would change.  Return code 1 means some files would be reformatted."
+        " Return code 123 means there was an internal error."
     ),
 )
 @click.option(
@@ -145,18 +390,56 @@ class Changed(Enum):
     is_flag=True,
     help="Don't write the files back, just output a diff for each file on stdout.",
 )
+@click.option(
+    "--color/--no-color",
+    is_flag=True,
+    help="Show colored diff. Only applies when `--diff` is given.",
+)
 @click.option(
     "--fast/--safe",
     is_flag=True,
     help="If --fast given, skip temporary sanity checks. [default: --safe]",
 )
+@click.option(
+    "--include",
+    type=str,
+    default=DEFAULT_INCLUDES,
+    help=(
+        "A regular expression that matches files and directories that should be"
+        " included on recursive searches.  An empty value means all files are included"
+        " regardless of the name.  Use forward slashes for directories on all platforms"
+        " (Windows, too).  Exclusions are calculated first, inclusions later."
+    ),
+    show_default=True,
+)
+@click.option(
+    "--exclude",
+    type=str,
+    default=DEFAULT_EXCLUDES,
+    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."
+    ),
+    show_default=True,
+)
 @click.option(
     "-q",
     "--quiet",
     is_flag=True,
     help=(
-        "Don't emit non-error messages to stderr. Errors are still emitted, "
-        "silence those with 2>/dev/null."
+        "Don't emit non-error messages to stderr. Errors are still emitted; silence"
+        " those with 2>/dev/null."
+    ),
+)
+@click.option(
+    "-v",
+    "--verbose",
+    is_flag=True,
+    help=(
+        "Also emit messages to stderr about files that were not changed or were ignored"
+        " due to --exclude=."
     ),
 )
 @click.version_option(version=__version__)
@@ -166,288 +449,550 @@ class Changed(Enum):
     type=click.Path(
         exists=True, file_okay=True, dir_okay=True, readable=True, allow_dash=True
     ),
+    is_eager=True,
+)
+@click.option(
+    "--config",
+    type=click.Path(
+        exists=True,
+        file_okay=True,
+        dir_okay=False,
+        readable=True,
+        allow_dash=False,
+        path_type=str,
+    ),
+    is_eager=True,
+    callback=read_pyproject_toml,
+    help="Read configuration from PATH.",
 )
 @click.pass_context
 def main(
     ctx: click.Context,
+    code: Optional[str],
     line_length: int,
+    target_version: List[TargetVersion],
     check: bool,
     diff: bool,
+    color: bool,
     fast: bool,
+    pyi: bool,
+    py36: bool,
+    skip_string_normalization: bool,
     quiet: bool,
-    src: List[str],
+    verbose: bool,
+    include: str,
+    exclude: str,
+    src: Tuple[str, ...],
+    config: Optional[str],
 ) -> None:
     """The uncompromising code formatter."""
-    sources: List[Path] = []
+    write_back = WriteBack.from_configuration(check=check, diff=diff, color=color)
+    if target_version:
+        if py36:
+            err("Cannot use both --target-version and --py36")
+            ctx.exit(2)
+        else:
+            versions = set(target_version)
+    elif py36:
+        err(
+            "--py36 is deprecated and will be removed in a future version. Use"
+            " --target-version py36 instead."
+        )
+        versions = PY36_VERSIONS
+    else:
+        # We'll autodetect later.
+        versions = set()
+    mode = Mode(
+        target_versions=versions,
+        line_length=line_length,
+        is_pyi=pyi,
+        string_normalization=not skip_string_normalization,
+    )
+    if config and verbose:
+        out(f"Using configuration from {config}.", bold=False, fg="blue")
+    if code is not None:
+        print(format_str(code, mode=mode))
+        ctx.exit(0)
+    try:
+        include_regex = re_compile_maybe_verbose(include)
+    except re.error:
+        err(f"Invalid regular expression for include given: {include!r}")
+        ctx.exit(2)
+    try:
+        exclude_regex = re_compile_maybe_verbose(exclude)
+    except re.error:
+        err(f"Invalid regular expression for exclude given: {exclude!r}")
+        ctx.exit(2)
+    report = Report(check=check, diff=diff, quiet=quiet, verbose=verbose)
+    root = find_project_root(src)
+    sources: Set[Path] = set()
+    path_empty(src, quiet, verbose, ctx)
     for s in src:
         p = Path(s)
         if p.is_dir():
-            sources.extend(gen_python_files_in_dir(p))
-        elif p.is_file():
+            sources.update(
+                gen_python_files_in_dir(
+                    p, root, include_regex, exclude_regex, report, get_gitignore(root)
+                )
+            )
+        elif p.is_file() or s == "-":
             # if a file was explicitly given, we don't care about its extension
-            sources.append(p)
-        elif s == "-":
-            sources.append(Path("-"))
+            sources.add(p)
         else:
             err(f"invalid path: {s}")
-
-    if check and not diff:
-        write_back = WriteBack.NO
-    elif diff:
-        write_back = WriteBack.DIFF
-    else:
-        write_back = WriteBack.YES
-    report = Report(check=check, quiet=quiet)
     if len(sources) == 0:
-        out("No paths given. Nothing to do 😴")
+        if verbose or not quiet:
+            out("No Python files are present to be formatted. Nothing to do 😴")
         ctx.exit(0)
-        return
 
-    elif len(sources) == 1:
-        reformat_one(sources[0], line_length, fast, write_back, report)
+    if len(sources) == 1:
+        reformat_one(
+            src=sources.pop(),
+            fast=fast,
+            write_back=write_back,
+            mode=mode,
+            report=report,
+        )
     else:
-        loop = asyncio.get_event_loop()
-        executor = ProcessPoolExecutor(max_workers=os.cpu_count())
-        try:
-            loop.run_until_complete(
-                schedule_formatting(
-                    sources, line_length, fast, write_back, report, loop, executor
-                )
-            )
-        finally:
-            shutdown(loop)
-        if not quiet:
-            out("All done! ✨ 🍰 ✨")
-            click.echo(str(report))
+        reformat_many(
+            sources=sources, fast=fast, write_back=write_back, mode=mode, report=report
+        )
+
+    if verbose or not quiet:
+        out("Oh no! 💥 💔 💥" if report.return_code else "All done! ✨ 🍰 ✨")
+        click.secho(str(report), err=True)
     ctx.exit(report.return_code)
 
 
+def path_empty(
+    src: Tuple[str, ...], quiet: bool, verbose: bool, ctx: click.Context
+) -> None:
+    """
+    Exit if there is no `src` provided for formatting
+    """
+    if not src:
+        if verbose or not quiet:
+            out("No Path provided. Nothing to do 😴")
+            ctx.exit(0)
+
+
 def reformat_one(
-    src: Path, line_length: int, fast: bool, write_back: WriteBack, report: "Report"
+    src: Path, fast: bool, write_back: WriteBack, mode: Mode, report: "Report"
 ) -> None:
     """Reformat a single file under `src` without spawning child processes.
 
-    If `quiet` is True, non-error messages are not output. `line_length`,
-    `write_back`, and `fast` options are passed to :func:`format_file_in_place`.
+    `fast`, `write_back`, and `mode` options are passed to
+    :func:`format_file_in_place` or :func:`format_stdin_to_stdout`.
     """
     try:
         changed = Changed.NO
         if not src.is_file() and str(src) == "-":
-            if format_stdin_to_stdout(
-                line_length=line_length, fast=fast, write_back=write_back
-            ):
+            if format_stdin_to_stdout(fast=fast, write_back=write_back, mode=mode):
                 changed = Changed.YES
         else:
             cache: Cache = {}
             if write_back != WriteBack.DIFF:
-                cache = read_cache(line_length)
-                src = src.resolve()
-                if src in cache and cache[src] == get_cache_info(src):
+                cache = read_cache(mode)
+                res_src = src.resolve()
+                if res_src in cache and cache[res_src] == get_cache_info(res_src):
                     changed = Changed.CACHED
             if changed is not Changed.CACHED and format_file_in_place(
-                src, line_length=line_length, fast=fast, write_back=write_back
+                src, fast=fast, write_back=write_back, mode=mode
             ):
                 changed = Changed.YES
-            if write_back == WriteBack.YES and changed is not Changed.NO:
-                write_cache(cache, [src], line_length)
+            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)
         report.done(src, changed)
     except Exception as 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."""
+    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, 61)
+    try:
+        executor = ProcessPoolExecutor(max_workers=worker_count)
+    except OSError:
+        # we arrive here if the underlying system does not support multi-processing
+        # like in AWS Lambda, in which case we gracefully fallback to the default
+        # mono-process Executor by using None
+        executor = None
+
+    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: List[Path],
-    line_length: int,
+    sources: Set[Path],
     fast: bool,
     write_back: WriteBack,
+    mode: Mode,
     report: "Report",
-    loop: BaseEventLoop,
-    executor: Executor,
+    loop: asyncio.AbstractEventLoop,
+    executor: Optional[Executor],
 ) -> None:
     """Run formatting of `sources` in parallel using the provided `executor`.
 
     (Use ProcessPoolExecutors for actual parallelism.)
 
-    `line_length`, `write_back`, and `fast` options are passed to
+    `write_back`, `fast`, and `mode` options are passed to
     :func:`format_file_in_place`.
     """
     cache: Cache = {}
     if write_back != WriteBack.DIFF:
-        cache = read_cache(line_length)
+        cache = read_cache(mode)
         sources, cached = filter_cached(cache, sources)
-        for src in cached:
+        for src in sorted(cached):
             report.done(src, Changed.CACHED)
+    if not sources:
+        return
+
     cancelled = []
-    formatted = []
-    if sources:
-        lock = None
-        if write_back == WriteBack.DIFF:
-            # For diff output, we need locks to ensure we don't interleave output
-            # from different processes.
-            manager = Manager()
-            lock = manager.Lock()
-        tasks = {
+    sources_to_cache = []
+    lock = None
+    if write_back == WriteBack.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, line_length, fast, write_back, lock
-            ): src
-            for src in sorted(sources)
-        }
-        pending: Iterable[asyncio.Task] = 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:
-                    formatted.append(src)
-                    report.done(src, Changed.YES if task.result() else Changed.NO)
+                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 write_back == WriteBack.YES and formatted:
-        write_cache(cache, formatted, line_length)
+    if sources_to_cache:
+        write_cache(cache, sources_to_cache, mode)
 
 
 def format_file_in_place(
     src: Path,
-    line_length: int,
     fast: bool,
+    mode: Mode,
     write_back: WriteBack = WriteBack.NO,
     lock: Any = None,  # multiprocessing.Manager().Lock() is some crazy proxy
 ) -> bool:
     """Format file under `src` path. Return True if changed.
 
