Changes default logger used by blib2to3 Driver (#732)

[etc/vim.git] / black.py

diff --git a/black.py b/black.py
index b3b37ef3295687460e646679c0683c753e89926a..906b5064f5992022fbd485e0a57cf19ffeffafe9 100644 (file)
--- a/black.py
+++ b/black.py
@@ -1,23 +1,27 @@
 import asyncio
 import asyncio

-import pickle

 from asyncio.base_events import BaseEventLoop
 from concurrent.futures import Executor, ProcessPoolExecutor
 from asyncio.base_events import BaseEventLoop
 from concurrent.futures import Executor, ProcessPoolExecutor

-from enum import Enum, Flag
-from functools import partial, wraps
-import keyword
+from datetime import datetime
+from enum import Enum
+from functools import lru_cache, partial, wraps
+import io
+import itertools

 import logging
 import logging

-from multiprocessing import Manager
+from multiprocessing import Manager, freeze_support

 import os
 from pathlib import Path
 import os
 from pathlib import Path

+import pickle

 import re
 import re

-import tokenize

 import signal
 import sys
 import signal
 import sys

+import tempfile
+import tokenize

 from typing import (
     Any,
     Callable,
     Collection,
     Dict,
 from typing import (
     Any,
     Callable,
     Collection,
     Dict,

+    Generator,

     Generic,
     Iterable,
     Iterator,
     Generic,
     Iterable,
     Iterator,


@@ -27,27 +31,28 @@ from typing import (
     Sequence,
     Set,
     Tuple,
     Sequence,
     Set,
     Tuple,

-    Type,

     TypeVar,
     Union,
     cast,
 )
 
 from appdirs import user_cache_dir
     TypeVar,
     Union,
     cast,
 )
 
 from appdirs import user_cache_dir

-from attr import dataclass, Factory
+from attr import dataclass, evolve, Factory

 import click
 import click

+import toml

 
 # lib2to3 fork
 from blib2to3.pytree import Node, Leaf, type_repr
 from blib2to3 import pygram, pytree
 from blib2to3.pgen2 import driver, token
 
 # 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 blib2to3.pgen2.parse import ParseError
 
 

-__version__ = "18.5b1"
+__version__ = "18.9b0"

 DEFAULT_LINE_LENGTH = 88
 DEFAULT_EXCLUDES = (
 DEFAULT_LINE_LENGTH = 88
 DEFAULT_EXCLUDES = (

-    r"/(\.git|\.hg|\.mypy_cache|\.tox|\.venv|_build|buck-out|build|dist)/"
+    r"/(\.eggs|\.git|\.hg|\.mypy_cache|\.nox|\.tox|\.venv|_build|buck-out|build|dist)/"

 )
 DEFAULT_INCLUDES = r"\.pyi?$"
 CACHE_DIR = Path(user_cache_dir("black", version=__version__))
 )
 DEFAULT_INCLUDES = r"\.pyi?$"
 CACHE_DIR = Path(user_cache_dir("black", version=__version__))


@@ -56,6 +61,7 @@ CACHE_DIR = Path(user_cache_dir("black", version=__version__))
 # types
 FileContent = str
 Encoding = str
 # types
 FileContent = str
 Encoding = str

+NewLine = str

 Depth = int
 NodeType = int
 LeafID = int
 Depth = int
 NodeType = int
 LeafID = int


@@ -75,54 +81,27 @@ syms = pygram.python_symbols
 
 
 class NothingChanged(UserWarning):
 
 
 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):
 
 
 class CannotSplit(Exception):

-    """A readable split that fits the allotted line length is impossible.
-
-    Raised by :func:`left_hand_split`, :func:`right_hand_split`, and
-    :func:`delimiter_split`.
-    """
-
-
-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.
-    """
-
-    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 :]
+    """A readable split that fits the allotted line length is impossible."""

 
 

-    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 FormatOn(FormatError):
-    """Found a comment like `# fmt: on` in the file."""
-
-
-class FormatOff(FormatError):
-    """Found a comment like `# fmt: off` in the file."""
+class InvalidInput(ValueError):
+    """Raised when input source code fails all parse attempts."""

 
 
 class WriteBack(Enum):
     NO = 0
     YES = 1
     DIFF = 2
 
 
 class WriteBack(Enum):
     NO = 0
     YES = 1
     DIFF = 2

+    CHECK = 3

 
     @classmethod
     def from_configuration(cls, *, check: bool, diff: bool) -> "WriteBack":
         if check and not diff:
 
     @classmethod
     def from_configuration(cls, *, check: bool, diff: bool) -> "WriteBack":
         if check and not diff:

-            return cls.NO
+            return cls.CHECK

 
         return cls.DIFF if diff else cls.YES
 
 
         return cls.DIFF if diff else cls.YES
 


@@ -133,42 +112,138 @@ class Changed(Enum):
     YES = 2
 
 
     YES = 2
 
 

-class FileMode(Flag):
-    AUTO_DETECT = 0
-    PYTHON36 = 1
-    PYI = 2
-    NO_STRING_NORMALIZATION = 4
+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 = 4
+
+
+VERSION_TO_FEATURES: Dict[TargetVersion, Set[Feature]] = {
+    TargetVersion.PY27: set(),
+    TargetVersion.PY33: {Feature.UNICODE_LITERALS},
+    TargetVersion.PY34: {Feature.UNICODE_LITERALS},
+    TargetVersion.PY35: {Feature.UNICODE_LITERALS, Feature.TRAILING_COMMA},
+    TargetVersion.PY36: {
+        Feature.UNICODE_LITERALS,
+        Feature.F_STRINGS,
+        Feature.NUMERIC_UNDERSCORES,
+        Feature.TRAILING_COMMA,
+    },
+    TargetVersion.PY37: {
+        Feature.UNICODE_LITERALS,
+        Feature.F_STRINGS,
+        Feature.NUMERIC_UNDERSCORES,
+        Feature.TRAILING_COMMA,
+    },
+    TargetVersion.PY38: {
+        Feature.UNICODE_LITERALS,
+        Feature.F_STRINGS,
+        Feature.NUMERIC_UNDERSCORES,
+        Feature.TRAILING_COMMA,
+    },
+}

 
 

-    @classmethod
-    def from_configuration(
-        cls, *, py36: bool, pyi: bool, skip_string_normalization: bool
-    ) -> "FileMode":
-        mode = cls.AUTO_DETECT
-        if py36:
-            mode |= cls.PYTHON36
-        if pyi:
-            mode |= cls.PYI
-        if skip_string_normalization:
-            mode |= cls.NO_STRING_NORMALIZATION
-        return mode
-
-
-@click.command()
+
+@dataclass
+class FileMode:
+    target_versions: Set[TargetVersion] = 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)
+
+
+def supports_feature(target_versions: Set[TargetVersion], feature: Feature) -> bool:
+    return all(feature in VERSION_TO_FEATURES[version] for version in target_versions)
+
+
+def read_pyproject_toml(
+    ctx: click.Context, param: click.Parameter, value: Union[str, int, bool, None]
+) -> 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.
+    """
+    assert not isinstance(value, (int, bool)), "Invalid parameter type passed"
+    if not value:
+        root = find_project_root(ctx.params.get("src", ()))
+        path = root / "pyproject.toml"
+        if path.is_file():
+            value = str(path)
+        else:
+            return None
+
+    try:
+        pyproject_toml = toml.load(value)
+        config = pyproject_toml.get("tool", {}).get("black", {})
+    except (toml.TomlDecodeError, OSError) as e:
+        raise click.FileError(
+            filename=value, hint=f"Error reading configuration file: {e}"
+        )
+
+    if not config:
+        return None
+
+    if ctx.default_map is None:
+        ctx.default_map = {}
+    ctx.default_map.update(  # type: ignore  # bad types in .pyi
+        {k.replace("--", "").replace("-", "_"): v for k, v in config.items()}
+    )
+    return value
+
+
+@click.command(context_settings=dict(help_option_names=["-h", "--help"]))

