Fix async blackd tests which won't fail currently (#966)

[etc/vim.git] / black.py

diff --git a/black.py b/black.py
index 85cb45bded5cd7794d6d69bc2f4adea22aa10dda..05edf1a2ae320c3bd3b9a11c57b2edd3a314a4cf 100644 (file)
--- a/black.py
+++ b/black.py
@@ -1,20 +1,23 @@
+import ast

 import asyncio
 import asyncio

-from asyncio.base_events import BaseEventLoop

 from concurrent.futures import Executor, ProcessPoolExecutor
 from concurrent.futures import Executor, ProcessPoolExecutor

+from contextlib import contextmanager

 from datetime import datetime
 from datetime import datetime

-from enum import Enum, Flag
+from enum import Enum

 from functools import lru_cache, partial, wraps
 import io
 from functools import lru_cache, partial, wraps
 import io

-import keyword
+import itertools

 import logging
 import logging

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

 import os
 from pathlib import Path
 import pickle
 import re
 import signal
 import sys
 import os
 from pathlib import Path
 import pickle
 import re
 import signal
 import sys

+import tempfile

 import tokenize
 import tokenize

+import traceback

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


@@ -36,21 +39,23 @@ from typing import (
 )
 
 from appdirs import user_cache_dir
 )
 
 from appdirs import user_cache_dir

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

 import click
 import toml
 import click
 import toml

+from typed_ast import ast3, ast27

 
 # 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__ = "18.6b4"
+__version__ = "19.3b0"

 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__))


@@ -66,7 +71,7 @@ LeafID = int
 Priority = int
 Index = int
 LN = Union[Leaf, Node]
 Priority = int
 Index = int
 LN = Union[Leaf, Node]

-SplitFunc = Callable[["Line", bool], Iterator["Line"]]
+SplitFunc = Callable[["Line", Collection["Feature"]], Iterator["Line"]]

 Timestamp = float
 FileSize = int
 CacheInfo = Tuple[Timestamp, FileSize]
 Timestamp = float
 FileSize = int
 CacheInfo = Tuple[Timestamp, FileSize]


@@ -79,15 +84,15 @@ 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.
+    """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 InvalidInput(ValueError):
+    """Raised when input source code fails all parse attempts."""

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


@@ -110,24 +115,101 @@ 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_IN_CALL = 4
+    TRAILING_COMMA_IN_DEF = 5
+    # The following two feature-flags are mutually exclusive, and exactly one should be
+    # set for every version of python.
+    ASYNC_IDENTIFIERS = 6
+    ASYNC_KEYWORDS = 7
+    ASSIGNMENT_EXPRESSIONS = 8
+    POS_ONLY_ARGUMENTS = 9
+
+
+VERSION_TO_FEATURES: Dict[TargetVersion, Set[Feature]] = {
+    TargetVersion.PY27: {Feature.ASYNC_IDENTIFIERS},
+    TargetVersion.PY33: {Feature.UNICODE_LITERALS, Feature.ASYNC_IDENTIFIERS},
+    TargetVersion.PY34: {Feature.UNICODE_LITERALS, Feature.ASYNC_IDENTIFIERS},
+    TargetVersion.PY35: {
+        Feature.UNICODE_LITERALS,
+        Feature.TRAILING_COMMA_IN_CALL,
+        Feature.ASYNC_IDENTIFIERS,
+    },
+    TargetVersion.PY36: {
+        Feature.UNICODE_LITERALS,
+        Feature.F_STRINGS,
+        Feature.NUMERIC_UNDERSCORES,
+        Feature.TRAILING_COMMA_IN_CALL,
+        Feature.TRAILING_COMMA_IN_DEF,
+        Feature.ASYNC_IDENTIFIERS,
+    },
+    TargetVersion.PY37: {
+        Feature.UNICODE_LITERALS,
+        Feature.F_STRINGS,
+        Feature.NUMERIC_UNDERSCORES,
+        Feature.TRAILING_COMMA_IN_CALL,
+        Feature.TRAILING_COMMA_IN_DEF,
+        Feature.ASYNC_KEYWORDS,
+    },
+    TargetVersion.PY38: {
+        Feature.UNICODE_LITERALS,
+        Feature.F_STRINGS,
+        Feature.NUMERIC_UNDERSCORES,
+        Feature.TRAILING_COMMA_IN_CALL,
+        Feature.TRAILING_COMMA_IN_DEF,
+        Feature.ASYNC_KEYWORDS,
+        Feature.ASSIGNMENT_EXPRESSIONS,
+        Feature.POS_ONLY_ARGUMENTS,
+    },
+}

 
 

-    @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
+
+@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(
 
 
 def read_pyproject_toml(


@@ -151,7 +233,9 @@ def read_pyproject_toml(
         pyproject_toml = toml.load(value)
         config = pyproject_toml.get("tool", {}).get("black", {})
     except (toml.TomlDecodeError, OSError) as e:
         pyproject_toml = toml.load(value)
         config = pyproject_toml.get("tool", {}).get("black", {})
     except (toml.TomlDecodeError, OSError) as e:

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

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


@@ -165,6 +249,7 @@ def read_pyproject_toml(
 
 
 @click.command(context_settings=dict(help_option_names=["-h", "--help"]))
 
 
 @click.command(context_settings=dict(help_option_names=["-h", "--help"]))

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

 @click.option(
     "-l",
     "--line-length",
 @click.option(
     "-l",
     "--line-length",


@@ -173,13 +258,25 @@ def read_pyproject_toml(
     help="How many characters per line to allow.",
     show_default=True,
 )
     help="How many characters per line to allow.",
     show_default=True,
 )

+@click.option(
+    "-t",
+    "--target-version",
+    type=click.Choice([v.name.lower() for v in TargetVersion]),
+    callback=lambda c, p, v: [TargetVersion[val.upper()] for val in v],
+    multiple=True,
+    help=(
+        "Python versions that should be supported by Black's output. [default: "
+        "per-file auto-detection]"
+    ),
+)

 @click.option(
     "--py36",
     is_flag=True,
     help=(
         "Allow using Python 3.6-only syntax on all input files.  This will put "
         "trailing commas in function signatures and calls also after *args and "
 @click.option(
     "--py36",
     is_flag=True,
     help=(
         "Allow using Python 3.6-only syntax on all input files.  This will put "
         "trailing commas in function signatures and calls also after *args and "

-        "**kwargs.  [default: per-file auto-detection]"
+        "**kwargs. Deprecated; use --target-version instead. "
+        "[default: per-file auto-detection]"

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


@@ -245,7 +342,7 @@ def read_pyproject_toml(
     "--quiet",
     is_flag=True,
     help=(
     "--quiet",
     is_flag=True,
     help=(

-        "Don't emit non-error messages to stderr. Errors are still emitted, "
+        "Don't emit non-error messages to stderr. Errors are still emitted; "

         "silence those with 2>/dev/null."
     ),
 )
         "silence those with 2>/dev/null."
     ),
 )


@@ -279,7 +376,9 @@ def read_pyproject_toml(
 @click.pass_context
 def main(
     ctx: click.Context,
 @click.pass_context
 def main(
     ctx: click.Context,

+    code: Optional[str],

     line_length: int,
     line_length: int,

+    target_version: List[TargetVersion],

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


@@ -295,11 +394,32 @@ def main(
 ) -> 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:
+        if py36:
+            err(f"Cannot use both --target-version and --py36")
+            ctx.exit(2)
+        else:
+            versions = set(target_version)
+    elif py36:
+        err(
+            "--py36 is deprecated and will be removed in a future version. "
+            "Use --target-version py36 instead."
+        )
+        versions = PY36_VERSIONS
+    else:
+        # We'll autodetect later.
+        versions = set()
+    mode = FileMode(
+        target_versions=versions,
+        line_length=line_length,
+        is_pyi=pyi,
+        string_normalization=not skip_string_normalization,

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

+    if code is not None:
+        print(format_str(code, mode=mode))
+        ctx.exit(0)

     try:
         include_regex = re_compile_maybe_verbose(include)
     except re.error:
     try:
         include_regex = re_compile_maybe_verbose(include)
     except re.error:


@@ -332,102 +452,105 @@ def main(
     if len(sources) == 1:
         reformat_one(
             src=sources.pop(),
     if len(sources) == 1:
         reformat_one(
             src=sources.pop(),

-            line_length=line_length,

             fast=fast,
             write_back=write_back,
             mode=mode,
             report=report,
         )
     else:
             fast=fast,
             write_back=write_back,
             mode=mode,
             report=report,
         )
     else:

-        loop = asyncio.get_event_loop()
-        executor = ProcessPoolExecutor(max_workers=os.cpu_count())
-        try:
-            loop.run_until_complete(
-                schedule_formatting(
-                    sources=sources,
-                    line_length=line_length,
-                    fast=fast,
-                    write_back=write_back,
-                    mode=mode,
-                    report=report,
-                    loop=loop,
-                    executor=executor,
-                )
-            )
-        finally:
-            shutdown(loop)
+        reformat_many(
+            sources=sources, fast=fast, write_back=write_back, mode=mode, report=report
+        )
+

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

-        bang = "💥 💔 💥" if report.return_code else "✨ 🍰 ✨"
-        out(f"All done! {bang}")
+        out("Oh no! 💥 💔 💥" if report.return_code else "All done! ✨ 🍰 ✨")

         click.secho(str(report), err=True)
     ctx.exit(report.return_code)
 
 
 def reformat_one(
         click.secho(str(report), err=True)
     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.
 

-    If `quiet` is True, non-error messages are not output. `line_length`,
-    `write_back`, `fast` and `pyi` options are passed to
+    `fast`, `write_back`, and `mode` options are passed to

     :func:`format_file_in_place` or :func:`format_stdin_to_stdout`.
     """
     try:
         changed = Changed.NO
         if not src.is_file() and str(src) == "-":
     :func:`format_file_in_place` or :func:`format_stdin_to_stdout`.
     """
     try:
         changed = Changed.NO
         if not src.is_file() and str(src) == "-":

-            if format_stdin_to_stdout(
-                line_length=line_length, fast=fast, write_back=write_back, 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
             if (write_back is WriteBack.YES and changed is not Changed.CACHED) or (
                 write_back is WriteBack.CHECK and changed is Changed.NO
             ):
             ):
                 changed = Changed.YES
             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], line_length, mode)
+                write_cache(cache, [src], mode)

         report.done(src, changed)
     except Exception as exc:
         report.failed(src, str(exc))
 
 
         report.done(src, changed)
     except Exception as exc:
         report.failed(src, str(exc))
 
 

+def reformat_many(
+    sources: Set[Path],
+    fast: bool,
+    write_back: WriteBack,
+    mode: FileMode,
+    report: "Report",
+) -> None:
+    """Reformat multiple files using a ProcessPoolExecutor."""
+    loop = asyncio.get_event_loop()
+    worker_count = os.cpu_count()
+    if sys.platform == "win32":
+        # Work around https://bugs.python.org/issue26903
+        worker_count = min(worker_count, 61)
+    executor = ProcessPoolExecutor(max_workers=worker_count)
+    try:
+        loop.run_until_complete(
+            schedule_formatting(
+                sources=sources,
+                fast=fast,
+                write_back=write_back,
+                mode=mode,
+                report=report,
+                loop=loop,
+                executor=executor,
+            )
+        )
+    finally:
+        shutdown(loop)
+        executor.shutdown()
+
+

 async def schedule_formatting(
     sources: Set[Path],
 async def schedule_formatting(
     sources: Set[Path],

-    line_length: int,

     fast: bool,
     write_back: WriteBack,
     mode: FileMode,
     report: "Report",
     fast: bool,
     write_back: WriteBack,
     mode: FileMode,
     report: "Report",

-    loop: BaseEventLoop,
+    loop: asyncio.AbstractEventLoop,

     executor: Executor,
 ) -> None:
     """Run formatting of `sources` in parallel using the provided `executor`.
 
     (Use ProcessPoolExecutors for actual parallelism.)
 
     executor: Executor,
 ) -> None:
     """Run formatting of `sources` in parallel using the provided `executor`.
 
     (Use ProcessPoolExecutors for actual parallelism.)
 

-    `line_length`, `write_back`, `fast`, and `pyi` options are passed to
+    `write_back`, `fast`, and `mode` options are passed to

     :func:`format_file_in_place`.
     """
     cache: Cache = {}
     if write_back != WriteBack.DIFF:
     :func:`format_file_in_place`.
     """
     cache: Cache = {}
     if write_back != WriteBack.DIFF:

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

         sources, cached = filter_cached(cache, sources)
         for src in sorted(cached):
             report.done(src, Changed.CACHED)
         sources, cached = filter_cached(cache, sources)
         for src in sorted(cached):
             report.done(src, Changed.CACHED)


@@ -443,19 +566,14 @@ async def schedule_formatting(
         manager = Manager()
         lock = manager.Lock()
     tasks = {
         manager = Manager()
         lock = manager.Lock()
     tasks = {

-        loop.run_in_executor(
-            executor,
-            format_file_in_place,
-            src,
-            line_length,
-            fast,
-            write_back,
-            mode,
-            lock,
+        asyncio.ensure_future(
+            loop.run_in_executor(
+                executor, format_file_in_place, src, fast, mode, write_back, lock
+            )

         ): src
         for src in sorted(sources)
     }
         ): src
         for src in sorted(sources)
     }

-    pending: Iterable[asyncio.Task] = tasks.keys()
+    pending: Iterable[asyncio.Future] = tasks.keys()

     try:
         loop.add_signal_handler(signal.SIGINT, cancel, pending)
         loop.add_signal_handler(signal.SIGTERM, cancel, pending)
     try:
         loop.add_signal_handler(signal.SIGINT, cancel, pending)
         loop.add_signal_handler(signal.SIGTERM, cancel, pending)


@@ -482,33 +600,30 @@ async def schedule_formatting(
     if cancelled:
         await asyncio.gather(*cancelled, loop=loop, return_exceptions=True)
     if sources_to_cache:
     if cancelled:
         await asyncio.gather(*cancelled, loop=loop, return_exceptions=True)
     if sources_to_cache:

-        write_cache(cache, sources_to_cache, line_length, mode)
+        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.
 
     If `write_back` is DIFF, write a diff to stdout. If it is YES, write reformatted
     code to the file.
     lock: Any = None,  # multiprocessing.Manager().Lock() is some crazy proxy
 ) -> bool:
     """Format file under `src` path. Return True if changed.
 
     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`.
+    `mode` and `fast` options are passed to :func:`format_file_contents`.

     """
     if src.suffix == ".pyi":
     """
     if src.suffix == ".pyi":

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

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

     except NothingChanged:
         return False
 
     except NothingChanged:
         return False
 


@@ -520,9 +635,8 @@ def format_file_in_place(
         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)
         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:
+
+        with lock or nullcontext():

             f = io.TextIOWrapper(
                 sys.stdout.buffer,
                 encoding=encoding,
             f = io.TextIOWrapper(
                 sys.stdout.buffer,
                 encoding=encoding,


@@ -531,30 +645,24 @@ def format_file_in_place(
             )
             f.write(diff_contents)
             f.detach()
             )
             f.write(diff_contents)
             f.detach()

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

     return True
 
 
 def format_stdin_to_stdout(
     return True
 
 
 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.
 
     If `write_back` is YES, write reformatted code back to stdout. If it is DIFF,
 ) -> bool:
     """Format file on stdin. Return True if changed.
 
     If `write_back` is YES, write reformatted code back to stdout. If it is DIFF,

-    write a diff to stdout.
-    `line_length`, `fast`, `is_pyi`, and `force_py36` arguments are passed to
+    write a diff to stdout. The `mode` argument is passed to

     :func:`format_file_contents`.
     """
     then = datetime.utcnow()
     src, encoding, newline = decode_bytes(sys.stdin.buffer.read())
     dst = src
     try:
     :func:`format_file_contents`.
     """
     then = datetime.utcnow()
     src, encoding, newline = decode_bytes(sys.stdin.buffer.read())
     dst = src
     try:

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

         return True
 
     except NothingChanged:
         return True
 
     except NothingChanged:


@@ -575,63 +683,66 @@ def format_stdin_to_stdout(
 
 
 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.
 