-    If `write_back` is True, write reformatted code back to stdout.
-    `line_length` and `fast` options are passed to :func:`format_file_contents`.
+    If `write_back` is DIFF, write a diff to stdout. If it is YES, write reformatted
+    code to the file.
+    `mode` and `fast` options are passed to :func:`format_file_contents`.
     """
-    is_pyi = src.suffix == ".pyi"
+    if src.suffix == ".pyi":
+        mode = replace(mode, is_pyi=True)
 
-    with tokenize.open(src) as src_buffer:
-        src_contents = src_buffer.read()
+    then = datetime.utcfromtimestamp(src.stat().st_mtime)
+    with open(src, "rb") as buf:
+        src_contents, encoding, newline = decode_bytes(buf.read())
     try:
-        dst_contents = format_file_contents(
-            src_contents, line_length=line_length, fast=fast, is_pyi=is_pyi
-        )
+        dst_contents = format_file_contents(src_contents, fast=fast, mode=mode)
     except NothingChanged:
         return False
 
-    if write_back == write_back.YES:
-        with open(src, "w", encoding=src_buffer.encoding) as f:
+    if write_back == WriteBack.YES:
+        with open(src, "w", encoding=encoding, newline=newline) as f:
             f.write(dst_contents)
-    elif write_back == write_back.DIFF:
-        src_name = f"{src}  (original)"
-        dst_name = f"{src}  (formatted)"
+    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)
-        if lock:
-            lock.acquire()
-        try:
-            sys.stdout.write(diff_contents)
-        finally:
-            if lock:
-                lock.release()
+
+        if write_back == write_back.COLOR_DIFF:
+            diff_contents = color_diff(diff_contents)
+
+        with lock or nullcontext():
+            f = io.TextIOWrapper(
+                sys.stdout.buffer,
+                encoding=encoding,
+                newline=newline,
+                write_through=True,
+            )
+            f = wrap_stream_for_windows(f)
+            f.write(diff_contents)
+            f.detach()
+
     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
+        if line.startswith("@@"):
+            line = "\033[36m" + line + "\033[0m"  # cyan, reset
+        if 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.AnsiToWin32"]:
+    """
+    Wrap the stream in colorama's wrap_stream so colors are shown on Windows.
+
+    If `colorama` is not found, then no change is made. If `colorama` does
+    exist, then it handles the logic to determine whether or not to change
+    things.
+    """
+    try:
+        from colorama import initialise
+
+        # We set `strip=False` so that we can don't have to modify
+        # test_express_diff_with_color.
+        f = initialise.wrap_stream(
+            f, convert=None, strip=False, autoreset=False, wrap=True
+        )
+
+        # wrap_stream returns a `colorama.AnsiToWin32.AnsiToWin32` object
+        # which does not have a `detach()` method. So we fake one.
+        f.detach = lambda *args, **kwargs: None  # type: ignore
+    except ImportError:
+        pass
+
+    return f
+
+
 def format_stdin_to_stdout(
-    line_length: int, fast: bool, write_back: WriteBack = WriteBack.NO
+    fast: bool, *, write_back: WriteBack = WriteBack.NO, mode: Mode
 ) -> bool:
     """Format file on stdin. Return True if changed.
 
-    If `write_back` is True, write reformatted code back to stdout.
-    `line_length` and `fast` arguments are 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`.
     """
-    src = sys.stdin.read()
+    then = datetime.utcnow()
+    src, encoding, newline = decode_bytes(sys.stdin.buffer.read())
     dst = src
     try:
-        dst = format_file_contents(src, line_length=line_length, fast=fast)
+        dst = format_file_contents(src, fast=fast, mode=mode)
         return True
 
     except NothingChanged:
         return False
 
     finally:
+        f = io.TextIOWrapper(
+            sys.stdout.buffer, encoding=encoding, newline=newline, write_through=True
+        )
         if write_back == WriteBack.YES:
-            sys.stdout.write(dst)
-        elif write_back == WriteBack.DIFF:
-            src_name = "<stdin>  (original)"
-            dst_name = "<stdin>  (formatted)"
-            sys.stdout.write(diff(src, dst, src_name, dst_name))
-
-
-def format_file_contents(
-    src_contents: str, *, line_length: int, fast: bool, is_pyi: bool = False
-) -> FileContent:
+            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"
+            d = diff(src, dst, src_name, dst_name)
+            if write_back == WriteBack.COLOR_DIFF:
+                d = color_diff(d)
+                f = wrap_stream_for_windows(f)
+            f.write(d)
+        f.detach()
+
+
+def format_file_contents(src_contents: str, *, fast: bool, mode: Mode) -> FileContent:
     """Reformat contents a file and return new contents.
 
     If `fast` is False, additionally confirm that the reformatted code is
     valid by calling :func:`assert_equivalent` and :func:`assert_stable` on it.
-    `line_length` is passed to :func:`format_str`.
+    `mode` is passed to :func:`format_str`.
     """
     if src_contents.strip() == "":
         raise NothingChanged
 
-    dst_contents = format_str(src_contents, line_length=line_length, is_pyi=is_pyi)
+    dst_contents = format_str(src_contents, mode=mode)
     if src_contents == dst_contents:
         raise NothingChanged
 
     if not fast:
         assert_equivalent(src_contents, dst_contents)
-        assert_stable(
-            src_contents, dst_contents, line_length=line_length, is_pyi=is_pyi
-        )
+        assert_stable(src_contents, dst_contents, mode=mode)
     return dst_contents
 
 
-def format_str(
-    src_contents: str, line_length: int, *, is_pyi: bool = False
-) -> FileContent:
+def format_str(src_contents: str, *, mode: Mode) -> FileContent:
     """Reformat a string and return new contents.
 
-    `line_length` determines how many characters per line are allowed.
+    `mode` determines formatting options, such as how many characters per line are
+    allowed.  Example:
+
+    >>> import black
+    >>> print(black.format_str("def f(arg:str='')->None:...", mode=Mode()))
+    def f(arg: str = "") -> None:
+        ...
+
+    A more complex example:
+    >>> print(
+    ...   black.format_str(
+    ...     "def f(arg:str='')->None: hey",
+    ...     mode=black.Mode(
+    ...       target_versions={black.TargetVersion.PY36},
+    ...       line_length=10,
+    ...       string_normalization=False,
+    ...       is_pyi=False,
+    ...     ),
+    ...   ),
+    ... )
+    def f(
+        arg: str = '',
+    ) -> None:
+        hey
+
     """
-    src_node = lib2to3_parse(src_contents)
-    dst_contents = ""
+    src_node = lib2to3_parse(src_contents.lstrip(), mode.target_versions)
+    dst_contents = []
     future_imports = get_future_imports(src_node)
-    elt = EmptyLineTracker(is_pyi=is_pyi)
-    py36 = is_python36(src_node)
+    if mode.target_versions:
+        versions = mode.target_versions
+    else:
+        versions = detect_target_versions(src_node)
+    normalize_fmt_off(src_node)
     lines = LineGenerator(
-        remove_u_prefix=py36 or "unicode_literals" in future_imports, is_pyi=is_pyi
+        remove_u_prefix="unicode_literals" in future_imports
+        or supports_feature(versions, Feature.UNICODE_LITERALS),
+        is_pyi=mode.is_pyi,
+        normalize_strings=mode.string_normalization,
     )
+    elt = EmptyLineTracker(is_pyi=mode.is_pyi)
     empty_line = Line()
     after = 0
+    split_line_features = {
+        feature
+        for feature in {Feature.TRAILING_COMMA_IN_CALL, Feature.TRAILING_COMMA_IN_DEF}
+        if supports_feature(versions, feature)
+    }
     for current_line in lines.visit(src_node):
-        for _ in range(after):
-            dst_contents += str(empty_line)
+        dst_contents.append(str(empty_line) * after)
         before, after = elt.maybe_empty_lines(current_line)
-        for _ in range(before):
-            dst_contents += str(empty_line)
-        for line in split_line(current_line, line_length=line_length, py36=py36):
-            dst_contents += str(line)
-    return dst_contents
+        dst_contents.append(str(empty_line) * before)
+        for line in transform_line(
+            current_line,
+            line_length=mode.line_length,
+            normalize_strings=mode.string_normalization,
+            features=split_line_features,
+        ):
+            dst_contents.append(str(line))
+    return "".join(dst_contents)
+
+
+def decode_bytes(src: bytes) -> Tuple[FileContent, Encoding, NewLine]:
+    """Return a tuple of (decoded_contents, encoding, newline).
+
+    `newline` is either CRLF or LF but `decoded_contents` is decoded with
+    universal newlines (i.e. only contains LF).
+    """
+    srcbuf = io.BytesIO(src)
+    encoding, lines = tokenize.detect_encoding(srcbuf.readline)
+    if not lines:
+        return "", encoding, "\n"
+
+    newline = "\r\n" if b"\r\n" == lines[0][-2:] else "\n"
+    srcbuf.seek(0)
+    with io.TextIOWrapper(srcbuf, encoding) as tiow:
+        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,
+        ]
 
-GRAMMARS = [
-    pygram.python_grammar_no_print_statement_no_exec_statement,
-    pygram.python_grammar_no_print_statement,
-    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) -> Node:
+def lib2to3_parse(src_txt: str, target_versions: Iterable[TargetVersion] = ()) -> Node:
     """Given a string with source, return the lib2to3 Node."""
-    grammar = pygram.python_grammar_no_print_statement
-    if src_txt[-1] != "\n":
-        nl = "\r\n" if "\r\n" in src_txt[:1024] else "\n"
-        src_txt += nl
-    for grammar in GRAMMARS:
+    if src_txt[-1:] != "\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)
@@ -460,7 +1005,7 @@ def lib2to3_parse(src_txt: str) -> Node:
                 faulty_line = lines[lineno - 1]
             except IndexError:
                 faulty_line = "<line number missing in source>"
-            exc = ValueError(f"Cannot parse: {lineno}:{column}: {faulty_line}")
+            exc = InvalidInput(f"Cannot parse: {lineno}:{column}: {faulty_line}")
     else:
         raise exc from None
 
@@ -475,9 +1020,6 @@ def lib2to3_unparse(node: Node) -> str:
     return code
 
 
-T = TypeVar("T")
-
-
 class Visitor(Generic[T]):
     """Basic lib2to3 visitor that yields things of type `T` on `visit()`."""
 