 @click.option(
     "-l",
     "--line-length",
     type=int,
     default=DEFAULT_LINE_LENGTH,
 @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(
     show_default=True,
 )
 @click.option(

-    "--py36",
-    is_flag=True,
+    "-t",
+    "--target-version",
+    type=click.Choice([v.name.lower() for v in TargetVersion]),
+    callback=lambda c, p, v: [TargetVersion[val.upper()] for val in v],
+    multiple=True,

     help=(
     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.  [default: per-file auto-detection]"
+        "Python versions that should be supported by Black's output. [default: "
+        "per-file auto-detection]"

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


@@ -254,61 +329,81 @@ class FileMode(Flag):
     type=click.Path(
         exists=True, file_okay=True, dir_okay=True, readable=True, allow_dash=True
     ),
     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=False, file_okay=True, dir_okay=False, readable=True, allow_dash=False
+    ),
+    is_eager=True,
+    callback=read_pyproject_toml,
+    help="Read configuration from PATH.",

 )
 @click.pass_context
 def main(
     ctx: click.Context,
     line_length: int,
 )
 @click.pass_context
 def main(
     ctx: click.Context,
     line_length: int,

+    target_version: List[TargetVersion],

     check: bool,
     diff: bool,
     fast: bool,
     pyi: bool,
     check: bool,
     diff: bool,
     fast: bool,
     pyi: bool,

-    py36: bool,

     skip_string_normalization: bool,
     quiet: bool,
     verbose: bool,
     include: str,
     exclude: str,
     skip_string_normalization: bool,
     quiet: bool,
     verbose: bool,
     include: str,
     exclude: str,

-    src: List[str],
+    src: Tuple[str],
+    config: Optional[str],

 ) -> None:
     """The uncompromising code formatter."""
     write_back = WriteBack.from_configuration(check=check, diff=diff)
 ) -> None:
     """The uncompromising code formatter."""
     write_back = WriteBack.from_configuration(check=check, diff=diff)

-    mode = FileMode.from_configuration(
-        py36=py36, pyi=pyi, skip_string_normalization=skip_string_normalization
+    if target_version:
+        versions = set(target_version)
+    else:
+        # We'll autodetect later.
+        versions = set()
+    mode = FileMode(
+        target_versions=versions,
+        line_length=line_length,
+        is_pyi=pyi,
+        string_normalization=not skip_string_normalization,

     )
     )

-    report = Report(check=check, quiet=quiet, verbose=verbose)
-    sources: List[Path] = []
+    if config and verbose:
+        out(f"Using configuration from {config}.", bold=False, fg="blue")

     try:
     try:

-        include_regex = re.compile(include)
+        include_regex = re_compile_maybe_verbose(include)

     except re.error:
         err(f"Invalid regular expression for include given: {include!r}")
         ctx.exit(2)
     try:
     except re.error:
         err(f"Invalid regular expression for include given: {include!r}")
         ctx.exit(2)
     try:

-        exclude_regex = re.compile(exclude)
+        exclude_regex = re_compile_maybe_verbose(exclude)

     except re.error:
         err(f"Invalid regular expression for exclude given: {exclude!r}")
         ctx.exit(2)
     except re.error:
         err(f"Invalid regular expression for exclude given: {exclude!r}")
         ctx.exit(2)

+    report = Report(check=check, quiet=quiet, verbose=verbose)

     root = find_project_root(src)
     root = find_project_root(src)

+    sources: Set[Path] = set()

     for s in src:
         p = Path(s)
         if p.is_dir():
     for s in src:
         p = Path(s)
         if p.is_dir():

-            sources.extend(
+            sources.update(

                 gen_python_files_in_dir(p, root, include_regex, exclude_regex, report)
             )
         elif p.is_file() or s == "-":
             # if a file was explicitly given, we don't care about its extension
                 gen_python_files_in_dir(p, root, include_regex, exclude_regex, report)
             )
         elif p.is_file() or s == "-":
             # if a file was explicitly given, we don't care about its extension

-            sources.append(p)
+            sources.add(p)

         else:
             err(f"invalid path: {s}")
     if len(sources) == 0:
         else:
             err(f"invalid path: {s}")
     if len(sources) == 0:

-        out("No paths given. Nothing to do 😴")
+        if verbose or not quiet:
+            out("No paths given. Nothing to do 😴")

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

-        return

 
 

-    elif len(sources) == 1:
+    if len(sources) == 1:

         reformat_one(
         reformat_one(

-            src=sources[0],
-            line_length=line_length,
+            src=sources.pop(),

             fast=fast,
             write_back=write_back,
             mode=mode,
             fast=fast,
             write_back=write_back,
             mode=mode,


@@ -321,7 +416,6 @@ def main(
             loop.run_until_complete(
                 schedule_formatting(
                     sources=sources,
             loop.run_until_complete(
                 schedule_formatting(
                     sources=sources,

-                    line_length=line_length,

                     fast=fast,
                     write_back=write_back,
                     mode=mode,
                     fast=fast,
                     write_back=write_back,
                     mode=mode,


@@ -332,19 +426,15 @@ def main(
             )
         finally:
             shutdown(loop)
             )
         finally:
             shutdown(loop)

-        if not quiet:
-            out("All done! ✨ 🍰 ✨")
-            click.echo(str(report))
+    if verbose or not quiet:
+        bang = "💥 💔 💥" if report.return_code else "✨ 🍰 ✨"
+        out(f"All done! {bang}")
+        click.secho(str(report), err=True)

     ctx.exit(report.return_code)
 
 
 def reformat_one(
     ctx.exit(report.return_code)
 
 
 def reformat_one(

-    src: Path,
-    line_length: int,
-    fast: bool,
-    write_back: WriteBack,
-    mode: FileMode,
-    report: "Report",
+    src: Path, fast: bool, write_back: WriteBack, mode: FileMode, report: "Report"

 ) -> None:
     """Reformat a single file under `src` without spawning child processes.
 
 ) -> None:
     """Reformat a single file under `src` without spawning child processes.
 


@@ -355,35 +445,30 @@ def reformat_one(
     try:
         changed = Changed.NO
         if not src.is_file() and str(src) == "-":
     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, mode=mode
-            ):
+            if format_stdin_to_stdout(fast=fast, write_back=write_back, mode=mode):

                 changed = Changed.YES
         else:
             cache: Cache = {}
             if write_back != WriteBack.DIFF:
                 changed = Changed.YES
         else:
             cache: Cache = {}
             if write_back != WriteBack.DIFF:

-                cache = read_cache(line_length, mode)
+                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(
                 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,
-                mode=mode,
+                src, fast=fast, write_back=write_back, mode=mode

             ):
                 changed = Changed.YES
             ):
                 changed = Changed.YES

-            if write_back == WriteBack.YES and changed is not Changed.NO:
-                write_cache(cache, [src], line_length, mode)
+            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))
 
 
 async def schedule_formatting(
         report.done(src, changed)
     except Exception as exc:
         report.failed(src, str(exc))
 
 
 async def schedule_formatting(

-    sources: List[Path],
-    line_length: int,
+    sources: Set[Path],

     fast: bool,
     write_back: WriteBack,
     mode: FileMode,
     fast: bool,
     write_back: WriteBack,
     mode: FileMode,


@@ -400,91 +485,100 @@ async def schedule_formatting(
     """
     cache: Cache = {}
     if write_back != WriteBack.DIFF:
     """
     cache: Cache = {}
     if write_back != WriteBack.DIFF:

-        cache = read_cache(line_length, mode)
+        cache = read_cache(mode)

         sources, cached = filter_cached(cache, sources)
         sources, cached = filter_cached(cache, sources)

-        for src in cached:
+        for src in sorted(cached):

             report.done(src, Changed.CACHED)
             report.done(src, Changed.CACHED)

+    if not sources:
+        return
+

     cancelled = []
     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 = {
-            loop.run_in_executor(
-                executor,
-                format_file_in_place,
-                src,
-                line_length,
-                fast,
-                write_back,
-                mode,
-                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)
+    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 = {
+        loop.run_in_executor(
+            executor, format_file_in_place, src, fast, mode, 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:
+                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 cancelled:
         await asyncio.gather(*cancelled, loop=loop, return_exceptions=True)

-    if write_back == WriteBack.YES and formatted:
-        write_cache(cache, formatted, line_length, mode)
+    if sources_to_cache:
+        write_cache(cache, sources_to_cache, mode)

 
 
 def format_file_in_place(
     src: Path,
 
 
 def format_file_in_place(
     src: Path,

-    line_length: int,

     fast: bool,
     fast: bool,

+    mode: FileMode,

     write_back: WriteBack = WriteBack.NO,
     write_back: WriteBack = WriteBack.NO,

-    mode: FileMode = FileMode.AUTO_DETECT,

     lock: Any = None,  # multiprocessing.Manager().Lock() is some crazy proxy
 ) -> bool:
     """Format file under `src` path. Return True if changed.
 
     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.
+    If `write_back` is DIFF, write a diff to stdout. If it is YES, write reformatted
+    code to the file.

     `line_length` and `fast` options are passed to :func:`format_file_contents`.
     """
     if src.suffix == ".pyi":
     `line_length` and `fast` options are passed to :func:`format_file_contents`.
     """
     if src.suffix == ".pyi":

-        mode |= FileMode.PYI
-    with tokenize.open(src) as src_buffer:
-        src_contents = src_buffer.read()
+        mode = evolve(mode, is_pyi=True)
+
+    then = datetime.utcfromtimestamp(src.stat().st_mtime)
+    with open(src, "rb") as buf:
+        src_contents, encoding, newline = decode_bytes(buf.read())

     try:
     try:

-        dst_contents = format_file_contents(
-            src_contents, line_length=line_length, fast=fast, mode=mode
-        )
+        dst_contents = format_file_contents(src_contents, fast=fast, mode=mode)

     except NothingChanged:
         return False
 
     if write_back == write_back.YES:
     except NothingChanged:
         return False
 
     if write_back == write_back.YES:

-        with open(src, "w", encoding=src_buffer.encoding) as f:
+        with open(src, "w", encoding=encoding, newline=newline) as f:

             f.write(dst_contents)
     elif write_back == write_back.DIFF:
             f.write(dst_contents)
     elif write_back == write_back.DIFF:

-        src_name = f"{src}  (original)"
-        dst_name = f"{src}  (formatted)"
+        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:
         diff_contents = diff(src_contents, dst_contents, src_name, dst_name)
         if lock:
             lock.acquire()
         try:

-            sys.stdout.write(diff_contents)
+            f = io.TextIOWrapper(
+                sys.stdout.buffer,
+                encoding=encoding,
+                newline=newline,
+                write_through=True,
+            )
+            f.write(diff_contents)
+            f.detach()

         finally:
             if lock:
                 lock.release()
         finally:
             if lock:
                 lock.release()


@@ -492,41 +586,40 @@ def format_file_in_place(
 
 
 def format_stdin_to_stdout(
 
 
 def format_stdin_to_stdout(

-    line_length: int,
-    fast: bool,
-    write_back: WriteBack = WriteBack.NO,
-    mode: FileMode = FileMode.AUTO_DETECT,
+    fast: bool, *, write_back: WriteBack = WriteBack.NO, mode: FileMode

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

-    If `write_back` is True, write reformatted code back to stdout.
-    `line_length`, `fast`, `is_pyi`, and `force_py36` arguments are passed to
+    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`.
     """
     :func:`format_file_contents`.
     """

-    src = sys.stdin.read()
+    then = datetime.utcnow()
+    src, encoding, newline = decode_bytes(sys.stdin.buffer.read())

     dst = src
     try:
     dst = src
     try:

-        dst = format_file_contents(src, line_length=line_length, fast=fast, mode=mode)
+        dst = format_file_contents(src, fast=fast, mode=mode)

         return True
 
     except NothingChanged:
         return False
 
     finally:
         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:
         if write_back == WriteBack.YES:

-            sys.stdout.write(dst)
+            f.write(dst)

         elif write_back == WriteBack.DIFF:
         elif write_back == WriteBack.DIFF:

-            src_name = "<stdin>  (original)"
-            dst_name = "<stdin>  (formatted)"
-            sys.stdout.write(diff(src, dst, src_name, dst_name))
+            now = datetime.utcnow()
+            src_name = f"STDIN\t{then} +0000"
+            dst_name = f"STDOUT\t{now} +0000"
+            f.write(diff(src, dst, src_name, dst_name))
+        f.detach()

 
 
 def format_file_contents(
 
 
 def format_file_contents(

-    src_contents: str,
-    *,
-    line_length: int,
-    fast: bool,
-    mode: FileMode = FileMode.AUTO_DETECT,
+    src_contents: str, *, fast: bool, mode: FileMode

 ) -> FileContent:
     """Reformat contents a file and return new contents.
 
 ) -> FileContent:
     """Reformat contents a file and return new contents.
 


@@ -537,35 +630,36 @@ def format_file_contents(
     if src_contents.strip() == "":
         raise NothingChanged
 
     if src_contents.strip() == "":
         raise NothingChanged
 

-    dst_contents = format_str(src_contents, line_length=line_length, mode=mode)
+    dst_contents = format_str(src_contents, mode=mode)

     if src_contents == dst_contents:
         raise NothingChanged
 
     if not fast:
         assert_equivalent(src_contents, dst_contents)
     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, mode=mode)
+        assert_stable(src_contents, dst_contents, mode=mode)

     return dst_contents
 
 
     return dst_contents
 
 

-def format_str(
-    src_contents: str, line_length: int, *, mode: FileMode = FileMode.AUTO_DETECT
-) -> FileContent:
+def format_str(src_contents: str, *, mode: FileMode) -> FileContent:

     """Reformat a string and return new contents.
 
     `line_length` determines how many characters per line are allowed.
     """
     """Reformat a string and return new contents.
 