     If `fast` is False, additionally confirm that the reformatted code is
     valid by calling :func:`assert_equivalent` and :func:`assert_stable` on it.
 ) -> FileContent:
     """Reformat contents a file and return new contents.
 
     If `fast` is False, additionally confirm that the reformatted code is
     valid by calling :func:`assert_equivalent` and :func:`assert_stable` on it.

-    `line_length` is passed to :func:`format_str`.
+    `mode` is passed to :func:`format_str`.

     """
     if src_contents.strip() == "":
         raise NothingChanged
 
     """
     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.
 
     """Reformat a string and return new contents.
 

-    `line_length` determines how many characters per line are allowed.
+    `mode` determines formatting options, such as how many characters per line are
+    allowed.

     """
     """

-    src_node = lib2to3_parse(src_contents)
-    dst_contents = ""
+    src_node = lib2to3_parse(src_contents.lstrip(), mode.target_versions)
+    dst_contents = []

     future_imports = get_future_imports(src_node)
     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(
     normalize_fmt_off(src_node)
     lines = LineGenerator(

-        remove_u_prefix=py36 or "unicode_literals" in future_imports,
-        is_pyi=is_pyi,
-        normalize_strings=normalize_strings,
-        allow_underscores=py36,
+        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
     empty_line = Line()
     after = 0

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

     for current_line in lines.visit(src_node):
         for _ in range(after):
     for current_line in lines.visit(src_node):
         for _ in range(after):

-            dst_contents += str(empty_line)
+            dst_contents.append(str(empty_line))

         before, after = elt.maybe_empty_lines(current_line)
         for _ in range(before):
         before, after = elt.maybe_empty_lines(current_line)
         for _ in range(before):

-            dst_contents += str(empty_line)
-        for line in split_line(current_line, line_length=line_length, py36=py36):
-            dst_contents += str(line)
-    return dst_contents
+            dst_contents.append(str(empty_line))
+        for line in split_line(
+            current_line, line_length=mode.line_length, features=split_line_features
+        ):
+            dst_contents.append(str(line))
+    return "".join(dst_contents)

 
 
 def decode_bytes(src: bytes) -> Tuple[FileContent, Encoding, NewLine]:
 
 
 def decode_bytes(src: bytes) -> Tuple[FileContent, Encoding, NewLine]:


@@ -651,19 +762,50 @@ def decode_bytes(src: bytes) -> Tuple[FileContent, Encoding, NewLine]:
         return tiow.read(), encoding, newline
 
 
         return tiow.read(), encoding, newline
 
 

-GRAMMARS = [
-    pygram.python_grammar_no_print_statement_no_exec_statement,
-    pygram.python_grammar_no_print_statement,
-    pygram.python_grammar,
-]
+def get_grammars(target_versions: Set[TargetVersion]) -> List[Grammar]:
+    if not target_versions:
+        # No target_version specified, so try all grammars.
+        return [
+            # Python 3.7+
+            pygram.python_grammar_no_print_statement_no_exec_statement_async_keywords,
+            # Python 3.0-3.6
+            pygram.python_grammar_no_print_statement_no_exec_statement,
+            # Python 2.7 with future print_function import
+            pygram.python_grammar_no_print_statement,
+            # Python 2.7
+            pygram.python_grammar,
+        ]
+    elif all(version.is_python2() for version in target_versions):
+        # Python 2-only code, so try Python 2 grammars.
+        return [
+            # Python 2.7 with future print_function import
+            pygram.python_grammar_no_print_statement,
+            # Python 2.7
+            pygram.python_grammar,
+        ]
+    else:
+        # Python 3-compatible code, so only try Python 3 grammar.
+        grammars = []
+        # If we have to parse both, try to parse async as a keyword first
+        if not supports_feature(target_versions, Feature.ASYNC_IDENTIFIERS):
+            # Python 3.7+
+            grammars.append(
+                pygram.python_grammar_no_print_statement_no_exec_statement_async_keywords  # noqa: B950
+            )
+        if not supports_feature(target_versions, Feature.ASYNC_KEYWORDS):
+            # Python 3.0-3.6
+            grammars.append(pygram.python_grammar_no_print_statement_no_exec_statement)
+        # At least one of the above branches must have been taken, because every Python
+        # version has exactly one of the two 'ASYNC_*' flags
+        return grammars

 
 
 
 

-def lib2to3_parse(src_txt: str) -> Node:
+def lib2to3_parse(src_txt: str, target_versions: Iterable[TargetVersion] = ()) -> Node:

     """Given a string with source, return the lib2to3 Node."""
     """Given a string with source, return the lib2to3 Node."""

-    grammar = pygram.python_grammar_no_print_statement

     if src_txt[-1:] != "\n":
         src_txt += "\n"
     if src_txt[-1:] != "\n":
         src_txt += "\n"

-    for grammar in GRAMMARS:
+
+    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)


@@ -676,7 +818,7 @@ 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
 


@@ -756,9 +898,7 @@ class DebugVisitor(Visitor[T]):
         list(v.visit(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,


@@ -797,6 +937,7 @@ MATH_OPERATORS = {
     token.DOUBLESTAR,
 }
 STARS = {token.STAR, token.DOUBLESTAR}
     token.DOUBLESTAR,
 }
 STARS = {token.STAR, token.DOUBLESTAR}

+VARARGS_SPECIALS = STARS | {token.SLASH}

 VARARGS_PARENTS = {
     syms.arglist,
     syms.argument,  # double star in arglist
 VARARGS_PARENTS = {
     syms.arglist,
     syms.argument,  # double star in arglist


@@ -877,8 +1018,8 @@ 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.


@@ -924,7 +1065,7 @@ class BracketTracker:
         """Return True if there is an yet unmatched open bracket on the line."""
         return bool(self.bracket_match)
 
         """Return True if there is an yet unmatched open bracket on the line."""
         return bool(self.bracket_match)
 

-    def max_delimiter_priority(self, exclude: Iterable[LeafID] = ()) -> int:
+    def max_delimiter_priority(self, exclude: Iterable[LeafID] = ()) -> Priority:

         """Return the highest priority of a delimiter found on the line.
 
         Values are consistent with what `is_split_*_delimiter()` return.
         """Return the highest priority of a delimiter found on the line.
 
         Values are consistent with what `is_split_*_delimiter()` return.


@@ -932,7 +1073,7 @@ class BracketTracker:
         """
         return max(v for k, v in self.delimiters.items() if k not in exclude)
 
         """
         return max(v for k, v in self.delimiters.items() if k not in exclude)
 

-    def delimiter_count_with_priority(self, priority: int = 0) -> int:
+    def delimiter_count_with_priority(self, priority: Priority = 0) -> int:

         """Return the number of delimiters with the given `priority`.
 
         If no `priority` is passed, defaults to max priority on the line.
         """Return the number of delimiters with the given `priority`.
 
         If no `priority` is passed, defaults to max priority on the line.