@@ -494,8 +1036,16 @@ class Visitor(Generic[T]):
         if node.type < 256:
             name = token.tok_name[node.type]
         else:
-            name = type_repr(node.type)
-        yield from getattr(self, f"visit_{name}", self.visit_default)(node)
+            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`."""
@@ -529,19 +1079,19 @@ class DebugVisitor(Visitor[T]):
             out(f" {node.value!r}", fg="blue", bold=False)
 
     @classmethod
-    def show(cls, code: str) -> None:
+    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()
-        list(v.visit(lib2to3_parse(code)))
+        if isinstance(code, str):
+            code = lib2to3_parse(code)
+        list(v.visit(code))
 
 
-KEYWORDS = set(keyword.kwlist)
-WHITESPACE = {token.DEDENT, token.INDENT, token.NEWLINE}
-FLOW_CONTROL = {"return", "raise", "break", "continue"}
-STATEMENT = {
+WHITESPACE: Final = {token.DEDENT, token.INDENT, token.NEWLINE}
+STATEMENT: Final = {
     syms.if_stmt,
     syms.while_stmt,
     syms.for_stmt,
@@ -551,9 +1101,10 @@ STATEMENT = {
     syms.funcdef,
     syms.classdef,
 }
-STANDALONE_COMMENT = 153
-LOGIC_OPERATORS = {"and", "or"}
-COMPARATORS = {
+STANDALONE_COMMENT: Final = 153
+token.tok_name[STANDALONE_COMMENT] = "STANDALONE_COMMENT"
+LOGIC_OPERATORS: Final = {"and", "or"}
+COMPARATORS: Final = {
     token.LESS,
     token.GREATER,
     token.EQEQUAL,
@@ -561,7 +1112,7 @@ COMPARATORS = {
     token.LESSEQUAL,
     token.GREATEREQUAL,
 }
-MATH_OPERATORS = {
+MATH_OPERATORS: Final = {
     token.VBAR,
     token.CIRCUMFLEX,
     token.AMPER,
@@ -577,21 +1128,23 @@ MATH_OPERATORS = {
     token.TILDE,
     token.DOUBLESTAR,
 }
-STARS = {token.STAR, token.DOUBLESTAR}
-VARARGS_PARENTS = {
+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 = {
+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 = {
+TEST_DESCENDANTS: Final = {
     syms.test,
     syms.lambdef,
     syms.or_test,
@@ -608,7 +1161,7 @@ TEST_DESCENDANTS = {
     syms.term,
     syms.power,
 }
-ASSIGNMENTS = {
+ASSIGNMENTS: Final = {
     "=",
     "+=",
     "-=",
@@ -624,13 +1177,13 @@ ASSIGNMENTS = {
     "**=",
     "//=",
 }
-COMPREHENSION_PRIORITY = 20
-COMMA_PRIORITY = 18
-TERNARY_PRIORITY = 16
-LOGIC_PRIORITY = 14
-STRING_PRIORITY = 12
-COMPARATOR_PRIORITY = 10
-MATH_PRIORITIES = {
+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,
@@ -646,7 +1199,7 @@ MATH_PRIORITIES = {
     token.TILDE: 3,
     token.DOUBLESTAR: 2,
 }
-DOT_PRIORITY = 1
+DOT_PRIORITY: Final = 1
 
 
 @dataclass
@@ -654,11 +1207,11 @@ class BracketTracker:
     """Keeps track of brackets on a line."""
 
     depth: int = 0
-    bracket_match: Dict[Tuple[Depth, NodeType], Leaf] = Factory(dict)
-    delimiters: Dict[LeafID, Priority] = Factory(dict)
+    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_variable: int = 0
-    _lambda_arguments: int = 0
+    _for_loop_depths: List[int] = field(default_factory=list)
+    _lambda_argument_depths: List[int] = field(default_factory=list)
 
     def mark(self, leaf: Leaf) -> None:
         """Mark `leaf` with bracket-related metadata. Keep track of delimiters.
@@ -704,7 +1257,7 @@ class BracketTracker:
         """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] = ()) -> int:
+    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.
@@ -712,7 +1265,7 @@ class BracketTracker:
         """
         return max(v for k, v in self.delimiters.items() if k not in exclude)
 
-    def delimiter_count_with_priority(self, priority: int = 0) -> int:
+    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.
@@ -731,16 +1284,21 @@ class BracketTracker:
         """
         if leaf.type == token.NAME and leaf.value == "for":
             self.depth += 1
-            self._for_loop_variable += 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_variable and leaf.type == token.NAME and leaf.value == "in":
+        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_variable -= 1
+            self._for_loop_depths.pop()
             return True
 
         return False
@@ -753,16 +1311,20 @@ class BracketTracker:
         """
         if leaf.type == token.NAME and leaf.value == "lambda":
             self.depth += 1
-            self._lambda_arguments += 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_arguments and leaf.type == token.COLON:
+        if (
+            self._lambda_argument_depths
+            and self._lambda_argument_depths[-1] == self.depth
+            and leaf.type == token.COLON
+        ):
             self.depth -= 1
-            self._lambda_arguments -= 1
+            self._lambda_argument_depths.pop()
             return True
 
         return False
@@ -777,9 +1339,10 @@ class Line:
     """Holds leaves and comments. Can be printed with `str(line)`."""
 
     depth: int = 0
-    leaves: List[Leaf] = Factory(list)
-    comments: List[Tuple[Index, Leaf]] = Factory(list)
-    bracket_tracker: BracketTracker = Factory(BracketTracker)
+    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_explode: bool = False
 
@@ -859,6 +1422,69 @@ class Line:
             Leaf(token.DOT, ".") for _ in range(3)
         ]
 
+    @property
+    def is_collection_with_optional_trailing_comma(self) -> bool:
+        """Is this line a collection literal with a trailing comma that's optional?
+
+        Note that the trailing comma in a 1-tuple is not optional.
+        """
+        if not self.leaves or len(self.leaves) < 4:
+            return False
+
+        # Look for and address a trailing colon.
+        if self.leaves[-1].type == token.COLON:
+            closer = self.leaves[-2]
+            close_index = -2
+        else:
+            closer = self.leaves[-1]
+            close_index = -1
+        if closer.type not in CLOSING_BRACKETS or self.inside_brackets:
+            return False
+
+        if closer.type == token.RPAR:
+            # Tuples require an extra check, because if there's only
+            # one element in the tuple removing the comma unmakes the
+            # tuple.
+            #
+            # We also check for parens before looking for the trailing
+            # comma because in some cases (eg assigning a dict
+            # literal) the literal gets wrapped in temporary parens
+            # during parsing. This case is covered by the
+            # collections.py test data.
+            opener = closer.opening_bracket
+            for _open_index, leaf in enumerate(self.leaves):
+                if leaf is opener:
+                    break
+
+            else:
+                # Couldn't find the matching opening paren, play it safe.
+                return False
+
+            commas = 0
+            comma_depth = self.leaves[close_index - 1].bracket_depth
+            for leaf in self.leaves[_open_index + 1 : close_index]:
+                if leaf.bracket_depth == comma_depth and leaf.type == token.COMMA:
+                    commas += 1
+            if commas > 1:
+                # We haven't looked yet for the trailing comma because
+                # we might also have caught noop parens.
+                return self.leaves[close_index - 1].type == token.COMMA
+
+            elif commas == 1:
+                return False  # it's either a one-tuple or didn't have a trailing comma
+
+            if self.leaves[close_index - 1].type in CLOSING_BRACKETS:
+                close_index -= 1
+                closer = self.leaves[close_index]
+                if closer.type == token.RPAR:
+                    # TODO: this is a gut feeling. Will we ever see this?
+                    return False
+
+        if self.leaves[close_index - 1].type != token.COMMA:
+            return False
+
+        return True
+
     @property
     def is_def(self) -> bool:
         """Is this a function definition? (Also returns True for async defs.)"""
@@ -878,27 +1504,6 @@ class Line:
             and second_leaf.value == "def"
         )
 
-    @property
-    def is_flow_control(self) -> bool:
-        """Is this line a flow control statement?
-
-        Those are `return`, `raise`, `break`, and `continue`.
-        """
-        return (
-            bool(self)
-            and self.leaves[0].type == token.NAME
-            and self.leaves[0].value in FLOW_CONTROL
-        )
-
-    @property
-    def is_yield(self) -> bool:
-        """Is this line a yield statement?"""
-        return (
-            bool(self)
-            and self.leaves[0].type == token.NAME
-            and self.leaves[0].value == "yield"
-        )
-
     @property
     def is_class_paren_empty(self) -> bool:
         """Is this a class with no base classes but using parentheses?
@@ -915,68 +1520,129 @@ class Line:
             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:
-                if leaf.bracket_depth <= depth_limit:
-                    return True
+            if leaf.type == STANDALONE_COMMENT and leaf.bracket_depth <= depth_limit:
+                return True
 
         return False
 
-    def maybe_remove_trailing_comma(self, closing: Leaf) -> bool:
-        """Remove trailing comma if there is one and it's safe."""
-        if not (
-            self.leaves
-            and self.leaves[-1].type == token.COMMA
-            and closing.type in CLOSING_BRACKETS
-        ):
+    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
 
-        if closing.type == token.RBRACE:
-            self.remove_trailing_comma()
-            return True
+        # 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
 
-        if closing.type == token.RSQB:
-            comma = self.leaves[-1]
-            if comma.parent and comma.parent.type == syms.listmaker:
-                self.remove_trailing_comma()
-                return True
+        return False
 
-        # For parens let's check if it's safe to remove the comma.
-        # Imports are always safe.
-        if self.is_import:
-            self.remove_trailing_comma()
-            return True
+    def contains_unsplittable_type_ignore(self) -> bool:
+        if not self.leaves:
+            return False
 
-        # Otheriwsse, if the trailing one is the only one, we might mistakenly
-        # change a tuple into a different type by removing the comma.
-        depth = closing.bracket_depth + 1
-        commas = 0
-        opening = closing.opening_bracket
-        for _opening_index, leaf in enumerate(self.leaves):
-            if leaf is opening:
-                break
+        # 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((l.lineno for l in self.leaves if l.lineno != 0), 0)
+        last_line = next((l.lineno for l in reversed(self.leaves) if l.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
 
-        else:
-            return False
+        return False
 
-        for leaf in self.leaves[_opening_index + 1 :]:
-            if leaf is closing:
-                break
+    def contains_multiline_strings(self) -> bool:
+        return any(is_multiline_string(leaf) for leaf in self.leaves)
 
-            bracket_depth = leaf.bracket_depth
-            if bracket_depth == depth and leaf.type == token.COMMA:
-                commas += 1
-                if leaf.parent and leaf.parent.type == syms.arglist:
-                    commas += 1
-                    break
+    def maybe_remove_trailing_comma(self, closing: Leaf) -> bool:
+        """Remove trailing comma if there is one and it's safe."""
+        if not (self.leaves and self.leaves[-1].type == token.COMMA):
+            return False
 