     `line_length` determines how many characters per line are allowed.
     """

-    src_node = lib2to3_parse(src_contents)
+    src_node = lib2to3_parse(src_contents.lstrip(), mode.target_versions)

     dst_contents = ""
     future_imports = get_future_imports(src_node)
     dst_contents = ""
     future_imports = get_future_imports(src_node)

-    is_pyi = bool(mode & FileMode.PYI)
-    py36 = bool(mode & FileMode.PYTHON36) or is_python36(src_node)
-    normalize_strings = not bool(mode & FileMode.NO_STRING_NORMALIZATION)
+    if mode.target_versions:
+        versions = mode.target_versions
+    else:
+        versions = detect_target_versions(src_node)
+    normalize_fmt_off(src_node)

     lines = LineGenerator(
     lines = LineGenerator(

-        remove_u_prefix=py36 or "unicode_literals" in future_imports,
-        is_pyi=is_pyi,
-        normalize_strings=normalize_strings,
+        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=is_pyi)
+    elt = EmptyLineTracker(is_pyi=mode.is_pyi)

     empty_line = Line()
     after = 0
     for current_line in lines.visit(src_node):
     empty_line = Line()
     after = 0
     for current_line in lines.visit(src_node):


@@ -574,11 +668,32 @@ def format_str(
         before, after = elt.maybe_empty_lines(current_line)
         for _ in range(before):
             dst_contents += str(empty_line)
         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):
+        for line in split_line(
+            current_line,
+            line_length=mode.line_length,
+            supports_trailing_commas=supports_feature(versions, Feature.TRAILING_COMMA),
+        ):

             dst_contents += str(line)
     return dst_contents
 
 
             dst_contents += str(line)
     return 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
+
+

 GRAMMARS = [
     pygram.python_grammar_no_print_statement_no_exec_statement,
     pygram.python_grammar_no_print_statement,
 GRAMMARS = [
     pygram.python_grammar_no_print_statement_no_exec_statement,
     pygram.python_grammar_no_print_statement,


@@ -586,13 +701,25 @@ GRAMMARS = [
 ]
 
 
 ]
 
 

-def lib2to3_parse(src_txt: str) -> Node:
+def get_grammars(target_versions: Set[TargetVersion]) -> List[Grammar]:
+    if not target_versions:
+        return GRAMMARS
+    elif all(not version.is_python2() for version in target_versions):
+        # Python 2-compatible code, so don't try Python 3 grammar.
+        return [
+            pygram.python_grammar_no_print_statement_no_exec_statement,
+            pygram.python_grammar_no_print_statement,
+        ]
+    else:
+        return [pygram.python_grammar]
+
+
+def lib2to3_parse(src_txt: str, target_versions: Iterable[TargetVersion] = ()) -> Node:

     """Given a string with source, return the lib2to3 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)
         drv = driver.Driver(grammar, pytree.convert)
         try:
             result = drv.parse_string(src_txt, True)


@@ -605,7 +732,7 @@ def lib2to3_parse(src_txt: str) -> Node:
                 faulty_line = lines[lineno - 1]
             except IndexError:
                 faulty_line = "<line number missing in source>"
                 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
 
     else:
         raise exc from None
 


@@ -674,18 +801,18 @@ class DebugVisitor(Visitor[T]):
             out(f" {node.value!r}", fg="blue", bold=False)
 
     @classmethod
             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()
         """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}
 WHITESPACE = {token.DEDENT, token.INDENT, token.NEWLINE}

-FLOW_CONTROL = {"return", "raise", "break", "continue"}

 STATEMENT = {
     syms.if_stmt,
     syms.while_stmt,
 STATEMENT = {
     syms.if_stmt,
     syms.while_stmt,


@@ -697,6 +824,7 @@ STATEMENT = {
     syms.classdef,
 }
 STANDALONE_COMMENT = 153
     syms.classdef,
 }
 STANDALONE_COMMENT = 153

+token.tok_name[STANDALONE_COMMENT] = "STANDALONE_COMMENT"

 LOGIC_OPERATORS = {"and", "or"}
 COMPARATORS = {
     token.LESS,
 LOGIC_OPERATORS = {"and", "or"}
 COMPARATORS = {
     token.LESS,


@@ -735,6 +863,7 @@ UNPACKING_PARENTS = {
     syms.dictsetmaker,
     syms.listmaker,
     syms.testlist_gexp,
     syms.dictsetmaker,
     syms.listmaker,
     syms.testlist_gexp,

+    syms.testlist_star_expr,

 }
 TEST_DESCENDANTS = {
     syms.test,
 }
 TEST_DESCENDANTS = {
     syms.test,


@@ -802,8 +931,8 @@ class BracketTracker:
     bracket_match: Dict[Tuple[Depth, NodeType], Leaf] = Factory(dict)
     delimiters: Dict[LeafID, Priority] = Factory(dict)
     previous: Optional[Leaf] = None
     bracket_match: Dict[Tuple[Depth, NodeType], Leaf] = Factory(dict)
     delimiters: Dict[LeafID, Priority] = Factory(dict)
     previous: Optional[Leaf] = None

-    _for_loop_variable: int = 0
-    _lambda_arguments: int = 0
+    _for_loop_depths: List[int] = Factory(list)
+    _lambda_argument_depths: List[int] = Factory(list)

 
     def mark(self, leaf: Leaf) -> None:
         """Mark `leaf` with bracket-related metadata. Keep track of delimiters.
 
     def mark(self, leaf: Leaf) -> None:
         """Mark `leaf` with bracket-related metadata. Keep track of delimiters.


@@ -876,16 +1005,21 @@ class BracketTracker:
         """
         if leaf.type == token.NAME and leaf.value == "for":
             self.depth += 1
         """
         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."""
             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.depth -= 1

-            self._for_loop_variable -= 1
+            self._for_loop_depths.pop()

             return True
 
         return False
             return True
 
         return False


@@ -898,16 +1032,20 @@ class BracketTracker:
         """
         if leaf.type == token.NAME and leaf.value == "lambda":
             self.depth += 1
         """
         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."""
             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.depth -= 1

-            self._lambda_arguments -= 1
+            self._lambda_argument_depths.pop()

             return True
 
         return False
             return True
 
         return False


@@ -923,7 +1061,9 @@ class Line:
 
     depth: int = 0
     leaves: List[Leaf] = Factory(list)
 
     depth: int = 0
     leaves: List[Leaf] = Factory(list)

-    comments: List[Tuple[Index, Leaf]] = Factory(list)
+    # The LeafID keys of comments must remain ordered by the corresponding leaf's index
+    # in leaves
+    comments: Dict[LeafID, List[Leaf]] = Factory(dict)

     bracket_tracker: BracketTracker = Factory(BracketTracker)
     inside_brackets: bool = False
     should_explode: bool = False
     bracket_tracker: BracketTracker = Factory(BracketTracker)
     inside_brackets: bool = False
     should_explode: bool = False


@@ -1057,6 +1197,13 @@ class Line:
 
         return False
 
 
         return False
 

+    def contains_multiline_strings(self) -> bool:
+        for leaf in self.leaves:
+            if is_multiline_string(leaf):
+                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 (
     def maybe_remove_trailing_comma(self, closing: Leaf) -> bool:
         """Remove trailing comma if there is one and it's safe."""
         if not (


@@ -1082,7 +1229,7 @@ class Line:
             self.remove_trailing_comma()
             return True
 
             self.remove_trailing_comma()
             return True
 

-        # Otheriwsse, if the trailing one is the only one, we might mistakenly
+        # Otherwise, 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
         # change a tuple into a different type by removing the comma.
         depth = closing.bracket_depth + 1
         commas = 0


@@ -1123,56 +1270,51 @@ class Line:
         if comment.type != token.COMMENT:
             return False
 
         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:
             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
-
+            leaf_id = id(self.leaves[-1])
+            if leaf_id not in self.comments:
+                self.comments[leaf_id] = [comment]

             else:
             else:

-                return
+                self.comments[leaf_id].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."""
 