@@ -951,16 +1092,21 @@ 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


@@ -973,16 +1119,20 @@ 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


@@ -998,7 +1148,7 @@ 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)
+    comments: Dict[LeafID, List[Leaf]] = Factory(dict)  # keys ordered like `leaves`

     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


@@ -1129,6 +1279,32 @@ class Line:
             if leaf.type == STANDALONE_COMMENT:
                 if leaf.bracket_depth <= depth_limit:
                     return True
             if leaf.type == STANDALONE_COMMENT:
                 if leaf.bracket_depth <= depth_limit:
                     return True

+        return False
+
+    def contains_inner_type_comments(self) -> bool:
+        ignored_ids = set()
+        try:
+            last_leaf = self.leaves[-1]
+            ignored_ids.add(id(last_leaf))
+            if last_leaf.type == token.COMMA or (
+                last_leaf.type == token.RPAR and not last_leaf.value
+            ):
+                # When trailing commas or optional parens are inserted by Black for
+                # consistency, comments after the previous last element are not moved
+                # (they don't have to, rendering will still be correct).  So we ignore
+                # trailing commas and invisible.
+                last_leaf = self.leaves[-2]
+                ignored_ids.add(id(last_leaf))
+        except IndexError:
+            return False
+
+        for leaf_id, comments in self.comments.items():
+            if leaf_id in ignored_ids:
+                continue
+
+            for comment in comments:
+                if is_type_comment(comment):
+                    return True

 
         return False
 
 
         return False
 


@@ -1183,7 +1359,10 @@ class Line:
             bracket_depth = leaf.bracket_depth
             if bracket_depth == depth and leaf.type == token.COMMA:
                 commas += 1
             bracket_depth = leaf.bracket_depth
             if bracket_depth == depth and leaf.type == token.COMMA:
                 commas += 1

-                if leaf.parent and leaf.parent.type == syms.arglist:
+                if leaf.parent and leaf.parent.type in {
+                    syms.arglist,
+                    syms.typedargslist,
+                }:

                     commas += 1
                     break
 
                     commas += 1
                     break
 


@@ -1205,44 +1384,41 @@ 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
 
             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 not self.comments:
-            return
-
-        if _index == -1:
-            for _index, _leaf in enumerate(self.leaves):
-                if leaf is _leaf:
-                    break
-
-            else:
-                return
+        last_leaf = self.leaves[-1]
+        if (
+            last_leaf.type == token.RPAR
+            and not last_leaf.value
+            and last_leaf.parent
+            and len(list(last_leaf.parent.leaves())) <= 3
+            and not is_type_comment(comment)
+        ):
+            # Comments on an optional parens wrapping a single leaf should belong to
+            # the wrapped node except if it's a type comment. Pinning the comment like
+            # this avoids unstable formatting caused by comment migration.
+            if len(self.leaves) < 2:
+                comment.type = STANDALONE_COMMENT
+                comment.prefix = ""
+                return False
+            last_leaf = self.leaves[-2]
+        self.comments.setdefault(id(last_leaf), []).append(comment)
+        return True

 
 

-        for index, comment_after in self.comments:
-            if _index == index:
-                yield comment_after
+    def comments_after(self, leaf: Leaf) -> List[Leaf]:
+        """Generate comments that should appear directly after `leaf`."""
+        return self.comments.get(id(leaf), [])

 
     def remove_trailing_comma(self) -> None:
         """Remove the trailing comma and moves the comments attached to it."""
 
     def remove_trailing_comma(self) -> None:
         """Remove the trailing comma and moves the comments attached to it."""

-        comma_index = len(self.leaves) - 1
-        for i in range(len(self.comments)):
-            comment_index, comment = self.comments[i]
-            if comment_index == comma_index:
-                self.comments[i] = (comma_index - 1, comment)
-        self.leaves.pop()
+        trailing_comma = self.leaves.pop()
+        trailing_comma_comments = self.comments.pop(id(trailing_comma), [])
+        self.comments.setdefault(id(self.leaves[-1]), []).extend(
+            trailing_comma_comments
+        )

 
     def is_complex_subscript(self, leaf: Leaf) -> bool:
         """Return True iff `leaf` is part of a slice with non-trivial exprs."""
 
     def is_complex_subscript(self, leaf: Leaf) -> bool:
         """Return True iff `leaf` is part of a slice with non-trivial exprs."""


@@ -1273,7 +1449,7 @@ 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"
 


@@ -1304,7 +1480,13 @@ class EmptyLineTracker:
         lines (two on module-level).
         """
         before, after = self._maybe_empty_lines(current_line)
         lines (two on module-level).
         """
         before, after = self._maybe_empty_lines(current_line)

-        before -= self.previous_after
+        before = (
+            # Black should not insert empty lines at the beginning
+            # of the file
+            0
+            if self.previous_line is None
+            else before - self.previous_after
+        )

         self.previous_after = after
         self.previous_line = current_line
         return before, after
         self.previous_after = after
         self.previous_line = current_line
         return before, after


@@ -1405,7 +1587,6 @@ class LineGenerator(Visitor[Line]):
     normalize_strings: bool = True
     current_line: Line = Factory(Line)
     remove_u_prefix: bool = False
     normalize_strings: bool = True
     current_line: Line = Factory(Line)
     remove_u_prefix: bool = False

-    allow_underscores: bool = False

 
     def line(self, indent: int = 0) -> Iterator[Line]:
         """Generate a line.
 
     def line(self, indent: int = 0) -> Iterator[Line]:
         """Generate a line.


@@ -1448,11 +1629,44 @@ class LineGenerator(Visitor[Line]):
                 normalize_string_prefix(node, remove_u_prefix=self.remove_u_prefix)
                 normalize_string_quotes(node)
             if node.type == token.NUMBER:
                 normalize_string_prefix(node, remove_u_prefix=self.remove_u_prefix)
                 normalize_string_quotes(node)
             if node.type == token.NUMBER:

-                normalize_numeric_literal(node, self.allow_underscores)
+                normalize_numeric_literal(node)

             if node.type not in WHITESPACE:
                 self.current_line.append(node)
         yield from super().visit_default(node)
 
             if node.type not in WHITESPACE:
                 self.current_line.append(node)
         yield from super().visit_default(node)
 

+    def visit_atom(self, node: Node) -> Iterator[Line]:
+        # Always make parentheses invisible around a single node, because it should
+        # not be needed (except in the case of yield, where removing the parentheses
+        # produces a SyntaxError).
+        if (
+            len(node.children) == 3
+            and isinstance(node.children[0], Leaf)
+            and node.children[0].type == token.LPAR
+            and isinstance(node.children[2], Leaf)
+            and node.children[2].type == token.RPAR
+            and isinstance(node.children[1], Leaf)
+            and not (
+                node.children[1].type == token.NAME
+                and node.children[1].value == "yield"
+            )
+        ):
+            node.children[0].value = ""
+            node.children[2].value = ""
+        yield from super().visit_default(node)
+
+    def visit_factor(self, node: Node) -> Iterator[Line]:
+        """Force parentheses between a unary op and a binary power:
+
+        -2 ** 8 -> -(2 ** 8)
+        """
+        child = node.children[1]
+        if child.type == syms.power and len(child.children) == 3:
+            lpar = Leaf(token.LPAR, "(")
+            rpar = Leaf(token.RPAR, ")")
+            index = child.remove() or 0
+            node.insert_child(index, Node(syms.atom, [lpar, child, rpar]))
+        yield from self.visit_default(node)
+

     def visit_INDENT(self, node: Node) -> Iterator[Line]:
         """Increase indentation level, maybe yield a line."""
         # In blib2to3 INDENT never holds comments.
     def visit_INDENT(self, node: Node) -> Iterator[Line]:
         """Increase indentation level, maybe yield a line."""
         # In blib2to3 INDENT never holds comments.


@@ -1572,6 +1786,7 @@ 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
 


@@ -1584,7 +1799,7 @@ 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


@@ -1641,7 +1856,7 @@ def whitespace(leaf: Leaf, *, complex_subscript: bool) -> str:  # noqa C901
                     # that, too.
                     return prevp.prefix
 
                     # that, too.
                     return prevp.prefix
 

-        elif prevp.type in STARS:
+        elif prevp.type in VARARGS_SPECIALS:

             if is_vararg(prevp, within=VARARGS_PARENTS | UNPACKING_PARENTS):
                 return NO
 
             if is_vararg(prevp, within=VARARGS_PARENTS | UNPACKING_PARENTS):
                 return NO
 


@@ -1731,7 +1946,7 @@ def whitespace(leaf: Leaf, *, complex_subscript: bool) -> str:  # noqa C901
             if not prevp or prevp.type == token.LPAR:
                 return NO
 
             if not prevp or prevp.type == token.LPAR:
                 return NO
 

-        elif prev.type in {token.EQUAL} | STARS:
+        elif prev.type in {token.EQUAL} | VARARGS_SPECIALS:

             return NO
 
     elif p.type == syms.decorator:
             return NO
 
     elif p.type == syms.decorator:


@@ -1865,7 +2080,7 @@ def container_of(leaf: Leaf) -> LN:
     return container
 
 
     return container
 
 

-def is_split_after_delimiter(leaf: Leaf, previous: Leaf = None) -> int:
+def is_split_after_delimiter(leaf: Leaf, previous: Optional[Leaf] = None) -> Priority:

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


@@ -1879,8 +2094,8 @@ 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) -> Priority:
+    """Return the priority of the `leaf` delimiter, given a line break before it.