-        if commas > 1:
-            self.remove_trailing_comma()
-            return True
+        # We remove trailing commas only in the case of importing a
+        # single name from a module.
+        if not (
+            self.leaves
+            and self.is_import
+            and len(self.leaves) > 4
+            and self.leaves[-1].type == token.COMMA
+            and closing.type in CLOSING_BRACKETS
+            and self.leaves[-4].type == token.NAME
+            and (
+                # regular `from foo import bar,`
+                self.leaves[-4].value == "import"
+                # `from foo import (bar as baz,)
+                or (
+                    len(self.leaves) > 6
+                    and self.leaves[-6].value == "import"
+                    and self.leaves[-3].value == "as"
+                )
+                # `from foo import bar as baz,`
+                or (
+                    len(self.leaves) > 5
+                    and self.leaves[-5].value == "import"
+                    and self.leaves[-3].value == "as"
+                )
+            )
+            and closing.type == token.RPAR
+        ):
+            return False
 
-        return False
+        self.remove_trailing_comma()
+        return True
 
     def append_comment(self, comment: Leaf) -> bool:
         """Add an inline or standalone comment to the line."""
@@ -990,60 +1656,68 @@ class Line:
         if comment.type != token.COMMENT:
             return False
 
-        after = len(self.leaves) - 1
-        if after == -1:
+        if not self.leaves:
             comment.type = STANDALONE_COMMENT
             comment.prefix = ""
             return False
 
-        else:
-            self.comments.append((after, comment))
-            return True
-
-    def comments_after(self, leaf: Leaf, _index: int = -1) -> Iterator[Leaf]:
-        """Generate comments that should appear directly after `leaf`.
-
-        Provide a non-negative leaf `_index` to speed up the function.
-        """
-        if _index == -1:
-            for _index, _leaf in enumerate(self.leaves):
-                if leaf is _leaf:
-                    break
-
-            else:
-                return
+        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
 
-        for index, comment_after in self.comments:
-            if _index == index:
-                yield comment_after
+    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."""
-        comma_index = len(self.leaves) - 1
-        for i in range(len(self.comments)):
-            comment_index, comment = self.comments[i]
-            if comment_index == comma_index:
-                self.comments[i] = (comma_index - 1, comment)
-        self.leaves.pop()
+        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 = (
-            leaf if leaf.type == token.LSQB else self.bracket_tracker.get_open_lsqb()
-        )
+        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)
-            and subscript_start.type == syms.subscriptlist
-        ):
-            subscript_start = child_towards(subscript_start, leaf)
+
+        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(
+            depth=self.depth,
+            inside_brackets=self.inside_brackets,
+            should_explode=self.should_explode,
+        )
+
     def __str__(self) -> str:
         """Render the line."""
         if not self:
@@ -1055,8 +1729,9 @@ class Line:
         res = f"{first.prefix}{indent}{first.value}"
         for leaf in leaves:
             res += str(leaf)
-        for _, comment in self.comments:
+        for comment in itertools.chain.from_iterable(self.comments.values()):
             res += str(comment)
+
         return res + "\n"
 
     def __bool__(self) -> bool:
@@ -1064,55 +1739,6 @@ class Line:
         return bool(self.leaves or self.comments)
 
 
-class UnformattedLines(Line):
-    """Just like :class:`Line` but stores lines which aren't reformatted."""
-
-    def append(self, leaf: Leaf, preformatted: bool = True) -> None:
-        """Just add a new `leaf` to the end of the lines.
-
-        The `preformatted` argument is ignored.
-
-        Keeps track of indentation `depth`, which is useful when the user
-        says `# fmt: on`. Otherwise, doesn't do anything with the `leaf`.
-        """
-        try:
-            list(generate_comments(leaf))
-        except FormatOn as f_on:
-            self.leaves.append(f_on.leaf_from_consumed(leaf))
-            raise
-
-        self.leaves.append(leaf)
-        if leaf.type == token.INDENT:
-            self.depth += 1
-        elif leaf.type == token.DEDENT:
-            self.depth -= 1
-
-    def __str__(self) -> str:
-        """Render unformatted lines from leaves which were added with `append()`.
-
-        `depth` is not used for indentation in this case.
-        """
-        if not self:
-            return "\n"
-
-        res = ""
-        for leaf in self.leaves:
-            res += str(leaf)
-        return res
-
-    def append_comment(self, comment: Leaf) -> bool:
-        """Not implemented in this class. Raises `NotImplementedError`."""
-        raise NotImplementedError("Unformatted lines don't store comments separately.")
-
-    def maybe_remove_trailing_comma(self, closing: Leaf) -> bool:
-        """Does nothing and returns False."""
-        return False
-
-    def maybe_increment_for_loop_variable(self, leaf: Leaf) -> bool:
-        """Does nothing and returns False."""
-        return False
-
-
 @dataclass
 class EmptyLineTracker:
     """Provides a stateful method that returns the number of potential extra
@@ -1122,23 +1748,26 @@ class EmptyLineTracker:
     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] = Factory(list)
+    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), as well as providing an extra empty line
-        after flow control keywords to make them more prominent.
+        lines (two on module-level).
         """
-        if isinstance(current_line, UnformattedLines):
-            return 0, 0
-
         before, after = self._maybe_empty_lines(current_line)
-        before -= self.previous_after
+        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
@@ -1162,39 +1791,8 @@ class EmptyLineTracker:
                 before = 0 if depth else 1
             else:
                 before = 1 if depth else 2
-        is_decorator = current_line.is_decorator
-        if is_decorator or current_line.is_def or current_line.is_class:
-            if not is_decorator:
-                self.previous_defs.append(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:
-                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:
-                        newlines = 0
-                    else:
-                        newlines = 1
-                else:
-                    newlines = 0
-            else:
-                newlines = 2
-            if current_line.depth and newlines:
-                newlines -= 1
-            return newlines, 0
+        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
@@ -1204,8 +1802,59 @@ class EmptyLineTracker:
         ):
             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:
+            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 and not self.previous_line.is_def:
+                # Blank line between a block of functions 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]):
@@ -1214,11 +1863,13 @@ class LineGenerator(Visitor[Line]):
     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.
     """
+
     is_pyi: bool = False
-    current_line: Line = Factory(Line)
+    normalize_strings: bool = True
+    current_line: Line = field(default_factory=Line)
     remove_u_prefix: bool = False
 
-    def line(self, indent: int = 0, type: Type[Line] = Line) -> Iterator[Line]:
+    def line(self, indent: int = 0) -> Iterator[Line]:
         """Generate a line.
 
         If the line is empty, only emit if it makes sense.
@@ -1227,76 +1878,50 @@ class LineGenerator(Visitor[Line]):
         If any lines were generated, set up a new current_line.
         """
         if not self.current_line:
-            if self.current_line.__class__ == type:
-                self.current_line.depth += indent
-            else:
-                self.current_line = type(depth=self.current_line.depth + indent)
+            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 = type(depth=complete_line.depth + indent)
+        self.current_line = Line(depth=complete_line.depth + indent)
         yield complete_line
 
-    def visit(self, node: LN) -> Iterator[Line]:
-        """Main method to visit `node` and its children.
-
-        Yields :class:`Line` objects.
-        """
-        if isinstance(self.current_line, UnformattedLines):
-            # File contained `# fmt: off`
-            yield from self.visit_unformatted(node)
-
-        else:
-            yield from super().visit(node)
-
     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()
-            try:
-                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()
-
-            except FormatOff as f_off:
-                f_off.trim_prefix(node)
-                yield from self.line(type=UnformattedLines)
-                yield from self.visit(node)
-
-            except FormatOn as f_on:
-                # This only happens here if somebody says "fmt: on" multiple
-                # times in a row.
-                f_on.trim_prefix(node)
-                yield from self.visit_default(node)
+            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:
-                normalize_prefix(node, inside_brackets=any_open_brackets)
-                if node.type == token.STRING:
-                    normalize_string_prefix(node, remove_u_prefix=self.remove_u_prefix)
-                    normalize_string_quotes(node)
-                if node.type not in WHITESPACE:
-                    self.current_line.append(node)
+                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.normalize_strings 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: Node) -> Iterator[Line]:
+    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: Node) -> Iterator[Line]:
+    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).
@@ -1375,32 +2000,6 @@ class LineGenerator(Visitor[Line]):
             yield from self.line()
             yield from self.visit(child)
 
-    def visit_import_from(self, node: Node) -> Iterator[Line]:
-        """Visit import_from and maybe put invisible parentheses.
-
-        This is separate from `visit_stmt` because import statements don't
-        support arbitrary atoms and thus handling of parentheses is custom.
-        """
-        check_lpar = False
-        for index, child in enumerate(node.children):
-            if check_lpar:
-                if child.type == token.LPAR:
-                    # make parentheses invisible
-                    child.value = ""  # type: ignore
-                    node.children[-1].value = ""  # type: ignore
-                else:
-                    # insert invisible parentheses
-                    node.insert_child(index, Leaf(token.LPAR, ""))
-                    node.append_child(Leaf(token.RPAR, ""))
-                break
-
-            check_lpar = (
-                child.type == token.NAME and child.value == "import"  # type: ignore
-            )
-
-        for child in node.children:
-            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()
@@ -1410,25 +2009,41 @@ class LineGenerator(Visitor[Line]):
         yield from self.visit_default(leaf)
         yield from self.line()
 
-    def visit_unformatted(self, node: LN) -> Iterator[Line]:
-        """Used when file contained a `# fmt: off`."""
-        if isinstance(node, Node):
-            for child in node.children:
-                yield from self.visit(child)
+    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)
 
-        else:
-            try:
-                self.current_line.append(node)
-            except FormatOn as f_on:
-                f_on.trim_prefix(node)
-                yield from self.line()
-                yield from self.visit(node)
+    def visit_factor(self, node: Node) -> Iterator[Line]:
+        """Force parentheses between a unary op and a binary power:
 
-            if node.type == token.ENDMARKER:
-                # somebody decided not to put a final `# fmt: on`
-                yield from self.line()
+        -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]:
+        # Check if it's a docstring
+        if prev_siblings_are(
+            leaf.parent, [None, token.NEWLINE, token.INDENT, syms.simple_stmt]
+        ) and is_multiline_string(leaf):
+            prefix = "    " * self.current_line.depth
+            docstring = fix_docstring(leaf.value[3:-3], prefix)
+            leaf.value = leaf.value[0:3] + docstring + leaf.value[-3:]
+            normalize_string_quotes(leaf)
 
-    def __attrs_post_init__(self) -> None:
+        yield from self.visit_default(leaf)
+
+    def __post_init__(self) -> None:
         """You are in a twisty little maze of passages."""
         v = self.visit_stmt
         Ø: Set[str] = set()
@@ -1447,6 +2062,8 @@ class LineGenerator(Visitor[Line]):
         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
 