     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)
+        # Remember, the LeafID keys of self.comments are ordered by the
+        # corresponding leaf's index in self.leaves
+        # If id(self.leaves[-2]) is in self.comments, the order doesn't change.
+        # Otherwise, we insert it into self.comments, and it becomes the last entry.
+        # However, since we delete id(self.leaves[-1]) from self.comments, the invariant
+        # is maintained
+        self.comments.setdefault(id(self.leaves[-2]), []).extend(
+            self.comments.get(id(self.leaves[-1]), [])
+        )
+        self.comments.pop(id(self.leaves[-1]), None)

         self.leaves.pop()
 
     def is_complex_subscript(self, leaf: Leaf) -> bool:
         """Return True iff `leaf` is part of a slice with non-trivial exprs."""
         self.leaves.pop()
 
     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 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()
         )
         return subscript_start is not None and any(
             n.type in TEST_DESCENDANTS for n in subscript_start.pre_order()
         )


@@ -1188,7 +1330,7 @@ class Line:
         res = f"{first.prefix}{indent}{first.value}"
         for leaf in leaves:
             res += str(leaf)
         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"
 
             res += str(comment)
         return res + "\n"
 


@@ -1197,55 +1339,6 @@ class Line:
         return bool(self.leaves or self.comments)
 
 
         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
 @dataclass
 class EmptyLineTracker:
     """Provides a stateful method that returns the number of potential extra


@@ -1267,9 +1360,6 @@ class EmptyLineTracker:
         This is for separating `def`, `async def` and `class` with extra empty
         lines (two on module-level).
         """
         This is for separating `def`, `async def` and `class` with extra empty
         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
         self.previous_after = after
         before, after = self._maybe_empty_lines(current_line)
         before -= self.previous_after
         self.previous_after = after


@@ -1295,44 +1385,8 @@ class EmptyLineTracker:
                 before = 0 if depth else 1
             else:
                 before = 1 if depth else 2
                 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.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:
-                        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
 
         if (
             self.previous_line


@@ -1351,6 +1405,50 @@ class EmptyLineTracker:
 
         return before, 0
 
 
         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]):
 
 @dataclass
 class LineGenerator(Visitor[Line]):


@@ -1365,7 +1463,7 @@ class LineGenerator(Visitor[Line]):
     current_line: Line = Factory(Line)
     remove_u_prefix: bool = False
 
     current_line: Line = 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.
         """Generate a line.
 
         If the line is empty, only emit if it makes sense.


@@ -1374,67 +1472,41 @@ class LineGenerator(Visitor[Line]):
         If any lines were generated, set up a new current_line.
         """
         if not self.current_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
             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
 
         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()
     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 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 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]:
         yield from super().visit_default(node)
 
     def visit_INDENT(self, node: Node) -> Iterator[Line]:


@@ -1531,23 +1603,10 @@ class LineGenerator(Visitor[Line]):
         yield from self.visit_default(leaf)
         yield from self.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)
-
-        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)
-
-            if node.type == token.ENDMARKER:
-                # somebody decided not to put a final `# fmt: on`
-                yield from self.line()
+    def visit_STANDALONE_COMMENT(self, leaf: Leaf) -> Iterator[Line]:
+        if not self.current_line.bracket_tracker.any_open_brackets():
+            yield from self.line()
+        yield from self.visit_default(leaf)

 
     def __attrs_post_init__(self) -> None:
         """You are in a twisty little maze of passages."""
 
     def __attrs_post_init__(self) -> None:
         """You are in a twisty little maze of passages."""


@@ -1569,6 +1628,7 @@ class LineGenerator(Visitor[Line]):
         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_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
 
         self.visit_async_funcdef = self.visit_async_stmt
         self.visit_decorated = self.visit_decorators
 


@@ -1581,7 +1641,7 @@ BRACKETS = OPENING_BRACKETS | CLOSING_BRACKETS
 ALWAYS_NO_SPACE = CLOSING_BRACKETS | {token.COMMA, STANDALONE_COMMENT}
 
 
 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
     """Return whitespace prefix if needed for the given `leaf`.
 
     `complex_subscript` signals whether the given leaf is part of a subscription


@@ -1790,7 +1850,7 @@ def whitespace(leaf: Leaf, *, complex_subscript: bool) -> str:  # noqa C901
             elif prevp.type == token.EQUAL and prevp_parent.type == syms.argument:
                 return NO
 
             elif 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:
             return NO
 
     elif p.type == syms.import_from:


@@ -1837,7 +1897,32 @@ def child_towards(ancestor: Node, descendant: LN) -> Optional[LN]:
     return node
 
 
     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) -> int:

     """Return the priority of the `leaf` delimiter, given a line break after it.
 
     The delimiter priorities returned here are from those delimiters that would
     """Return the priority of the `leaf` delimiter, given a line break after it.
 
     The delimiter priorities returned here are from those delimiters that would


@@ -1851,8 +1936,8 @@ def is_split_after_delimiter(leaf: Leaf, previous: Leaf = None) -> int:
     return 0
 
 
     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) -> int:
+    """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.
 
     The delimiter priorities returned here are from those delimiters that would
     cause a line break before themselves.


@@ -1889,15 +1974,20 @@ def is_split_before_delimiter(leaf: Leaf, previous: Leaf = None) -> int:
     ):
         return STRING_PRIORITY
 
     ):
         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}
         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"
 
     if (
         leaf.value == "if"


@@ -1942,6 +2032,10 @@ def is_split_before_delimiter(leaf: Leaf, previous: Leaf = None) -> int:
     return 0
 
 
     return 0
 
 

+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.
 
 def generate_comments(leaf: LN) -> Iterator[Leaf]:
     """Clean the prefix of the `leaf` and generate comments from it, if any.
 


@@ -1961,16 +2055,38 @@ def generate_comments(leaf: LN) -> Iterator[Leaf]:
     Inline comments are emitted as regular token.COMMENT leaves.  Standalone
     are emitted with a fake STANDALONE_COMMENT token identifier.
     """
     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)
+
+
+@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

 
 

-    if "#" not in p:
-        return
+
+@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
 
     consumed = 0
     nlines = 0

-    for index, line in enumerate(p.split("\n")):
+    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:
         consumed += len(line) + 1  # adding the length of the split '\n'
         line = line.lstrip()
         if not line:


@@ -1978,32 +2094,25 @@ def generate_comments(leaf: LN) -> Iterator[Leaf]:
         if not line.startswith("#"):
             continue
 
         if not line.startswith("#"):
             continue
 

-        if index == 0 and leaf.type != token.ENDMARKER:
+        if index == 0 and not is_endmarker:

             comment_type = token.COMMENT  # simple trailing comment
         else:
             comment_type = STANDALONE_COMMENT
         comment = make_comment(line)
             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
         nlines = 0

+    return result

 
 
 def make_comment(content: str) -> str:
     """Return a consistently formatted comment from the given `content` string.
 
 
 
 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.
     """
 
     If `content` didn't start with a hash sign, one is provided.
     """


@@ -2013,13 +2122,16 @@ def make_comment(content: str) -> str:
 
     if content[0] == "#":
         content = content[1:]
 
     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(
         content = " " + content
     return "#" + content
 
 
 def split_line(

-    line: Line, line_length: int, inner: bool = False, py36: bool = False
+    line: Line,
+    line_length: int,
+    inner: bool = False,
+    supports_trailing_commas: bool = False,

 ) -> Iterator[Line]:
     """Split a `line` into potentially many lines.
 
 ) -> Iterator[Line]:
     """Split a `line` into potentially many lines.
 


@@ -2028,16 +2140,26 @@ def split_line(
     current `line`, possibly transitively. This means we can fallback to splitting
     by delimiters if the LHS/RHS don't yield any results.
 
     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.
+    If `supports_trailing_commas` is True, splitting may use the TRAILING_COMMA feature.