 
     The delimiter priorities returned here are from those delimiters that would
     cause a line break before themselves.
 
     The delimiter priorities returned here are from those delimiters that would
     cause a line break before themselves.


@@ -1917,15 +2132,20 @@ 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"


@@ -1999,6 +2219,16 @@ def generate_comments(leaf: LN) -> Iterator[Leaf]:
 
 @dataclass
 class ProtoComment:
 
 @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
     type: int  # token.COMMENT or STANDALONE_COMMENT
     value: str  # content of the comment
     newlines: int  # how many newlines before the comment


@@ -2007,21 +2237,28 @@ class ProtoComment:
 
 @lru_cache(maxsize=4096)
 def list_comments(prefix: str, *, is_endmarker: bool) -> List[ProtoComment]:
 
 @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
     result: List[ProtoComment] = []
     if not prefix or "#" not in prefix:
         return result
 
     consumed = 0
     nlines = 0

+    ignored_lines = 0

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

+            # Escaped newlines outside of a comment are not really newlines at
+            # all. We treat a single-line comment following an escaped newline
+            # as a simple trailing comment.
+            if line.endswith("\\"):
+                ignored_lines += 1

             continue
 
             continue
 

-        if index == 0 and not is_endmarker:
+        if index == ignored_lines and not is_endmarker:

             comment_type = token.COMMENT  # simple trailing comment
         else:
             comment_type = STANDALONE_COMMENT
             comment_type = token.COMMENT  # simple trailing comment
         else:
             comment_type = STANDALONE_COMMENT


@@ -2038,8 +2275,8 @@ def list_comments(prefix: str, *, is_endmarker: bool) -> List[ProtoComment]:
 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.
     """


@@ -2049,13 +2286,16 @@ 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,
+    features: Collection[Feature] = (),

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


@@ -2064,16 +2304,18 @@ 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.
+    `features` are syntactical features that may be used in the output.

     """
     if line.is_comment:
         yield line
         return
 
     line_str = str(line).strip("\n")
     """
     if line.is_comment:
         yield line
         return
 
     line_str = str(line).strip("\n")

-    if not line.should_explode and is_line_short_enough(
-        line, line_length=line_length, line_str=line_str
+
+    if (
+        not line.contains_inner_type_comments()
+        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


@@ -2083,9 +2325,9 @@ 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, features: Collection[Feature]) -> 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, features, 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


@@ -2093,7 +2335,7 @@ 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, line_length, features=features)

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


@@ -2105,14 +2347,16 @@ 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, features):

                 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, features=features
+                    )

                 )
                 )

-        except CannotSplit as cs:
+        except CannotSplit:

             continue
 
         else:
             continue
 
         else:


@@ -2123,16 +2367,13 @@ def split_line(
         yield line
 
 
         yield line
 
 

-def left_hand_split(line: Line, py36: bool = False) -> Iterator[Line]:
+def left_hand_split(line: Line, features: Collection[Feature] = ()) -> Iterator[Line]:

     """Split line into many lines, starting with the first matching bracket pair.
 
     Note: this usually looks weird, only use this for function definitions.
     Prefer RHS otherwise.  This is why this function is not symmetrical with
     :func:`right_hand_split` which also handles optional parentheses.
     """
     """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] = []


@@ -2150,15 +2391,12 @@ 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:


@@ -2166,7 +2404,10 @@ 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,
+    features: Collection[Feature] = (),
+    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.
 


@@ -2176,9 +2417,6 @@ 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] = []


@@ -2195,25 +2433,18 @@ 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)
-    assert opening_bracket and closing_bracket
-    body.should_explode = should_explode(body, opening_bracket)
+    tail_leaves.reverse()
+    body_leaves.reverse()
+    head_leaves.reverse()
+    head = bracket_split_build_line(head_leaves, line, opening_bracket)
+    body = bracket_split_build_line(body_leaves, line, opening_bracket, is_body=True)
+    tail = bracket_split_build_line(tail_leaves, line, opening_bracket)

     bracket_split_succeeded_or_raise(head, body, tail)
     if (
         # the body shouldn't be exploded
     bracket_split_succeeded_or_raise(head, body, tail)
     if (
         # the body shouldn't be exploded


@@ -2234,7 +2465,7 @@ def right_hand_split(
     ):
         omit = {id(closing_bracket), *omit}
         try:
     ):
         omit = {id(closing_bracket), *omit}
         try:

-            yield from right_hand_split(line, line_length, py36=py36, omit=omit)
+            yield from right_hand_split(line, line_length, features=features, omit=omit)

             return
 
         except CannotSplit:
             return
 
         except CannotSplit:


@@ -2287,6 +2518,46 @@ 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 for imports and standalone function arguments, but
+            # be careful not to add one after any comments.
+            no_commas = original.is_def and not any(
+                l.type == token.COMMA for l in leaves
+            )
+
+            if original.is_import or no_commas:
+                for i in range(len(leaves) - 1, -1, -1):
+                    if leaves[i].type == STANDALONE_COMMENT:
+                        continue
+                    elif leaves[i].type == token.COMMA:
+                        break
+                    else:
+                        leaves.insert(i + 1, Leaf(token.COMMA, ","))
+                        break
+    # Populate the line
+    for leaf in leaves:
+        result.append(leaf, preformatted=True)
+        for comment_after in original.comments_after(leaf):
+            result.append(comment_after, preformatted=True)
+    if is_body:
+        result.should_explode = should_explode(result, opening_bracket)
+    return result
+
+

 def dont_increase_indentation(split_func: SplitFunc) -> SplitFunc:
     """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`.
 


@@ -2294,8 +2565,8 @@ 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, features: Collection[Feature] = ()) -> Iterator[Line]:
+        for l in split_func(line, features):

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


@@ -2303,11 +2574,11 @@ 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, features: Collection[Feature] = ()) -> 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 the appropriate Features are given, the split will add trailing commas
+    also in function signatures and calls that contain `*` and `**`.

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


@@ -2333,23 +2604,29 @@ 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)
             yield from append_to_line(comment_after)
 
         lowest_depth = min(lowest_depth, leaf.bracket_depth)

-        if leaf.bracket_depth == lowest_depth and is_vararg(
-            leaf, within=VARARGS_PARENTS
-        ):
-            trailing_comma_safe = trailing_comma_safe and py36
+        if leaf.bracket_depth == lowest_depth:
+            if is_vararg(leaf, within={syms.typedargslist}):
+                trailing_comma_safe = (
+                    trailing_comma_safe and Feature.TRAILING_COMMA_IN_DEF in features
+                )
+            elif is_vararg(leaf, within={syms.arglist, syms.argument}):
+                trailing_comma_safe = (
+                    trailing_comma_safe and Feature.TRAILING_COMMA_IN_CALL in features
+                )
+

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


@@ -2367,7 +2644,9 @@ 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, features: Collection[Feature] = ()
+) -> Iterator[Line]:

     """Split standalone comments from the rest of the line."""
     if not line.contains_standalone_comments(0):
         raise CannotSplit("Line does not have any standalone comments")
     """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")


@@ -2379,16 +2658,16 @@ 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:


@@ -2409,6 +2688,14 @@ def is_import(leaf: Leaf) -> bool:
     )
 
 
     )
 
 

+def is_type_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 t in {token.COMMENT, 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.