@@ -1459,7 +2076,7 @@ BRACKETS = OPENING_BRACKETS | CLOSING_BRACKETS
 ALWAYS_NO_SPACE = CLOSING_BRACKETS | {token.COMMA, STANDALONE_COMMENT}
 
 
-def whitespace(leaf: Leaf, *, complex_subscript: bool) -> str:  # noqa C901
+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
@@ -1516,7 +2133,7 @@ def whitespace(leaf: Leaf, *, complex_subscript: bool) -> str:  # noqa C901
                     # that, too.
                     return prevp.prefix
 
-        elif prevp.type in STARS:
+        elif prevp.type in VARARGS_SPECIALS:
             if is_vararg(prevp, within=VARARGS_PARENTS | UNPACKING_PARENTS):
                 return NO
 
@@ -1606,7 +2223,7 @@ def whitespace(leaf: Leaf, *, complex_subscript: bool) -> str:  # noqa C901
             if not prevp or prevp.type == token.LPAR:
                 return NO
 
-        elif prev.type in {token.EQUAL} | STARS:
+        elif prev.type in {token.EQUAL} | VARARGS_SPECIALS:
             return NO
 
     elif p.type == syms.decorator:
@@ -1668,7 +2285,7 @@ def whitespace(leaf: Leaf, *, complex_subscript: bool) -> str:  # noqa C901
             elif prevp.type == token.EQUAL and prevp_parent.type == syms.argument:
                 return NO
 
-        elif t == token.NAME or t == token.NUMBER:
+        elif t in {token.NAME, token.NUMBER, token.STRING}:
             return NO
 
     elif p.type == syms.import_from:
@@ -1707,6 +2324,22 @@ def preceding_leaf(node: Optional[LN]) -> Optional[Leaf]:
     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
@@ -1715,7 +2348,32 @@ def child_towards(ancestor: Node, descendant: LN) -> Optional[LN]:
     return node
 
 
-def is_split_after_delimiter(leaf: Leaf, previous: Leaf = None) -> int:
+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
@@ -1729,8 +2387,8 @@ def is_split_after_delimiter(leaf: Leaf, previous: Leaf = None) -> int:
     return 0
 
 
-def is_split_before_delimiter(leaf: Leaf, previous: Leaf = None) -> int:
-    """Return the priority of the `leaf` delimiter, given a line before after it.
+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.
@@ -1767,15 +2425,20 @@ def is_split_before_delimiter(leaf: Leaf, previous: Leaf = None) -> int:
     ):
         return STRING_PRIORITY
 
-    if leaf.type != token.NAME:
+    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
     ):
-        return COMPREHENSION_PRIORITY
+        if (
+            not isinstance(leaf.prev_sibling, Leaf)
+            or leaf.prev_sibling.value != "async"
+        ):
+            return COMPREHENSION_PRIORITY
 
     if (
         leaf.value == "if"
@@ -1820,7 +2483,11 @@ def is_split_before_delimiter(leaf: Leaf, previous: Leaf = None) -> int:
     return 0
 
 
-def generate_comments(leaf: Leaf) -> Iterator[Leaf]:
+FMT_OFF = {"# fmt: off", "# fmt:off", "# yapf: disable"}
+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
@@ -1839,49 +2506,70 @@ def generate_comments(leaf: Leaf) -> Iterator[Leaf]:
     Inline comments are emitted as regular token.COMMENT leaves.  Standalone
     are emitted with a fake STANDALONE_COMMENT token identifier.
     """
-    p = leaf.prefix
-    if not p:
-        return
+    for pc in list_comments(leaf.prefix, is_endmarker=leaf.type == token.ENDMARKER):
+        yield Leaf(pc.type, pc.value, prefix="\n" * pc.newlines)
 
-    if "#" not in p:
-        return
+
+@dataclass
+class ProtoComment:
+    """Describes a piece of syntax that is a comment.
+
+    It's not a :class:`blib2to3.pytree.Leaf` so that:
+
+    * 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``.
+    """
+
+    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
-    for index, line in enumerate(p.split("\n")):
+    ignored_lines = 0
+    for index, line in enumerate(prefix.split("\n")):
         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 == 0 and leaf.type != token.ENDMARKER:
+        if index == ignored_lines and not is_endmarker:
             comment_type = token.COMMENT  # simple trailing comment
         else:
             comment_type = STANDALONE_COMMENT
         comment = make_comment(line)
-        yield Leaf(comment_type, comment, prefix="\n" * nlines)
-
-        if comment in {"# fmt: on", "# yapf: enable"}:
-            raise FormatOn(consumed)
-
-        if comment in {"# fmt: off", "# yapf: disable"}:
-            if comment_type == STANDALONE_COMMENT:
-                raise FormatOff(consumed)
-
-            prev = preceding_leaf(leaf)
-            if not prev or prev.type in WHITESPACE:  # standalone comment in disguise
-                raise FormatOff(consumed)
-
+        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.
+    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.
     """
@@ -1891,43 +2579,58 @@ def make_comment(content: str) -> str:
 
     if content[0] == "#":
         content = content[1:]
-    if content and content[0] not in " !:#":
+    if content and content[0] not in " !:#'%":
         content = " " + content
     return "#" + content
 
 
-def split_line(
-    line: Line, line_length: int, inner: bool = False, py36: bool = False
+def transform_line(
+    line: Line,
+    line_length: int,
+    normalize_strings: bool,
+    features: Collection[Feature] = (),
 ) -> Iterator[Line]:
-    """Split a `line` into potentially many lines.
+    """Transform a `line`, potentially splitting it into many lines.
 
     They should fit in the allotted `line_length` but might not be able to.
-    `inner` signifies that there were a pair of brackets somewhere around the
-    current `line`, possibly transitively. This means we can fallback to splitting
-    by delimiters if the LHS/RHS don't yield any results.
 
-    If `py36` is True, splitting may generate syntax that is only compatible
-    with Python 3.6 and later.
+    `features` are syntactical features that may be used in the output.
     """
-    if isinstance(line, UnformattedLines) or line.is_comment:
+    if line.is_comment:
         yield line
         return
 
-    line_str = str(line).strip("\n")
-    if not line.should_explode and is_line_short_enough(
-        line, line_length=line_length, line_str=line_str
-    ):
-        yield line
-        return
+    line_str = line_to_string(line)
+
+    def init_st(ST: Type[StringTransformer]) -> StringTransformer:
+        """Initialize StringTransformer"""
+        return ST(line_length, normalize_strings)
 
-    split_funcs: List[SplitFunc]
-    if line.is_def:
-        split_funcs = [left_hand_split]
+    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_explode
+        and not line.is_collection_with_optional_trailing_comma
+        and (
+            is_line_short_enough(line, line_length=line_length, line_str=line_str)
+            or line.contains_unsplittable_type_ignore()
+        )
+        and not (line.contains_standalone_comments() and line.inside_brackets)
+    ):
+        # Only apply basic string preprocessing, since lines shouldn't be split here.
+        transformers = [string_merge, string_paren_strip]
+    elif line.is_def:
+        transformers = [left_hand_split]
     else:
 
-        def rhs(line: Line, py36: bool = False) -> Iterator[Line]:
+        def rhs(line: Line, features: Collection[Feature]) -> Iterator[Line]:
             for omit in generate_trailers_to_omit(line, line_length):
-                lines = list(right_hand_split(line, line_length, py36, omit=omit))
+                lines = list(right_hand_split(line, line_length, features, omit=omit))
                 if is_line_short_enough(lines[0], line_length=line_length):
                     yield from lines
                     return
@@ -1935,28 +2638,51 @@ def split_line(
             # 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.
-            yield from right_hand_split(line, py36)
+            # line_length=1 here was historically a bug that somehow became a feature.
+            # See #762 and #781 for the full story.
+            yield from right_hand_split(line, line_length=1, features=features)
 
         if line.inside_brackets:
-            split_funcs = [delimiter_split, standalone_comment_split, rhs]
+            transformers = [
+                string_merge,
+                string_paren_strip,
+                delimiter_split,
+                standalone_comment_split,
+                string_split,
+                string_paren_wrap,
+                rhs,
+            ]
         else:
-            split_funcs = [rhs]
-    for split_func in split_funcs:
+            transformers = [
+                string_merge,
+                string_paren_strip,
+                string_split,
+                string_paren_wrap,
+                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.
         result: List[Line] = []
         try:
-            for l in split_func(line, py36):
+            for l in transform(line, features):
                 if str(l).strip("\n") == line_str:
-                    raise CannotSplit("Split function returned an unchanged result")
+                    raise CannotTransform(
+                        "Line transformer returned an unchanged result"
+                    )
 
                 result.extend(
-                    split_line(l, line_length=line_length, inner=True, py36=py36)
+                    transform_line(
+                        l,
+                        line_length=line_length,
+                        normalize_strings=normalize_strings,
+                        features=features,
+                    )
                 )
-        except CannotSplit as cs:
+        except CannotTransform:
             continue
-
         else:
             yield from result
             break
@@ -1965,121 +2691,2057 @@ def split_line(
         yield line
 
 
-def left_hand_split(line: Line, py36: bool = False) -> Iterator[Line]:
-    """Split line into many lines, starting with the first matching bracket pair.
+@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(...)`.
 