     """
     """

-    if isinstance(line, UnformattedLines) or line.is_comment:
+    if line.is_comment:

         yield line
         return
 
     line_str = str(line).strip("\n")
         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
+
+    # we don't want to split special comments like type annotations
+    # https://github.com/python/typing/issues/186
+    has_special_comment = False
+    for leaf in line.leaves:
+        for comment in line.comments_after(leaf):
+            if leaf.type == token.COMMA and is_special_comment(comment):
+                has_special_comment = True
+
+    if (
+        not has_special_comment
+        and not line.should_explode
+        and is_line_short_enough(line, line_length=line_length, line_str=line_str)

     ):
         yield line
         return
     ):
         yield line
         return


@@ -2047,9 +2169,13 @@ def split_line(
         split_funcs = [left_hand_split]
     else:
 
         split_funcs = [left_hand_split]
     else:
 

-        def rhs(line: Line, py36: bool = False) -> Iterator[Line]:
+        def rhs(line: Line, supports_trailing_commas: bool = False) -> Iterator[Line]:

             for omit in generate_trailers_to_omit(line, line_length):
             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, supports_trailing_commas, omit=omit
+                    )
+                )

                 if is_line_short_enough(lines[0], line_length=line_length):
                     yield from lines
                     return
                 if is_line_short_enough(lines[0], line_length=line_length):
                     yield from lines
                     return


@@ -2057,7 +2183,7 @@ 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.
             # 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)
+            yield from right_hand_split(line, supports_trailing_commas)

 
         if line.inside_brackets:
             split_funcs = [delimiter_split, standalone_comment_split, rhs]
 
         if line.inside_brackets:
             split_funcs = [delimiter_split, standalone_comment_split, rhs]


@@ -2069,14 +2195,19 @@ def split_line(
         # split altogether.
         result: List[Line] = []
         try:
         # split altogether.
         result: List[Line] = []
         try:

-            for l in split_func(line, py36):
+            for l in split_func(line, supports_trailing_commas):

                 if str(l).strip("\n") == line_str:
                     raise CannotSplit("Split function returned an unchanged result")
 
                 result.extend(
                 if str(l).strip("\n") == line_str:
                     raise CannotSplit("Split function returned an unchanged result")
 
                 result.extend(

-                    split_line(l, line_length=line_length, inner=True, py36=py36)
+                    split_line(
+                        l,
+                        line_length=line_length,
+                        inner=True,
+                        supports_trailing_commas=supports_trailing_commas,
+                    )

                 )
                 )

-        except CannotSplit as cs:
+        except CannotSplit:

             continue
 
         else:
             continue
 
         else:


@@ -2087,16 +2218,15 @@ def split_line(
         yield line
 
 
         yield line
 
 

-def left_hand_split(line: Line, py36: bool = False) -> Iterator[Line]:
+def left_hand_split(
+    line: Line, supports_trailing_commas: bool = False
+) -> 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.
     """
     """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.
     """

-    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] = []
     tail_leaves: List[Leaf] = []
     body_leaves: List[Leaf] = []
     head_leaves: List[Leaf] = []


@@ -2114,15 +2244,12 @@ def left_hand_split(line: Line, py36: bool = False) -> Iterator[Line]:
             if leaf.type in OPENING_BRACKETS:
                 matching_bracket = leaf
                 current_leaves = body_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)
+    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:
     bracket_split_succeeded_or_raise(head, body, tail)
     for result in (head, body, tail):
         if result:


@@ -2130,7 +2257,10 @@ def left_hand_split(line: Line, py36: bool = False) -> Iterator[Line]:
 
 
 def right_hand_split(
 
 
 def right_hand_split(

-    line: Line, line_length: int, py36: bool = False, omit: Collection[LeafID] = ()
+    line: Line,
+    line_length: int,
+    supports_trailing_commas: bool = False,
+    omit: Collection[LeafID] = (),

 ) -> Iterator[Line]:
     """Split line into many lines, starting with the last matching bracket pair.
 
 ) -> Iterator[Line]:
     """Split line into many lines, starting with the last matching bracket pair.
 


@@ -2140,9 +2270,6 @@ def right_hand_split(
 
     Note: running this function modifies `bracket_depth` on the leaves of `line`.
     """
 
     Note: running this function modifies `bracket_depth` on the leaves of `line`.
     """

-    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] = []
     tail_leaves: List[Leaf] = []
     body_leaves: List[Leaf] = []
     head_leaves: List[Leaf] = []


@@ -2159,49 +2286,65 @@ def right_hand_split(
                 opening_bracket = leaf.opening_bracket
                 closing_bracket = leaf
                 current_leaves = body_leaves
                 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
+    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")
 
         # 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)
+    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)
     bracket_split_succeeded_or_raise(head, body, tail)

-    assert opening_bracket and closing_bracket

     if (
     if (

+        # the body shouldn't be exploded
+        not body.should_explode

         # the opening bracket is an optional paren
         # the opening bracket is an optional paren

-        opening_bracket.type == token.LPAR
+        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
         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
         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}
     ):
         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,
+                supports_trailing_commas=supports_trailing_commas,
+                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)
 
     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
     for result in (head, body, tail):
         if result:
             yield result


@@ -2233,6 +2376,35 @@ def bracket_split_succeeded_or_raise(head: Line, body: Line, tail: Line) -> None
             )
 
 
             )
 
 

+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 when expected.
+            if original.is_import:
+                if leaves[-1].type != token.COMMA:
+                    leaves.append(Leaf(token.COMMA, ","))
+    # 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:
     """Normalize prefix of the first leaf in every line returned by `split_func`.
 
 def dont_increase_indentation(split_func: SplitFunc) -> SplitFunc:
     """Normalize prefix of the first leaf in every line returned by `split_func`.
 


@@ -2240,8 +2412,10 @@ def dont_increase_indentation(split_func: SplitFunc) -> SplitFunc:
     """
 
     @wraps(split_func)
     """
 
     @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, supports_trailing_commas: bool = False
+    ) -> Iterator[Line]:
+        for l in split_func(line, supports_trailing_commas):

             normalize_prefix(l.leaves[0], inside_brackets=True)
             yield l
 
             normalize_prefix(l.leaves[0], inside_brackets=True)
             yield l
 


@@ -2249,11 +2423,13 @@ def dont_increase_indentation(split_func: SplitFunc) -> SplitFunc:
 
 
 @dont_increase_indentation
 
 
 @dont_increase_indentation

-def delimiter_split(line: Line, py36: bool = False) -> Iterator[Line]:
+def delimiter_split(
+    line: Line, supports_trailing_commas: bool = False
+) -> Iterator[Line]:

     """Split according to delimiters of the highest priority.
 
     """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 `supports_trailing_commas` is True, the split will add trailing commas
+    also in function signatures that contain `*` and `**`.

     """
     try:
         last_leaf = line.leaves[-1]
     """
     try:
         last_leaf = line.leaves[-1]


@@ -2279,23 +2455,23 @@ def delimiter_split(line: Line, py36: bool = False) -> Iterator[Line]:
         nonlocal current_line
         try:
             current_line.append_safe(leaf, preformatted=True)
         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)
 
             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)
 
         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
         ):
             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
+            trailing_comma_safe = trailing_comma_safe and supports_trailing_commas

         leaf_priority = bt.delimiters.get(id(leaf))
         if leaf_priority == delimiter_priority:
             yield current_line
         leaf_priority = bt.delimiters.get(id(leaf))
         if leaf_priority == delimiter_priority:
             yield current_line


@@ -2313,7 +2489,9 @@ def delimiter_split(line: Line, py36: bool = False) -> Iterator[Line]:
 
 
 @dont_increase_indentation
 
 
 @dont_increase_indentation

-def standalone_comment_split(line: Line, py36: bool = False) -> Iterator[Line]:
+def standalone_comment_split(
+    line: Line, supports_trailing_commas: bool = False
+) -> 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")
     """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")


@@ -2325,16 +2503,16 @@ def standalone_comment_split(line: Line, py36: bool = False) -> Iterator[Line]:
         nonlocal current_line
         try:
             current_line.append_safe(leaf, preformatted=True)
         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)
 
             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)
 
         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:
             yield from append_to_line(comment_after)
 
     if current_line:


@@ -2355,6 +2533,16 @@ def is_import(leaf: Leaf) -> bool:
     )
 
 
     )
 
 

+def is_special_comment(leaf: Leaf) -> 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 bool(
+        (t == token.COMMENT or t == STANDALONE_COMMENT) and (v.startswith("# type:"))
+    )
+
+

 def normalize_prefix(leaf: Leaf, *, inside_brackets: bool) -> None:
     """Leave existing extra newlines if not `inside_brackets`. Remove everything
     else.
 def normalize_prefix(leaf: Leaf, *, inside_brackets: bool) -> None:
     """Leave existing extra newlines if not `inside_brackets`. Remove everything
     else.