@@ -2491,7 +2778,15 @@ def normalize_string_quotes(leaf: Leaf) -> None:
         new_body = sub_twice(escaped_orig_quote, rf"\1\2{orig_quote}", new_body)
         new_body = sub_twice(unescaped_new_quote, rf"\1\\{new_quote}", new_body)
     if "f" in prefix.casefold():
         new_body = sub_twice(escaped_orig_quote, rf"\1\2{orig_quote}", new_body)
         new_body = sub_twice(unescaped_new_quote, rf"\1\\{new_quote}", new_body)
     if "f" in prefix.casefold():

-        matches = re.findall(r"[^{]\{(.*?)\}[^}]", new_body)
+        matches = re.findall(
+            r"""
+            (?:[^{]|^)\{  # start of the string or a non-{ followed by a single {
+                ([^{].*?)  # contents of the brackets except if begins with {{
+            \}(?:[^}]|$)  # A } followed by end of the string or a non-}
+            """,
+            new_body,
+            re.VERBOSE,
+        )

         for m in matches:
             if "\\" in str(m):
                 # Do not introduce backslashes in interpolated expressions
         for m in matches:
             if "\\" in str(m):
                 # Do not introduce backslashes in interpolated expressions


@@ -2510,16 +2805,20 @@ 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, allow_underscores: bool) -> None:
+def normalize_numeric_literal(leaf: Leaf) -> None:

     """Normalizes numeric (float, int, and complex) literals.
 
     """Normalizes numeric (float, int, and complex) literals.
 

-    All letters used in the representation are normalized to lowercase, long number
-    literals are split using underscores.
+    All letters used in the representation are normalized to lowercase (except
+    in Python 2 long literals).

     """
     text = leaf.value.lower()
     """
     text = leaf.value.lower()

-    if text.startswith(("0o", "0x", "0b")):
-        # Leave octal, hex, and binary literals alone.
+    if text.startswith(("0o", "0b")):
+        # Leave octal and binary literals alone.

         pass
         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 = ""
     elif "e" in text:
         before, after = text.split("e")
         sign = ""


@@ -2528,51 +2827,33 @@ def normalize_numeric_literal(leaf: Leaf, allow_underscores: bool) -> None:
             sign = "-"
         elif after.startswith("+"):
             after = after[1:]
             sign = "-"
         elif after.startswith("+"):
             after = after[1:]

-        before = format_float_or_int_string(before, allow_underscores)
-        after = format_int_string(after, allow_underscores)
+        before = format_float_or_int_string(before)

         text = f"{before}e{sign}{after}"
     elif text.endswith(("j", "l")):
         number = text[:-1]
         suffix = text[-1]
         text = f"{before}e{sign}{after}"
     elif text.endswith(("j", "l")):
         number = text[:-1]
         suffix = text[-1]

-        text = f"{format_float_or_int_string(number, allow_underscores)}{suffix}"
+        # 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:
     else:

-        text = format_float_or_int_string(text, allow_underscores)
+        text = format_float_or_int_string(text)

     leaf.value = text
 
 
     leaf.value = text
 
 

-def format_float_or_int_string(text: str, allow_underscores: bool) -> str:
+def format_float_or_int_string(text: str) -> str:

     """Formats a float string like "1.0"."""
     if "." not in text:
     """Formats a float string like "1.0"."""
     if "." not in text:

-        return format_int_string(text, allow_underscores)
-
-    before, after = text.split(".")
-    before = format_int_string(before, allow_underscores) if before else "0"
-    after = format_int_string(after, allow_underscores) if after else "0"
-    return f"{before}.{after}"
-
-
-def format_int_string(text: str, allow_underscores: bool) -> str:
-    """Normalizes underscores in a string to e.g. 1_000_000.
-
-    Input must be a string of at least six digits and optional underscores.
-    """
-    if not allow_underscores:

         return text
 
         return text
 

-    text = text.replace("_", "")
-    if len(text) <= 6:
-        # No underscores for numbers <= 6 digits long.
-        return text
-
-    # Avoid removing leading zeros, which are important if we're formatting
-    # part of a number like "0.001".
-    return format(int("1" + text), "3_")[1:].lstrip("_")
+    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.
 

-    `parens_after` is a set of string leaf values immeditely after which parens
+    `parens_after` is a set of string leaf values immediately after which parens

     should be put.
 
     Standardizes on visible parentheses for single-element tuples, and keeps
     should be put.
 
     Standardizes on visible parentheses for single-element tuples, and keeps


@@ -2585,9 +2866,19 @@ def normalize_invisible_parens(node: Node, parens_after: Set[str]) -> None:
 
     check_lpar = False
     for index, child in enumerate(list(node.children)):
 
     check_lpar = False
     for index, child in enumerate(list(node.children)):

+        # Add parentheses around long tuple unpacking in assignments.
+        if (
+            index == 0
+            and isinstance(child, Node)
+            and child.type == syms.testlist_star_expr
+        ):
+            check_lpar = True
+

         if check_lpar:
         if check_lpar:

+            if is_walrus_assignment(child):
+                continue

             if child.type == syms.atom:
             if child.type == syms.atom:

-                if maybe_make_parens_invisible_in_atom(child):
+                if maybe_make_parens_invisible_in_atom(child, parent=node):

                     lpar = Leaf(token.LPAR, "")
                     rpar = Leaf(token.RPAR, "")
                     index = child.remove() or 0
                     lpar = Leaf(token.LPAR, "")
                     rpar = Leaf(token.RPAR, "")
                     index = child.remove() or 0


@@ -2616,7 +2907,11 @@ def normalize_invisible_parens(node: Node, parens_after: Set[str]) -> None:
                 lpar = Leaf(token.LPAR, "")
                 rpar = Leaf(token.RPAR, "")
                 index = child.remove() or 0
                 lpar = Leaf(token.LPAR, "")
                 rpar = Leaf(token.RPAR, "")
                 index = child.remove() or 0

-                node.insert_child(index, Node(syms.atom, [lpar, child, rpar]))
+                prefix = child.prefix
+                child.prefix = ""
+                new_child = Node(syms.atom, [lpar, child, rpar])
+                new_child.prefix = prefix
+                node.insert_child(index, new_child)

 
         check_lpar = isinstance(child, Leaf) and child.value in parens_after
 
 
         check_lpar = isinstance(child, Leaf) and child.value in parens_after
 


@@ -2698,7 +2993,7 @@ def generate_ignored_nodes(leaf: Leaf) -> Iterator[LN]:
         container = container.next_sibling
 
 
         container = container.next_sibling
 
 

-def maybe_make_parens_invisible_in_atom(node: LN) -> bool:
+def maybe_make_parens_invisible_in_atom(node: LN, parent: LN) -> bool:

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


@@ -2708,7 +3003,7 @@ def maybe_make_parens_invisible_in_atom(node: LN) -> bool:
         node.type != syms.atom
         or is_empty_tuple(node)
         or is_one_tuple(node)
         node.type != syms.atom
         or is_empty_tuple(node)
         or is_one_tuple(node)

-        or is_yield(node)
+        or (is_yield(node) and parent.type != syms.expr_stmt)

         or max_delimiter_priority_in_atom(node) >= COMMA_PRIORITY
     ):
         return False
         or max_delimiter_priority_in_atom(node) >= COMMA_PRIORITY
     ):
         return False


@@ -2719,8 +3014,7 @@ def maybe_make_parens_invisible_in_atom(node: LN) -> bool:
         # make parentheses invisible
         first.value = ""  # type: ignore
         last.value = ""  # type: ignore
         # make parentheses invisible
         first.value = ""  # type: ignore
         last.value = ""  # type: ignore

-        if len(node.children) > 1:
-            maybe_make_parens_invisible_in_atom(node.children[1])
+        maybe_make_parens_invisible_in_atom(node.children[1], parent=parent)

         return False
 
     return True
         return False
 
     return True


@@ -2736,18 +3030,24 @@ def is_empty_tuple(node: LN) -> bool:
     )
 
 
     )
 
 