-    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.
+    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 requirments, 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.
     """
-    head = Line(depth=line.depth)
-    body = Line(depth=line.depth + 1, inside_brackets=True)
-    tail = Line(depth=line.depth)
-    tail_leaves: List[Leaf] = []
-    body_leaves: List[Leaf] = []
-    head_leaves: List[Leaf] = []
-    current_leaves = head_leaves
-    matching_bracket = 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
-    # Since body is a new indent level, remove spurious leading whitespace.
-    if body_leaves:
-        normalize_prefix(body_leaves[0], inside_brackets=True)
-    # Build the new lines.
-    for result, leaves in (head, head_leaves), (body, body_leaves), (tail, tail_leaves):
-        for leaf in leaves:
-            result.append(leaf, preformatted=True)
-            for comment_after in line.comments_after(leaf):
-                result.append(comment_after, preformatted=True)
-    bracket_split_succeeded_or_raise(head, body, tail)
-    for result in (head, body, tail):
-        if result:
-            yield result
 
+    line_length: int
+    normalize_strings: bool
 
-def right_hand_split(
-    line: Line, line_length: int, py36: bool = False, omit: Collection[LeafID] = ()
-) -> Iterator[Line]:
-    """Split line into many lines, starting with the last matching bracket pair.
+    @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.
+        """
 
-    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.
+    @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.)
+        """
 