@@ -2416,8 +2604,8 @@ def normalize_string_quotes(leaf: Leaf) -> None:
 
     prefix = leaf.value[:first_quote_pos]
     unescaped_new_quote = re.compile(rf"(([^\\]|^)(\\\\)*){new_quote}")
 
     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):
     body = leaf.value[first_quote_pos + len(orig_quote) : -len(orig_quote)]
     if "r" in prefix.casefold():
         if unescaped_new_quote.search(body):


@@ -2428,15 +2616,21 @@ def normalize_string_quotes(leaf: Leaf) -> None:
         # Do not introduce or remove backslashes in raw strings
         new_body = body
     else:
         # 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:
         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)
             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"[^{]\{(.*?)\}[^}]", new_body)
+        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("\\")
         # edge case:
         new_body = new_body[:-1] + '\\"'
     orig_escape_count = body.count("\\")


@@ -2450,6 +2644,51 @@ def normalize_string_quotes(leaf: Leaf) -> None:
     leaf.value = f"{prefix}{new_quote}{new_body}{new_quote}"
 
 
     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.
 
 def normalize_invisible_parens(node: Node, parens_after: Set[str]) -> None:
     """Make existing optional parentheses invisible or create new ones.
 


@@ -2459,16 +2698,20 @@ def normalize_invisible_parens(node: Node, parens_after: Set[str]) -> None:
     Standardizes on visible parentheses for single-element tuples, and keeps
     existing visible parentheses for other tuples and generator expressions.
     """
     Standardizes on visible parentheses for single-element tuples, and keeps
     existing visible parentheses for other tuples and generator expressions.
     """

-    try:
-        list(generate_comments(node))
-    except FormatOff:
-        return  # This `node` has a prefix with `# fmt: off`, don't mess with parens.
+    for pc in list_comments(node.prefix, is_endmarker=False):
+        if pc.value in FMT_OFF:
+            # This `node` has a prefix with `# fmt: off`, don't mess with parens.
+            return

 
     check_lpar = False
     for index, child in enumerate(list(node.children)):
         if check_lpar:
             if child.type == syms.atom:
 
     check_lpar = False
     for index, child in enumerate(list(node.children)):
         if check_lpar:
             if child.type == syms.atom:

-                maybe_make_parens_invisible_in_atom(child)
+                if maybe_make_parens_invisible_in_atom(child):
+                    lpar = Leaf(token.LPAR, "")
+                    rpar = Leaf(token.RPAR, "")
+                    index = child.remove() or 0
+                    node.insert_child(index, Node(syms.atom, [lpar, child, rpar]))

             elif is_one_tuple(child):
                 # wrap child in visible parentheses
                 lpar = Leaf(token.LPAR, "(")
             elif is_one_tuple(child):
                 # wrap child in visible parentheses
                 lpar = Leaf(token.LPAR, "(")


@@ -2498,8 +2741,89 @@ def normalize_invisible_parens(node: Node, parens_after: Set[str]) -> None:
         check_lpar = isinstance(child, Leaf) and child.value in parens_after
 
 
         check_lpar = isinstance(child, Leaf) and child.value in parens_after
 
 

+def normalize_fmt_off(node: Node) -> None:
+    """Convert content between `# fmt: off`/`# fmt: on` into standalone comments."""
+    try_again = True
+    while try_again:
+        try_again = convert_one_fmt_off_pair(node)
+
+
+def convert_one_fmt_off_pair(node: Node) -> bool:
+    """Convert content of a single `# fmt: off`/`# fmt: on` into a standalone comment.
+
+    Returns True if a pair was converted.
+    """
+    for leaf in node.leaves():
+        previous_consumed = 0
+        for comment in list_comments(leaf.prefix, is_endmarker=False):
+            if comment.value 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 = 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:
+        for comment in list_comments(container.prefix, is_endmarker=False):
+            if comment.value in FMT_ON:
+                return
+
+        yield container
+
+        container = container.next_sibling
+
+

 def maybe_make_parens_invisible_in_atom(node: LN) -> bool:
 def maybe_make_parens_invisible_in_atom(node: LN) -> bool:

-    """If it's safe, make the parens in the atom `node` invisible, recusively."""
+    """If it's safe, make the parens in the atom `node` invisible, recursively.
+
+    Returns whether the node should itself be wrapped in invisible parentheses.
+
+    """

     if (
         node.type != syms.atom
         or is_empty_tuple(node)
     if (
         node.type != syms.atom
         or is_empty_tuple(node)


@@ -2517,9 +2841,9 @@ def maybe_make_parens_invisible_in_atom(node: LN) -> bool:
         last.value = ""  # type: ignore
         if len(node.children) > 1:
             maybe_make_parens_invisible_in_atom(node.children[1])
         last.value = ""  # type: ignore
         if len(node.children) > 1:
             maybe_make_parens_invisible_in_atom(node.children[1])

-        return True
+        return False

 
 

-    return False
+    return True

 
 
 def is_empty_tuple(node: LN) -> bool:
 
 
 def is_empty_tuple(node: LN) -> bool:


@@ -2676,6 +3000,7 @@ 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?"""
 
 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}
     if not (
         opening_bracket.parent
         and opening_bracket.parent.type in {syms.atom, syms.import_from}


@@ -2693,18 +3018,24 @@ def should_explode(line: Line, opening_bracket: Leaf) -> bool:
     return max_priority == COMMA_PRIORITY
 
 
     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:
 
     Currently looking for:

-    - f-strings; and
+    - f-strings;
+    - underscores in numeric literals; and

     - trailing commas after * or ** in function signatures and calls.
     """
     - trailing commas after * or ** in function signatures and calls.
     """

+    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"}:
     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 in {syms.typedargslist, syms.arglist}
 
         elif (
             n.type in {syms.typedargslist, syms.arglist}


@@ -2713,14 +3044,22 @@ def is_python36(node: Node) -> bool:
         ):
             for ch in n.children:
                 if ch.type in STARS:
         ):
             for ch in n.children:
                 if ch.type in STARS:

-                    return True
+                    features.add(Feature.TRAILING_COMMA)

 
                 if ch.type == syms.argument:
                     for argch in ch.children:
                         if argch.type in STARS:
 
                 if ch.type == syms.argument:
                     for argch in ch.children:
                         if argch.type in STARS:

-                            return True
+                            features.add(Feature.TRAILING_COMMA)

 
 

-    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]]:
 
 
 def generate_trailers_to_omit(line: Line, line_length: int) -> Iterator[Set[LeafID]]:


@@ -2739,7 +3078,6 @@ def generate_trailers_to_omit(line: Line, line_length: int) -> Iterator[Set[Leaf
     length = 4 * line.depth
     opening_bracket = None
     closing_bracket = None
     length = 4 * line.depth
     opening_bracket = None
     closing_bracket = None

-    optional_brackets: Set[LeafID] = set()

     inner_brackets: Set[LeafID] = set()
     for index, leaf, leaf_length in enumerate_with_length(line, reversed=True):
         length += leaf_length
     inner_brackets: Set[LeafID] = set()
     for index, leaf, leaf_length in enumerate_with_length(line, reversed=True):
         length += leaf_length


@@ -2750,17 +3088,12 @@ def generate_trailers_to_omit(line: Line, line_length: int) -> Iterator[Set[Leaf
         if leaf.type == STANDALONE_COMMENT or has_inline_comment:
             break
 
         if leaf.type == STANDALONE_COMMENT or has_inline_comment:
             break
 

-        optional_brackets.discard(id(leaf))

         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 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
             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


@@ -2768,18 +3101,36 @@ def generate_trailers_to_omit(line: Line, line_length: int) -> Iterator[Set[Leaf
                 inner_brackets.add(id(leaf))
                 continue
 
                 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
             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."""
 