+def unwrap_singleton_parenthesis(node: LN) -> Optional[LN]:
+    """Returns `wrapped` if `node` is of the shape ( wrapped ).
+
+    Parenthesis can be optional. Returns None otherwise"""
+    if len(node.children) != 3:
+        return None
+    lpar, wrapped, rpar = node.children
+    if not (lpar.type == token.LPAR and rpar.type == token.RPAR):
+        return None
+
+    return wrapped
+
+

 def is_one_tuple(node: LN) -> bool:
     """Return True if `node` holds a tuple with one element, with or without parens."""
     if node.type == syms.atom:
 def is_one_tuple(node: LN) -> bool:
     """Return True if `node` holds a tuple with one element, with or without parens."""
     if node.type == syms.atom:

-        if len(node.children) != 3:
-            return False
-
-        lpar, gexp, rpar = node.children
-        if not (
-            lpar.type == token.LPAR
-            and gexp.type == syms.testlist_gexp
-            and rpar.type == token.RPAR
-        ):
+        gexp = unwrap_singleton_parenthesis(node)
+        if gexp is None or gexp.type != syms.testlist_gexp:

             return False
 
         return len(gexp.children) == 2 and gexp.children[1].type == token.COMMA
             return False
 
         return len(gexp.children) == 2 and gexp.children[1].type == token.COMMA


@@ -2759,6 +3059,12 @@ def is_one_tuple(node: LN) -> bool:
     )
 
 
     )
 
 

+def is_walrus_assignment(node: LN) -> bool:
+    """Return True iff `node` is of the shape ( test := test )"""
+    inner = unwrap_singleton_parenthesis(node)
+    return inner is not None and inner.type == syms.namedexpr_test
+
+

 def is_yield(node: LN) -> bool:
     """Return True if `node` holds a `yield` or `yield from` expression."""
     if node.type == syms.yield_expr:
 def is_yield(node: LN) -> bool:
     """Return True if `node` holds a `yield` or `yield from` expression."""
     if node.type == syms.yield_expr:


@@ -2788,7 +3094,7 @@ def is_vararg(leaf: Leaf, within: Set[NodeType]) -> bool:
     extended iterable unpacking (PEP 3132) and additional unpacking
     generalizations (PEP 448).
     """
     extended iterable unpacking (PEP 3132) and additional unpacking
     generalizations (PEP 448).
     """

-    if leaf.type not in STARS or not leaf.parent:
+    if leaf.type not in VARARGS_SPECIALS or not leaf.parent:

         return False
 
     p = leaf.parent
         return False
 
     p = leaf.parent


@@ -2838,7 +3144,7 @@ def is_stub_body(node: LN) -> bool:
     )
 
 
     )
 
 

-def max_delimiter_priority_in_atom(node: LN) -> int:
+def max_delimiter_priority_in_atom(node: LN) -> Priority:

     """Return maximum delimiter priority inside `node`.
 
     This is specific to atoms with contents contained in a pair of parentheses.
     """Return maximum delimiter priority inside `node`.
 
     This is specific to atoms with contents contained in a pair of parentheses.


@@ -2870,7 +3176,7 @@ def ensure_visible(leaf: Leaf) -> None:
     """Make sure parentheses are visible.
 
     They could be invisible as part of some statements (see
     """Make sure parentheses are visible.
 
     They could be invisible as part of some statements (see

-    :func:`normalize_invible_parens` and :func:`visit_import_from`).
+    :func:`normalize_invisible_parens` and :func:`visit_import_from`).

     """
     if leaf.type == token.LPAR:
         leaf.value = "("
     """
     if leaf.type == token.LPAR:
         leaf.value = "("


@@ -2880,6 +3186,7 @@ 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}


@@ -2897,39 +3204,61 @@ 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:
     - f-strings;
 
     Currently looking for:
     - f-strings;

-    - underscores in numeric literals; and
-    - trailing commas after * or ** in function signatures and calls.
+    - underscores in numeric literals;
+    - trailing commas after * or ** in function signatures and calls;
+    - positional only arguments in function signatures and lambdas;

     """
     """

+    features: Set[Feature] = set()

     for n in node.pre_order():
         if n.type == token.STRING:
             value_head = n.value[:2]  # type: ignore
             if value_head in {'f"', 'F"', "f'", "F'", "rf", "fr", "RF", "FR"}:
     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
 
         elif n.type == token.NUMBER:
             if "_" in n.value:  # type: ignore

-                return True
+                features.add(Feature.NUMERIC_UNDERSCORES)
+
+        elif n.type == token.SLASH:
+            if n.parent and n.parent.type in {syms.typedargslist, syms.arglist}:
+                features.add(Feature.POS_ONLY_ARGUMENTS)
+
+        elif n.type == token.COLONEQUAL:
+            features.add(Feature.ASSIGNMENT_EXPRESSIONS)

 
         elif (
             n.type in {syms.typedargslist, syms.arglist}
             and n.children
             and n.children[-1].type == token.COMMA
         ):
 
         elif (
             n.type in {syms.typedargslist, syms.arglist}
             and n.children
             and n.children[-1].type == token.COMMA
         ):

+            if n.type == syms.typedargslist:
+                feature = Feature.TRAILING_COMMA_IN_DEF
+            else:
+                feature = Feature.TRAILING_COMMA_IN_CALL
+

             for ch in n.children:
                 if ch.type in STARS:
             for ch in n.children:
                 if ch.type in STARS:

-                    return True
+                    features.add(feature)

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

-                            return True
+                            features.add(feature)

 
 

-    return False
+    return features
+
+
+def detect_target_versions(node: Node) -> Set[TargetVersion]:
+    """Detect the version to target based on the nodes used."""
+    features = get_features_used(node)
+    return {
+        version for version in TargetVersion if features <= VERSION_TO_FEATURES[version]
+    }

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


@@ -2948,7 +3277,6 @@ 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


@@ -2959,17 +3287,12 @@ 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


@@ -2977,13 +3300,15 @@ 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]:
 
 
 def get_future_imports(node: Node) -> Set[str]:


@@ -3003,7 +3328,7 @@ def get_future_imports(node: Node) -> Set[str]:
             elif child.type == syms.import_as_names:
                 yield from get_imports_from_children(child.children)
             else:
             elif child.type == syms.import_as_names:
                 yield from get_imports_from_children(child.children)
             else:

-                assert False, "Invalid syntax parsing imports"
+                raise AssertionError("Invalid syntax parsing imports")

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


@@ -3187,17 +3512,58 @@ class Report:
         return ", ".join(report) + "."
 
 
         return ", ".join(report) + "."
 
 

+def parse_ast(src: str) -> Union[ast.AST, ast3.AST, ast27.AST]:
+    filename = "<unknown>"
+    if sys.version_info >= (3, 8):
+        # TODO: support Python 4+ ;)
+        for minor_version in range(sys.version_info[1], 4, -1):
+            try:
+                return ast.parse(src, filename, feature_version=(3, minor_version))
+            except SyntaxError:
+                continue
+    else:
+        for feature_version in (7, 6):
+            try:
+                return ast3.parse(src, filename, feature_version=feature_version)
+            except SyntaxError:
+                continue
+
+    return ast27.parse(src)
+
+
+def _fixup_ast_constants(
+    node: Union[ast.AST, ast3.AST, ast27.AST]
+) -> Union[ast.AST, ast3.AST, ast27.AST]:
+    """Map ast nodes deprecated in 3.8 to Constant."""
+    # casts are required until this is released:
+    # https://github.com/python/typeshed/pull/3142
+    if isinstance(node, (ast.Str, ast3.Str, ast27.Str, ast.Bytes, ast3.Bytes)):
+        return cast(ast.AST, ast.Constant(value=node.s))
+    elif isinstance(node, (ast.Num, ast3.Num, ast27.Num)):
+        return cast(ast.AST, ast.Constant(value=node.n))
+    elif isinstance(node, (ast.NameConstant, ast3.NameConstant)):
+        return cast(ast.AST, ast.Constant(value=node.value))
+    return node
+
+

 def assert_equivalent(src: str, dst: str) -> None:
     """Raise AssertionError if `src` and `dst` aren't equivalent."""
 
 def assert_equivalent(src: str, dst: str) -> None:
     """Raise AssertionError if `src` and `dst` aren't equivalent."""
 