-    Note: running this function modifies `bracket_depth` on the leaves of `line`.
+    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)
+        ```
     """
-    head = Line(depth=line.depth)
-    body = Line(depth=line.depth + 1, inside_brackets=True)
-    tail = Line(depth=line.depth)
-    tail_leaves: List[Leaf] = []
-    body_leaves: List[Leaf] = []
-    head_leaves: List[Leaf] = []
-    current_leaves = tail_leaves
-    opening_bracket = None
-    closing_bracket = 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
-    tail_leaves.reverse()
-    body_leaves.reverse()
-    head_leaves.reverse()
-    # Since body is a new indent level, remove spurious leading whitespace.
-    if body_leaves:
-        normalize_prefix(body_leaves[0], inside_brackets=True)
-    if not head_leaves:
-        # No `head` means the split failed. Either `tail` has all content or
-        # the matching `opening_bracket` wasn't available on `line` anymore.
-        raise CannotSplit("No brackets found")
 
-    # Build the new lines.
-    for result, leaves in (head, head_leaves), (body, body_leaves), (tail, tail_leaves):
-        for leaf in leaves:
-            result.append(leaf, preformatted=True)
-            for comment_after in line.comments_after(leaf):
-                result.append(comment_after, preformatted=True)
-    bracket_split_succeeded_or_raise(head, body, tail)
-    assert opening_bracket and closing_bracket
-    if (
-        # the opening bracket is an optional paren
-        opening_bracket.type == token.LPAR
-        and not opening_bracket.value
-        # the closing bracket is an optional paren
+    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 at most one substring
+        has inline comments AND none of those inline comments are pragmas AND
+        the set of all substring prefixes is either of length 1 or equal to
+        {"", "f"} AND none of the substrings are raw strings (i.e. are prefixed
+        with 'r').
+            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
+        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 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.
+        """
+        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 RPAR is NOT followed by an attribute access (i.e. a dot).
+
+    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
+
+            # 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)
+
+            # 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 a '.' symbol.
+                if is_valid_index(next_idx + 1) and LL[next_idx + 1].type == 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."
+                )
+
+        new_line = line.clone()
+        new_line.comments = line.comments.copy()
+
+        append_leaves(new_line, line, LL[: string_idx - 1])
+
+        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.
+                for leaf in LL[: p_idx + 1]:
+                    offset += len(str(leaf))
+
+        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).
+
+        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"""
+    (?<!\{)\{
+        (?:
+            [^\{\}]
+            | \{\{
+            | \}\}
+        )+?
+    (?<!\})(?:\}\})*\}(?!\})
+    """
+
+    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
+        # containes 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 algorthmically 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_big_enough = (
+                len(string[i:]) >= self.MIN_SUBSTR_SIZE
+                and len(string[:i]) >= self.MIN_SUBSTR_SIZE
+            )
+            return is_space 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 = None
+        string_idx = string_idx or self._return_match(LL)
+        string_idx = string_idx or self._else_match(LL)
+        string_idx = string_idx or self._assert_match(LL)
+        string_idx = string_idx or self._assign_match(LL)
+        string_idx = string_idx 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 = len(LL) - 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(
+            depth=line.depth + 1,
+            inside_brackets=True,
+            should_explode=line.should_explode,
+        )
+        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-existant, 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 paramater 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]) -> 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 attatched 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:
+        assert old_leaf in old_line.leaves
+
+        new_leaf = Leaf(old_leaf.type, old_leaf.value)
+        replace_child(old_leaf, new_leaf)
+        new_line.append(new_leaf)
+
+        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 (
+        # the body shouldn't be exploded
+        not body.should_explode
+        # 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
-        # there are no standalone comments in the body
-        and not line.contains_standalone_comments(0)
-        # and 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)
+        # 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 = {id(closing_bracket), *omit}
-        if can_omit_invisible_parens(body, line_length):
-            try:
-                yield from right_hand_split(line, line_length, py36=py36, omit=omit)
-                return
-            except CannotSplit:
-                pass
+        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)
-    body.should_explode = should_explode(body, opening_bracket)
     for result in (head, body, tail):
         if result:
             yield result
@@ -2106,20 +4768,62 @@ def bracket_split_succeeded_or_raise(head: Line, body: Line, tail: Line) -> None
 
         elif tail_len < 3:
             raise CannotSplit(
-                f"Splitting brackets on an empty body to save "
-                f"{tail_len} characters is not worth it"
+                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(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(l.type == token.COMMA for l 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:
+                        leaves.insert(i + 1, Leaf(token.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:
+        result.should_explode = should_explode(result, opening_bracket)
+    return result
+
 
-def dont_increase_indentation(split_func: SplitFunc) -> SplitFunc:
+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, py36: bool = False) -> Iterator[Line]:
-        for l in split_func(line, py36):
+    def split_wrapper(line: Line, features: Collection[Feature] = ()) -> Iterator[Line]:
+        for l in split_func(line, features):
             normalize_prefix(l.leaves[0], inside_brackets=True)
             yield l
 
@@ -2127,11 +4831,11 @@ def dont_increase_indentation(split_func: SplitFunc) -> SplitFunc:
 
 
 @dont_increase_indentation
-def delimiter_split(line: Line, py36: bool = False) -> Iterator[Line]:
+def delimiter_split(line: Line, features: Collection[Feature] = ()) -> Iterator[Line]:
     """Split according to delimiters of the highest priority.
 
-    If `py36` is True, the split will add trailing commas also in function
-    signatures that contain `*` and `**`.
+    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]
@@ -2157,23 +4861,29 @@ def delimiter_split(line: Line, py36: bool = False) -> Iterator[Line]:
         nonlocal current_line
         try:
             current_line.append_safe(leaf, preformatted=True)
-        except ValueError as ve:
+        except ValueError:
             yield current_line
 
             current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)
             current_line.append(leaf)
 
-    for index, leaf in enumerate(line.leaves):
+    for leaf in line.leaves:
         yield from append_to_line(leaf)
 
-        for comment_after in line.comments_after(leaf, index):
+        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 and is_vararg(
-            leaf, within=VARARGS_PARENTS
-        ):
-            trailing_comma_safe = trailing_comma_safe and py36
+        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
@@ -2191,7 +4901,9 @@ def delimiter_split(line: Line, py36: bool = False) -> Iterator[Line]:
 
 
 @dont_increase_indentation
-def standalone_comment_split(line: Line, py36: bool = False) -> Iterator[Line]:
+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")
@@ -2203,16 +4915,16 @@ def standalone_comment_split(line: Line, py36: bool = False) -> Iterator[Line]:
         nonlocal current_line
         try:
             current_line.append_safe(leaf, preformatted=True)
-        except ValueError as ve:
+        except ValueError:
             yield current_line
 
             current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)
             current_line.append(leaf)
 
-    for index, leaf in enumerate(line.leaves):
+    for leaf in line.leaves:
         yield from append_to_line(leaf)
 
-        for comment_after in line.comments_after(leaf, index):
+        for comment_after in line.comments_after(leaf):
             yield from append_to_line(comment_after)
 
     if current_line:
@@ -2233,6 +4945,14 @@ def is_import(leaf: Leaf) -> bool:
     )
 
 
+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.
@@ -2258,10 +4978,10 @@ def normalize_string_prefix(leaf: Leaf, remove_u_prefix: bool = False) -> None:
 
     Note: Mutates its argument.
     """
-    match = re.match(r"^([furbFURB]*)(.*)$", leaf.value, re.DOTALL)
+    match = re.match(r"^([" + STRING_PREFIX_CHARS + r"]*)(.*)$", 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.lower()
+    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)}"
@@ -2275,7 +4995,7 @@ def normalize_string_quotes(leaf: Leaf) -> None:
 
     Note: Mutates its argument.
     """
-    value = leaf.value.lstrip("furbFURB")
+    value = leaf.value.lstrip(STRING_PREFIX_CHARS)
     if value[:3] == '"""':
         return
 
@@ -2294,8 +5014,8 @@ def normalize_string_quotes(leaf: Leaf) -> None:
 
     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}")
+    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):
@@ -2306,15 +5026,30 @@ def normalize_string_quotes(leaf: Leaf) -> None:
         # Do not introduce or remove backslashes in raw strings
         new_body = body
     else:
-        # remove unnecessary quotes
+        # 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 quotes as the original
+            # 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 new_quote == '"""' and new_body[-1] == '"':
+    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("\\")
@@ -2328,43 +5063,233 @@ def normalize_string_quotes(leaf: Leaf) -> None:
     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"):
+        # Change hex literals to upper case.
+        before, after = text[:2], text[2:]
+        text = f"{before}{after.upper()}"
+    elif "e" in text:
+        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)
+        text = f"{before}e{sign}{after}"
+    elif text.endswith(("j", "l")):
+        number = text[:-1]
+        suffix = text[-1]
+        # Capitalize in "2L" because "l" looks too similar to "1".
+        if suffix == "l":
+            suffix = "L"
+        text = f"{format_float_or_int_string(number)}{suffix}"
+    else:
+        text = format_float_or_int_string(text)
+    leaf.value = text
+
+
+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 immeditely after which parens
+    `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 child in list(node.children):
+    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 is_walrus_assignment(child):
+                continue
+
             if child.type == syms.atom:
-                maybe_make_parens_invisible_in_atom(child)
+                if maybe_make_parens_invisible_in_atom(child, parent=node):
+                    wrap_in_parentheses(node, child, visible=False)
             elif is_one_tuple(child):
-                # wrap child in visible parentheses
-                lpar = Leaf(token.LPAR, "(")
-                rpar = Leaf(token.RPAR, ")")
-                index = child.remove() or 0
-                node.insert_child(index, Node(syms.atom, [lpar, child, rpar]))
+                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 child in invisible parentheses
-                lpar = Leaf(token.LPAR, "")
-                rpar = Leaf(token.RPAR, "")
-                index = child.remove() or 0
-                node.insert_child(index, Node(syms.atom, [lpar, child, rpar]))
+                wrap_in_parentheses(node, child, visible=False)
 
         check_lpar = isinstance(child, Leaf) and child.value in parens_after
 
 
-def maybe_make_parens_invisible_in_atom(node: LN) -> bool:
-    """If it's safe, make the parens in the atom `node` invisible, recusively."""
+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 in FMT_OFF:
+                # 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.type != STANDALONE_COMMENT:
+                    prev = preceding_leaf(leaf)
+                    if prev and prev.type not in WHITESPACE:
+                        continue
+
+                ignored_nodes = list(generate_ignored_nodes(leaf))
+                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 = (
+                    comment.value + "\n" + "".join(str(n) for n in ignored_nodes)
+                )
+                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
+
+            previous_consumed = comment.consumed
+
+    return False
+
+
+def generate_ignored_nodes(leaf: Leaf) -> Iterator[LN]:
+    """Starting from the container of `leaf`, generate all leaves until `# fmt: on`.
+
+    Stops at the end of the block.
+    """
+    container: Optional[LN] = container_of(leaf)
+    while container is not None and container.type != token.ENDMARKER:
+        if 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 fmt_on(container: LN) -> bool:
+    is_fmt_on = False
+    for comment in list_comments(container.prefix, is_endmarker=False):
+        if comment.value in FMT_ON:
+            is_fmt_on = True
+        elif comment.value in FMT_OFF:
+            is_fmt_on = False
+    return is_fmt_on
+
+
+def contains_fmt_on_at_column(container: LN, column: int) -> bool:
+    for child in container.children:
+        if (
+            isinstance(child, Node)
+            and first_leaf_column(child) == column
+            or isinstance(child, Leaf)
+            and child.column == column
+        ):
+            if fmt_on(child):
+                return True
+
+    return False
+
+
+def first_leaf_column(node: Node) -> Optional[int]:
+    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)
+        or (is_yield(node) and parent.type != syms.expr_stmt)
         or max_delimiter_priority_in_atom(node) >= COMMA_PRIORITY
     ):
         return False
@@ -2372,14 +5297,38 @@ def maybe_make_parens_invisible_in_atom(node: LN) -> bool:
     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
-        if len(node.children) > 1:
-            maybe_make_parens_invisible_in_atom(node.children[1])
-        return True
+        maybe_make_parens_invisible_in_atom(middle, parent=parent)
 
-    return False
+        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:
@@ -2392,18 +5341,43 @@ def is_empty_tuple(node: LN) -> bool:
     )
 
 
+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 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:
-        if len(node.children) != 3:
-            return False
-
-        lpar, gexp, rpar = node.children
-        if not (
-            lpar.type == token.LPAR
-            and gexp.type == syms.testlist_gexp
-            and rpar.type == token.RPAR
-        ):
+        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
@@ -2415,6 +5389,12 @@ def is_one_tuple(node: LN) -> bool:
     )
 
 
+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_yield(node: LN) -> bool:
     """Return True if `node` holds a `yield` or `yield from` expression."""
     if node.type == syms.yield_expr:
@@ -2444,7 +5424,7 @@ def is_vararg(leaf: Leaf, within: Set[NodeType]) -> bool:
     extended iterable unpacking (PEP 3132) and additional unpacking
     generalizations (PEP 448).
     """
-    if leaf.type not in STARS or not leaf.parent:
+    if leaf.type not in VARARGS_SPECIALS or not leaf.parent:
         return False
 
     p = leaf.parent
@@ -2461,8 +5441,7 @@ def is_vararg(leaf: Leaf, within: Set[NodeType]) -> bool:
 
 def is_multiline_string(leaf: Leaf) -> bool:
     """Return True if `leaf` is a multiline string that actually spans many lines."""
-    value = leaf.value.lstrip("furbFURB")
-    return value[:3] in {'"""', "'''"} and "\n" in value
+    return has_triple_quotes(leaf.value) and "\n" in leaf.value
 
 
 def is_stub_suite(node: Node) -> bool:
@@ -2494,7 +5473,7 @@ def is_stub_body(node: LN) -> bool:
     )
 
 
-def max_delimiter_priority_in_atom(node: LN) -> int:
+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.
@@ -2526,7 +5505,7 @@ def ensure_visible(leaf: Leaf) -> None:
     """Make sure parentheses are visible.
 
     They could be invisible as part of some statements (see
-    :func:`normalize_invible_parens` and :func:`visit_import_from`).
+    :func:`normalize_invisible_parens` and :func:`visit_import_from`).
     """
     if leaf.type == token.LPAR:
         leaf.value = "("
@@ -2536,6 +5515,7 @@ def ensure_visible(leaf: Leaf) -> None:
 
 def should_explode(line: Line, opening_bracket: Leaf) -> bool:
     """Should `line` immediately be split with `delimiter_split()` after RHS?"""
+
     if not (
         opening_bracket.parent
         and opening_bracket.parent.type in {syms.atom, syms.import_from}
@@ -2553,34 +5533,61 @@ def should_explode(line: Line, opening_bracket: Leaf) -> bool:
     return max_priority == COMMA_PRIORITY
 
 
-def is_python36(node: Node) -> bool:
-    """Return True if the current file is using Python 3.6+ features.
+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; and
-    - trailing commas after * or ** in function signatures and calls.
+    - f-strings;
+    - underscores in numeric literals;
+    - trailing commas after * or ** in function signatures and calls;
+    - positional only arguments in function signatures and lambdas;
     """
+    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"}:
-                return True
+                features.add(Feature.F_STRINGS)
+
+        elif n.type == token.NUMBER:
+            if "_" in n.value:  # type: ignore
+                features.add(Feature.NUMERIC_UNDERSCORES)
+
+        elif n.type == token.SLASH:
+            if n.parent and n.parent.type in {syms.typedargslist, syms.arglist}:
+                features.add(Feature.POS_ONLY_ARGUMENTS)
+
+        elif n.type == token.COLONEQUAL:
+            features.add(Feature.ASSIGNMENT_EXPRESSIONS)
 
         elif (
             n.type in {syms.typedargslist, syms.arglist}
             and n.children
             and n.children[-1].type == token.COMMA
         ):
+            if n.type == syms.typedargslist:
+                feature = Feature.TRAILING_COMMA_IN_DEF
+            else:
+                feature = Feature.TRAILING_COMMA_IN_CALL
+
             for ch in n.children:
                 if ch.type in STARS:
-                    return True
+                    features.add(feature)
 
                 if ch.type == syms.argument:
                     for argch in ch.children:
                         if argch.type in STARS:
-                            return True
+                            features.add(feature)
 
-    return False
+    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]
+    }
 
 
 def generate_trailers_to_omit(line: Line, line_length: int) -> Iterator[Set[LeafID]]:
@@ -2597,26 +5604,24 @@ def generate_trailers_to_omit(line: Line, line_length: int) -> Iterator[Set[Leaf
     yield omit
 
     length = 4 * line.depth
-    opening_bracket = None
-    closing_bracket = None
-    optional_brackets: Set[LeafID] = set()
+    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
 
-        optional_brackets.discard(id(leaf))
+        has_inline_comment = leaf_length > len(leaf.value) + len(leaf.prefix)
+        if leaf.type == STANDALONE_COMMENT or has_inline_comment:
+            break
+
         if opening_bracket:
             if leaf is opening_bracket:
                 opening_bracket = None
             elif leaf.type in CLOSING_BRACKETS:
                 inner_brackets.add(id(leaf))
         elif leaf.type in CLOSING_BRACKETS:
-            if not leaf.value:
-                optional_brackets.add(id(opening_bracket))
-                continue
-
             if index > 0 and line.leaves[index - 1].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
@@ -2624,21 +5629,43 @@ def generate_trailers_to_omit(line: Line, line_length: int) -> Iterator[Set[Leaf
                 inner_brackets.add(id(leaf))
                 continue
 
-            opening_bracket = leaf.opening_bracket
             if closing_bracket:
                 omit.add(id(closing_bracket))
                 omit.update(inner_brackets)
                 inner_brackets.clear()
                 yield omit
-            closing_bracket = leaf
+
+            if leaf.value:
+                opening_bracket = leaf.opening_bracket
+                closing_bracket = leaf
 
 
 def get_future_imports(node: Node) -> Set[str]:
     """Return a set of __future__ imports in the file."""
-    imports = set()
+    imports: Set[str] = set()
+
+    def get_imports_from_children(children: List[LN]) -> Generator[str, None, None]:
+        for child in children:
+            if isinstance(child, Leaf):
+                if child.type == token.NAME:
+                    yield child.value
+
+            elif child.type == syms.import_as_name:
+                orig_name = child.children[0]
+                assert isinstance(orig_name, Leaf), "Invalid syntax parsing imports"
+                assert orig_name.type == token.NAME, "Invalid syntax parsing imports"
+                yield orig_name.value
+
+            elif child.type == syms.import_as_names:
+                yield from get_imports_from_children(child.children)
+
+            else:
+                raise AssertionError("Invalid syntax parsing imports")
+
     for child in node.children:
         if child.type != syms.simple_stmt:
             break
+
         first_child = child.children[0]
         if isinstance(first_child, Leaf):
             # Continue looking if we see a docstring; otherwise stop.
@@ -2648,60 +5675,127 @@ def get_future_imports(node: Node) -> Set[str]:
                 and child.children[1].type == token.NEWLINE
             ):
                 continue
-            else:
-                break
+
+            break
+
         elif first_child.type == syms.import_from:
             module_name = first_child.children[1]
             if not isinstance(module_name, Leaf) or module_name.value != "__future__":
                 break
-            for import_from_child in first_child.children[3:]:
-                if isinstance(import_from_child, Leaf):
-                    if import_from_child.type == token.NAME:
-                        imports.add(import_from_child.value)
-                else:
-                    assert import_from_child.type == syms.import_as_names
-                    for leaf in import_from_child.children:
-                        if isinstance(leaf, Leaf) and leaf.type == token.NAME:
-                            imports.add(leaf.value)
+
+            imports |= set(get_imports_from_children(first_child.children[3:]))
         else:
             break
+
     return imports
 
 
-PYTHON_EXTENSIONS = {".py", ".pyi"}
-BLACKLISTED_DIRECTORIES = {
-    "build",
-    "buck-out",
-    "dist",
-    "_build",
-    ".git",
-    ".hg",
-    ".mypy_cache",
-    ".tox",
-    ".venv",
-}
+@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 gen_python_files_in_dir(path: Path) -> Iterator[Path]:
-    """Generate all files under `path` which aren't under BLACKLISTED_DIRECTORIES
-    and have one of the PYTHON_EXTENSIONS.
+def gen_python_files_in_dir(
+    path: Path,
+    root: Path,
+    include: Pattern[str],
+    exclude: Pattern[str],
+    report: "Report",
+    gitignore: PathSpec,
+) -> Iterator[Path]:
+    """Generate all files under `path` whose paths are not excluded by the
+    `exclude` regex, 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 path.iterdir():
-        if child.is_dir():
-            if child.name in BLACKLISTED_DIRECTORIES:
+        # First ignore files matching .gitignore
+        if gitignore.match_file(child.as_posix()):
+            report.path_ignored(child, "matches the .gitignore file content")
+            continue
+
+        # Then ignore with `exclude` option.
+        try:
+            normalized_path = "/" + child.resolve().relative_to(root).as_posix()
+        except OSError as e:
+            report.path_ignored(child, f"cannot be read because {e}")
+            continue
+
+        except ValueError:
+            if child.is_symlink():
+                report.path_ignored(
+                    child, f"is a symbolic link that points outside {root}"
+                )
                 continue
 
-            yield from gen_python_files_in_dir(child)
+            raise
+
+        if child.is_dir():
+            normalized_path += "/"
+
+        exclude_match = exclude.search(normalized_path)
+        if exclude_match and exclude_match.group(0):
+            report.path_ignored(child, "matches the --exclude regular expression")
+            continue
+
+        if child.is_dir():
+            yield from gen_python_files_in_dir(
+                child, root, include, exclude, report, gitignore
+            )
+
+        elif child.is_file():
+            include_match = include.search(normalized_path)
+            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 can be one of the directories passed in `srcs` or their
+    common parent.
+
+    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()
 
-        elif child.is_file() and child.suffix in PYTHON_EXTENSIONS:
-            yield child
+    common_base = min(Path(src).resolve() for src in srcs)
+    if common_base.is_dir():
+        # Append a fake file so `parents` below returns `common_base_dir`, too.
+        common_base /= "fake-file"
+    for directory in 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
 
 
 @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
@@ -2709,12 +5803,12 @@ class Report:
     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 else "reformatted"
-            if not self.quiet:
+            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 not self.quiet:
+            if self.verbose:
                 if changed is Changed.NO:
                     msg = f"{src} already well formatted, good job."
                 else:
@@ -2727,6 +5821,10 @@ class Report:
         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.
@@ -2737,7 +5835,7 @@ class Report:
         - otherwise return 0.
         """
         # According to http://tldp.org/LDP/abs/html/exitcodes.html starting with
-        # 126 we have special returncodes reserved by the shell.
+        # 126 we have special return codes reserved by the shell.
         if self.failure_count:
             return 123
 
@@ -2751,7 +5849,7 @@ class Report:
 
         Use `click.unstyle` to remove colors.
         """
-        if self.check:
+        if self.check or self.diff:
             reformatted = "would be reformatted"
             unchanged = "would be left unchanged"
             failed = "would fail to reformat"
@@ -2776,89 +5874,148 @@ class Report:
         return ", ".join(report) + "."
 
 
-def assert_equivalent(src: str, dst: str) -> None:
-    """Raise AssertionError if `src` and `dst` aren't equivalent."""
+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
 
-    import ast
-    import traceback
+    return ast27.parse(src)
 
-    def _v(node: ast.AST, depth: int = 0) -> Iterator[str]:
-        """Simple visitor generating strings to compare ASTs by content."""
-        yield f"{'  ' * depth}{node.__class__.__name__}("
 
-        for field in sorted(node._fields):
-            try:
-                value = getattr(node, field)
-            except AttributeError:
-                continue
+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
 
-            yield f"{'  ' * (depth+1)}{field}="
+        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)
 
-            if isinstance(value, list):
-                for item in value:
-                    if isinstance(item, ast.AST):
-                        yield from _v(item, depth + 2)
+                elif isinstance(item, (ast.AST, ast3.AST, ast27.AST)):
+                    yield from _stringify_ast(item, depth + 2)
 
-            elif isinstance(value, ast.AST):
-                yield from _v(value, 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
+            if (
+                isinstance(node, ast.Constant)
+                and field == "value"
+                and isinstance(value, str)
+            ):
+                normalized = re.sub(r"\n[ \t]+", "\n ", value)
             else:
-                yield f"{'  ' * (depth+2)}{value!r},  # {value.__class__.__name__}"
+                normalized = value
+            yield f"{'  ' * (depth+2)}{normalized!r},  # {value.__class__.__name__}"
+
+    yield f"{'  ' * depth})  # /{node.__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 = ast.parse(src)
+        src_ast = parse_ast(src)
     except Exception as exc:
-        major, minor = sys.version_info[:2]
         raise AssertionError(
-            f"cannot use --safe with this file; failed to parse source file "
-            f"with Python {major}.{minor}'s builtin AST. Re-run with --fast "
-            f"or stop using deprecated Python 2 syntax. AST error message: {exc}"
+            "cannot use --safe with this file; failed to parse source file.  AST"
+            f" error message: {exc}"
         )
 
     try:
-        dst_ast = ast.parse(dst)
+        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}. "
-            f"Please report a bug on https://github.com/ambv/black/issues.  "
-            f"This invalid output might be helpful: {log}"
+            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}"
         ) from None
 
-    src_ast_str = "\n".join(_v(src_ast))
-    dst_ast_str = "\n".join(_v(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(
-            f"INTERNAL ERROR: Black produced code that is not equivalent to "
-            f"the source.  "
-            f"Please report a bug on https://github.com/ambv/black/issues.  "
-            f"This diff might be helpful: {log}"
+            "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}"
         ) from None
 
 
-def assert_stable(src: str, dst: str, line_length: int, is_pyi: bool = False) -> None:
+def assert_stable(src: str, dst: str, mode: Mode) -> None:
     """Raise AssertionError if `dst` reformats differently the second time."""
-    newdst = format_str(dst, line_length=line_length, is_pyi=is_pyi)
+    newdst = format_str(dst, mode=mode)
     if dst != newdst:
         log = dump_to_file(
             diff(src, dst, "source", "first pass"),
             diff(dst, newdst, "first pass", "second pass"),
         )
         raise AssertionError(
-            f"INTERNAL ERROR: Black produced different code on the second pass "
-            f"of the formatter.  "
-            f"Please report a bug on https://github.com/ambv/black/issues.  "
-            f"This diff might be helpful: {log}"
+            "INTERNAL ERROR: Black produced different code on the second pass of the"
+            " formatter.  Please report a bug on https://github.com/psf/black/issues."
+            f"  This diff might be helpful: {log}"
         ) from None
 
 
+@mypyc_attr(patchable=True)
 def dump_to_file(*output: str) -> str:
     """Dump `output` to a temporary file. Return path to the file."""
-    import tempfile
-
     with tempfile.NamedTemporaryFile(
         mode="w", prefix="blk_", suffix=".log", delete=False, encoding="utf8"
     ) as f:
@@ -2869,29 +6026,42 @@ def dump_to_file(*output: str) -> str:
     return f.name
 
 
+@contextmanager
+def nullcontext() -> Iterator[None]:
+    """Return an empty context manager.
+
+    To be used like `nullcontext` in Python 3.7.
+    """
+    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 + "\n" for line in a.split("\n")]
-    b_lines = [line + "\n" for line in b.split("\n")]
+    a_lines = [line + "\n" for line in a.splitlines()]
+    b_lines = [line + "\n" for line in b.splitlines()]
     return "".join(
         difflib.unified_diff(a_lines, b_lines, fromfile=a_name, tofile=b_name, n=5)
     )
 
 
-def cancel(tasks: Iterable[asyncio.Task]) -> None:
+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: BaseEventLoop) -> None:
+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 asyncio.Task.all_tasks(loop) if not task.done()]
+        to_cancel = [task for task in all_tasks(loop) if not task.done()]
         if not to_cancel:
             return
 
@@ -2918,6 +6088,17 @@ def sub_twice(regex: Pattern[str], replacement: str, original: str) -> str:
     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
@@ -2942,11 +6123,7 @@ def enumerate_with_length(
         if "\n" in leaf.value:
             return  # Multiline strings, we can't continue.
 
-        comment: Optional[Leaf]
-        for comment in line.comments_after(leaf, index):
-            if "\n" in comment.prefix:
-                return  # Oops, standalone comment!
-
+        for comment in line.comments_after(leaf):
             length += len(comment.value)
 
         yield index, leaf, length
@@ -2958,7 +6135,7 @@ def is_line_short_enough(line: Line, *, line_length: int, line_str: str = "") ->
     Uses the provided `line_str` rendering, if any, otherwise computes a new one.
     """
     if not line_str:
-        line_str = str(line).strip("\n")
+        line_str = line_to_string(line)
     return (
         len(line_str) <= line_length
         and "\n" not in line_str  # multiline strings
@@ -2966,6 +6143,42 @@ def is_line_short_enough(line: Line, *, line_length: int, line_str: str = "") ->
     )
 
 
+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) -> bool:
     """Does `line` have a shape safe to reformat without optional parens around it?
 
@@ -3055,23 +6268,23 @@ def can_omit_invisible_parens(line: Line, line_length: int) -> bool:
     return False
 
 
-def get_cache_file(line_length: int) -> Path:
-    return CACHE_DIR / f"cache.{line_length}.pickle"
+def get_cache_file(mode: Mode) -> Path:
+    return CACHE_DIR / f"cache.{mode.get_cache_key()}.pickle"
 
 
-def read_cache(line_length: int) -> Cache:
+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(line_length)
+    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:
+        except (pickle.UnpicklingError, ValueError):
             return {}
 
     return cache
@@ -3083,36 +6296,89 @@ def get_cache_info(path: Path) -> CacheInfo:
     return stat.st_mtime, stat.st_size
 
 
-def filter_cached(
-    cache: Cache, sources: Iterable[Path]
-) -> Tuple[List[Path], List[Path]]:
-    """Split a list of paths into two.
+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 list contains paths of files that modified on disk or are not in the
-    cache. The other list contains paths to non-modified files.
+    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 = [], []
+    todo, done = set(), set()
     for src in sources:
         src = src.resolve()
         if cache.get(src) != get_cache_info(src):
-            todo.append(src)
+            todo.add(src)
         else:
-            done.append(src)
+            done.add(src)
     return todo, done
 
 
-def write_cache(cache: Cache, sources: List[Path], line_length: int) -> None:
+def write_cache(cache: Cache, sources: Iterable[Path], mode: Mode) -> None:
     """Update the cache file."""
-    cache_file = get_cache_file(line_length)
+    cache_file = get_cache_file(mode)
     try:
-        if not CACHE_DIR.exists():
-            CACHE_DIR.mkdir(parents=True)
+        CACHE_DIR.mkdir(parents=True, exist_ok=True)
         new_cache = {**cache, **{src.resolve(): get_cache_info(src) for src in sources}}
-        with cache_file.open("wb") as fobj:
-            pickle.dump(new_cache, fobj, protocol=pickle.HIGHEST_PROTOCOL)
+        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
 
 
-if __name__ == "__main__":
+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:
+    freeze_support()
+    patch_click()
     main()
+
+
+def fix_docstring(docstring: str, prefix: str) -> str:
+    # https://www.python.org/dev/peps/pep-0257/#handling-docstring-indentation
+    if not docstring:
+        return ""
+    # Convert tabs to spaces (following the normal Python rules)
+    # and split into a list of lines:
+    lines = docstring.expandtabs().splitlines()
+    # 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 a single string:
+    return "\n".join(trimmed)
+
+
+if __name__ == "__main__":
+    patched_main()