 
 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:
+                assert False, "Invalid syntax parsing imports"
+

     for child in node.children:
         if child.type != syms.simple_stmt:
             break
     for child in node.children:
         if child.type != syms.simple_stmt:
             break


@@ -2798,15 +3149,7 @@ def get_future_imports(node: Node) -> Set[str]:
             module_name = first_child.children[1]
             if not isinstance(module_name, Leaf) or module_name.value != "__future__":
                 break
             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
         else:
             break
     return imports


@@ -2822,16 +3165,28 @@ def gen_python_files_in_dir(
     """Generate all files under `path` whose paths are not excluded by the
     `exclude` regex, but are included by the `include` regex.
 
     """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():
     `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():

-        normalized_path = child.resolve().relative_to(root).as_posix()
+        try:
+            normalized_path = "/" + child.resolve().relative_to(root).as_posix()
+        except ValueError:
+            if child.is_symlink():
+                report.path_ignored(
+                    child, f"is a symbolic link that points outside {root}"
+                )
+                continue
+
+            raise
+

         if child.is_dir():
             normalized_path += "/"
         exclude_match = exclude.search(normalized_path)
         if exclude_match and exclude_match.group(0):
         if child.is_dir():
             normalized_path += "/"
         exclude_match = exclude.search(normalized_path)
         if exclude_match and exclude_match.group(0):

-            report.path_ignored(child, f"matches --exclude={exclude.pattern}")
+            report.path_ignored(child, f"matches the --exclude regular expression")

             continue
 
         if child.is_dir():
             continue
 
         if child.is_dir():


@@ -2843,7 +3198,8 @@ def gen_python_files_in_dir(
                 yield child
 
 
                 yield child
 
 

-def find_project_root(srcs: List[str]) -> Path:
+@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
     """Return a directory containing .git, .hg, or pyproject.toml.
 
     That directory can be one of the directories passed in `srcs` or their


@@ -2918,7 +3274,7 @@ class Report:
         - otherwise return 0.
         """
         # According to http://tldp.org/LDP/abs/html/exitcodes.html starting with
         - 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
 
         if self.failure_count:
             return 123
 


@@ -2977,7 +3333,16 @@ def assert_equivalent(src: str, dst: str) -> None:
 
             if isinstance(value, list):
                 for item in value:
 
             if isinstance(value, list):
                 for item in value:

-                    if isinstance(item, ast.AST):
+                    # Ignore nested tuples within del statements, because we may insert
+                    # parentheses and they change the AST.
+                    if (
+                        field == "targets"
+                        and isinstance(node, ast.Delete)
+                        and isinstance(item, ast.Tuple)
+                    ):
+                        for item in item.elts:
+                            yield from _v(item, depth + 2)
+                    elif isinstance(item, ast.AST):

                         yield from _v(item, depth + 2)
 
             elif isinstance(value, ast.AST):
                         yield from _v(item, depth + 2)
 
             elif isinstance(value, ast.AST):


@@ -3020,11 +3385,9 @@ def assert_equivalent(src: str, dst: str) -> None:
         ) from None
 
 
         ) from None
 
 

-def assert_stable(
-    src: str, dst: str, line_length: int, mode: FileMode = FileMode.AUTO_DETECT
-) -> None:
+def assert_stable(src: str, dst: str, mode: FileMode) -> None:

     """Raise AssertionError if `dst` reformats differently the second time."""
     """Raise AssertionError if `dst` reformats differently the second time."""

-    newdst = format_str(dst, line_length=line_length, mode=mode)
+    newdst = format_str(dst, mode=mode)

     if dst != newdst:
         log = dump_to_file(
             diff(src, dst, "source", "first pass"),
     if dst != newdst:
         log = dump_to_file(
             diff(src, dst, "source", "first pass"),


@@ -3101,6 +3464,16 @@ def sub_twice(regex: Pattern[str], replacement: str, original: str) -> str:
     return regex.sub(replacement, regex.sub(replacement, original))
 
 
     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
+    return re.compile(regex)
+
+

 def enumerate_reversed(sequence: Sequence[T]) -> Iterator[Tuple[Index, T]]:
     """Like `reversed(enumerate(sequence))` if that were possible."""
     index = len(sequence) - 1
 def enumerate_reversed(sequence: Sequence[T]) -> Iterator[Tuple[Index, T]]:
     """Like `reversed(enumerate(sequence))` if that were possible."""
     index = len(sequence) - 1


@@ -3126,7 +3499,7 @@ def enumerate_with_length(
             return  # Multiline strings, we can't continue.
 
         comment: Optional[Leaf]
             return  # Multiline strings, we can't continue.
 
         comment: Optional[Leaf]

-        for comment in line.comments_after(leaf, index):
+        for comment in line.comments_after(leaf):

             length += len(comment.value)
 
         yield index, leaf, length
             length += len(comment.value)
 
         yield index, leaf, length


@@ -3146,6 +3519,42 @@ 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?
 
 def can_omit_invisible_parens(line: Line, line_length: int) -> bool:
     """Does `line` have a shape safe to reformat without optional parens around it?
 


@@ -3235,21 +3644,16 @@ def can_omit_invisible_parens(line: Line, line_length: int) -> bool:
     return False
 
 
     return False
 
 

-def get_cache_file(line_length: int, mode: FileMode) -> Path:
-    pyi = bool(mode & FileMode.PYI)
-    py36 = bool(mode & FileMode.PYTHON36)
-    return (
-        CACHE_DIR
-        / f"cache.{line_length}{'.pyi' if pyi else ''}{'.py36' if py36 else ''}.pickle"
-    )
+def get_cache_file(mode: FileMode) -> Path:
+    return CACHE_DIR / f"cache.{mode.get_cache_key()}.pickle"

 
 
 
 

-def read_cache(line_length: int, mode: FileMode) -> Cache:
+def read_cache(mode: FileMode) -> 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.
     """
     """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, mode)
+    cache_file = get_cache_file(mode)

     if not cache_file.exists():
         return {}
 
     if not cache_file.exists():
         return {}
 


@@ -3268,38 +3672,62 @@ def get_cache_info(path: Path) -> CacheInfo:
     return stat.st_mtime, stat.st_size
 
 
     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):
     for src in sources:
         src = src.resolve()
         if cache.get(src) != get_cache_info(src):

-            todo.append(src)
+            todo.add(src)

         else:
         else:

-            done.append(src)
+            done.add(src)

     return todo, done
 
 
     return todo, done
 
 

-def write_cache(
-    cache: Cache, sources: List[Path], line_length: int, mode: FileMode
-) -> None:
+def write_cache(cache: Cache, sources: Iterable[Path], mode: FileMode) -> None:

     """Update the cache file."""
     """Update the cache file."""

-    cache_file = get_cache_file(line_length, mode)
+    cache_file = get_cache_file(mode)

     try:
     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}}
         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=pickle.HIGHEST_PROTOCOL)
+        os.replace(f.name, cache_file)

     except OSError:
         pass
 
 
     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()
     main()

+
+
+if __name__ == "__main__":
+    patched_main()