-    import ast
-    import traceback
-
-    def _v(node: ast.AST, depth: int = 0) -> Iterator[str]:
+    def _v(node: Union[ast.AST, ast3.AST, ast27.AST], depth: int = 0) -> Iterator[str]:

         """Simple visitor generating strings to compare ASTs by content."""
         """Simple visitor generating strings to compare ASTs by content."""

+
+        node = _fixup_ast_constants(node)
+

         yield f"{'  ' * depth}{node.__class__.__name__}("
 
         for field in sorted(node._fields):
         yield f"{'  ' * depth}{node.__class__.__name__}("
 
         for field in sorted(node._fields):

+            # TypeIgnore has only one field 'lineno' which breaks this comparison
+            type_ignore_classes = (ast3.TypeIgnore, ast27.TypeIgnore)
+            if sys.version_info >= (3, 8):
+                type_ignore_classes += (ast.TypeIgnore,)
+            if isinstance(node, type_ignore_classes):
+                break
+

             try:
                 value = getattr(node, field)
             except AttributeError:
             try:
                 value = getattr(node, field)
             except AttributeError:


@@ -3207,10 +3573,19 @@ 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, ast3.Delete, ast27.Delete))
+                        and isinstance(item, (ast.Tuple, ast3.Tuple, ast27.Tuple))
+                    ):
+                        for item in item.elts:
+                            yield from _v(item, depth + 2)
+                    elif isinstance(item, (ast.AST, ast3.AST, ast27.AST)):

                         yield from _v(item, depth + 2)
 
                         yield from _v(item, depth + 2)
 

-            elif isinstance(value, ast.AST):
+            elif isinstance(value, (ast.AST, ast3.AST, ast27.AST)):

                 yield from _v(value, depth + 2)
 
             else:
                 yield from _v(value, depth + 2)
 
             else:


@@ -3219,22 +3594,20 @@ def assert_equivalent(src: str, dst: str) -> None:
         yield f"{'  ' * depth})  # /{node.__class__.__name__}"
 
     try:
         yield f"{'  ' * depth})  # /{node.__class__.__name__}"
 
     try:

-        src_ast = ast.parse(src)
+        src_ast = parse_ast(src)

     except Exception as exc:
     except Exception as exc:

-        major, minor = sys.version_info[:2]

         raise AssertionError(
         raise AssertionError(

-            f"cannot use --safe with this file; failed to parse source file "
-            f"with Python {major}.{minor}'s builtin AST. Re-run with --fast "
-            f"or stop using deprecated Python 2 syntax. AST error message: {exc}"
+            f"cannot use --safe with this file; failed to parse source file.  "
+            f"AST error message: {exc}"

         )
 
     try:
         )
 
     try:

-        dst_ast = ast.parse(dst)
+        dst_ast = parse_ast(dst)

     except Exception as exc:
         log = dump_to_file("".join(traceback.format_tb(exc.__traceback__)), dst)
         raise AssertionError(
             f"INTERNAL ERROR: Black produced invalid code: {exc}. "
     except Exception as exc:
         log = dump_to_file("".join(traceback.format_tb(exc.__traceback__)), dst)
         raise AssertionError(
             f"INTERNAL ERROR: Black produced invalid code: {exc}. "

-            f"Please report a bug on https://github.com/ambv/black/issues.  "
+            f"Please report a bug on https://github.com/psf/black/issues.  "

             f"This invalid output might be helpful: {log}"
         ) from None
 
             f"This invalid output might be helpful: {log}"
         ) from None
 


@@ -3245,16 +3618,14 @@ def assert_equivalent(src: str, dst: str) -> None:
         raise AssertionError(
             f"INTERNAL ERROR: Black produced code that is not equivalent to "
             f"the source.  "
         raise AssertionError(
             f"INTERNAL ERROR: Black produced code that is not equivalent to "
             f"the source.  "

-            f"Please report a bug on https://github.com/ambv/black/issues.  "
+            f"Please report a bug on https://github.com/psf/black/issues.  "

             f"This diff might be helpful: {log}"
         ) from None
 
 
             f"This diff might be helpful: {log}"
         ) 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"),


@@ -3263,15 +3634,13 @@ def assert_stable(
         raise AssertionError(
             f"INTERNAL ERROR: Black produced different code on the second pass "
             f"of the formatter.  "
         raise AssertionError(
             f"INTERNAL ERROR: Black produced different code on the second pass "
             f"of the formatter.  "

-            f"Please report a bug on https://github.com/ambv/black/issues.  "
+            f"Please report a bug on https://github.com/psf/black/issues.  "

             f"This diff might be helpful: {log}"
         ) from None
 
 
 def dump_to_file(*output: str) -> str:
     """Dump `output` to a temporary file. Return path to the file."""
             f"This diff might be helpful: {log}"
         ) from None
 
 
 def dump_to_file(*output: str) -> str:
     """Dump `output` to a temporary file. Return path to the file."""

-    import tempfile
-

     with tempfile.NamedTemporaryFile(
         mode="w", prefix="blk_", suffix=".log", delete=False, encoding="utf8"
     ) as f:
     with tempfile.NamedTemporaryFile(
         mode="w", prefix="blk_", suffix=".log", delete=False, encoding="utf8"
     ) as f:


@@ -3282,6 +3651,13 @@ def dump_to_file(*output: str) -> str:
     return f.name
 
 
     return f.name
 
 

+@contextmanager
+def nullcontext() -> Iterator[None]:
+    """Return context manager that does nothing.
+    Similar to `nullcontext` from python 3.7"""
+    yield
+
+

 def diff(a: str, b: str, a_name: str, b_name: str) -> str:
     """Return a unified diff string between strings `a` and `b`."""
     import difflib
 def diff(a: str, b: str, a_name: str, b_name: str) -> str:
     """Return a unified diff string between strings `a` and `b`."""
     import difflib


@@ -3300,11 +3676,15 @@ def cancel(tasks: Iterable[asyncio.Task]) -> None:
         task.cancel()
 
 
         task.cancel()
 
 

-def shutdown(loop: BaseEventLoop) -> None:
+def shutdown(loop: asyncio.AbstractEventLoop) -> None:

     """Cancel all pending tasks on `loop`, wait for them, and close the loop."""
     try:
     """Cancel all pending tasks on `loop`, wait for them, and close the loop."""
     try:

+        if sys.version_info[:2] >= (3, 7):
+            all_tasks = asyncio.all_tasks
+        else:
+            all_tasks = asyncio.Task.all_tasks

         # This part is borrowed from asyncio/runners.py in Python 3.7b2.
         # This part is borrowed from asyncio/runners.py in Python 3.7b2.

-        to_cancel = [task for task in asyncio.Task.all_tasks(loop) if not task.done()]
+        to_cancel = [task for task in all_tasks(loop) if not task.done()]

         if not to_cancel:
             return
 
         if not to_cancel:
             return
 


@@ -3365,8 +3745,7 @@ def enumerate_with_length(
         if "\n" in leaf.value:
             return  # Multiline strings, we can't continue.
 
         if "\n" in leaf.value:
             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


@@ -3511,16 +3890,16 @@ 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:
-    return CACHE_DIR / f"cache.{line_length}.{mode.value}.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 {}
 


@@ -3555,17 +3934,15 @@ def filter_cached(cache: Cache, sources: Iterable[Path]) -> Tuple[Set[Path], Set
     return todo, done
 
 
     return todo, done
 
 

-def write_cache(
-    cache: Cache, sources: Iterable[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
 


@@ -3592,6 +3969,11 @@ def patch_click() -> None:
             module._verify_python3_env = lambda: None
 
 
             module._verify_python3_env = lambda: None
 
 

-if __name__ == "__main__":
+def patched_main() -> None:
+    freeze_support()

     patch_click()
     main()
     patch_click()
     main()

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