Bump sphinx from 7.2.3 to 7.2.5 in /docs (#3882)

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

diff --git a/src/black/__init__.py b/src/black/__init__.py
index a985926afa52384d414e0073e8d7a48c11632b43..188a4f79f0e51f027f445525bda3b2e68fd93081 100644 (file)
--- a/src/black/__init__.py
+++ b/src/black/__init__.py
@@ -1,58 +1,87 @@
-import asyncio
-from concurrent.futures import Executor, ThreadPoolExecutor, ProcessPoolExecutor
-from contextlib import contextmanager
-from datetime import datetime
-from enum import Enum

 import io
 import io

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

 import sys
 import tokenize
 import traceback
 import sys
 import tokenize
 import traceback

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

 from typing import (
     Any,
     Dict,
     Generator,
     Iterator,
     List,
 from typing import (
     Any,
     Dict,
     Generator,
     Iterator,
     List,

+    MutableMapping,

     Optional,
     Pattern,
     Optional,
     Pattern,

+    Sequence,

     Set,
     Sized,
     Tuple,
     Union,
 )
 
     Set,
     Sized,
     Tuple,
     Union,
 )
 

-from dataclasses import replace

 import click
 import click

+from click.core import ParameterSource
+from mypy_extensions import mypyc_attr
+from pathspec import PathSpec
+from pathspec.patterns.gitwildmatch import GitWildMatchPatternError

 
 

-from black.const import DEFAULT_LINE_LENGTH, DEFAULT_INCLUDES, DEFAULT_EXCLUDES
-from black.const import STDIN_PLACEHOLDER
-from black.nodes import STARS, syms, is_simple_decorator_expression
-from black.lines import Line, EmptyLineTracker
-from black.linegen import transform_line, LineGenerator, LN
+from _black_version import version as __version__
+from black.cache import Cache

 from black.comments import normalize_fmt_off
 from black.comments import normalize_fmt_off

-from black.mode import Mode, TargetVersion
-from black.mode import Feature, supports_feature, VERSION_TO_FEATURES
-from black.cache import read_cache, write_cache, get_cache_info, filter_cached, Cache
-from black.concurrency import cancel, shutdown, maybe_install_uvloop
-from black.output import dump_to_file, diff, color_diff, out, err
-from black.report import Report, Changed
-from black.files import find_project_root, find_pyproject_toml, parse_pyproject_toml
-from black.files import gen_python_files, get_gitignore, normalize_path_maybe_ignore
-from black.files import wrap_stream_for_windows
+from black.const import (
+    DEFAULT_EXCLUDES,
+    DEFAULT_INCLUDES,
+    DEFAULT_LINE_LENGTH,
+    STDIN_PLACEHOLDER,
+)
+from black.files import (
+    find_project_root,
+    find_pyproject_toml,
+    find_user_pyproject_toml,
+    gen_python_files,
+    get_gitignore,
+    normalize_path_maybe_ignore,
+    parse_pyproject_toml,
+    wrap_stream_for_windows,
+)
+from black.handle_ipynb_magics import (
+    PYTHON_CELL_MAGICS,
+    TRANSFORMED_MAGICS,
+    jupyter_dependencies_are_installed,
+    mask_cell,
+    put_trailing_semicolon_back,
+    remove_trailing_semicolon,
+    unmask_cell,
+)
+from black.linegen import LN, LineGenerator, transform_line
+from black.lines import EmptyLineTracker, LinesBlock
+from black.mode import FUTURE_FLAG_TO_FEATURE, VERSION_TO_FEATURES, Feature
+from black.mode import Mode as Mode  # re-exported
+from black.mode import TargetVersion, supports_feature
+from black.nodes import (
+    STARS,
+    is_number_token,
+    is_simple_decorator_expression,
+    is_string_token,
+    syms,
+)
+from black.output import color_diff, diff, dump_to_file, err, ipynb_diff, out

 from black.parsing import InvalidInput  # noqa F401
 from black.parsing import lib2to3_parse, parse_ast, stringify_ast
 from black.parsing import InvalidInput  # noqa F401
 from black.parsing import lib2to3_parse, parse_ast, stringify_ast

-
-
-# lib2to3 fork
-from blib2to3.pytree import Node, Leaf
+from black.report import Changed, NothingChanged, Report
+from black.trans import iter_fexpr_spans

 from blib2to3.pgen2 import token
 from blib2to3.pgen2 import token

+from blib2to3.pytree import Leaf, Node

 
 

-from _black_version import version as __version__
+COMPILED = Path(__file__).suffix in (".pyd", ".so")

 
 # types
 FileContent = str
 
 # types
 FileContent = str


@@ -60,10 +89,6 @@ Encoding = str
 NewLine = str
 
 
 NewLine = str
 
 

-class NothingChanged(UserWarning):
-    """Raised when reformatted code is the same as source."""
-
-

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


@@ -97,7 +122,9 @@ def read_pyproject_toml(
     otherwise.
     """
     if not value:
     otherwise.
     """
     if not value:

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

         if value is None:
             return None
 
         if value is None:
             return None
 


@@ -106,7 +133,7 @@ def read_pyproject_toml(
     except (OSError, ValueError) as e:
         raise click.FileError(
             filename=value, hint=f"Error reading configuration file: {e}"
     except (OSError, ValueError) as e:
         raise click.FileError(
             filename=value, hint=f"Error reading configuration file: {e}"

-        )
+        ) from None

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


@@ -125,6 +152,16 @@ def read_pyproject_toml(
             "target-version", "Config key target-version must be a list"
         )
 
             "target-version", "Config key target-version must be a list"
         )
 

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

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


@@ -160,14 +197,19 @@ def validate_regex(
     ctx: click.Context,
     param: click.Parameter,
     value: Optional[str],
     ctx: click.Context,
     param: click.Parameter,
     value: Optional[str],

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

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

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

 
 
 
 

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

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


@@ -184,8 +226,9 @@ def validate_regex(
     callback=target_version_option_callback,
     multiple=True,
     help=(
     callback=target_version_option_callback,
     multiple=True,
     help=(

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

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


@@ -196,6 +239,30 @@ def validate_regex(
         " when piping source on standard input)."
     ),
 )
         " when piping source on standard input)."
     ),
 )

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

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


@@ -212,9 +279,14 @@ def validate_regex(
     "--experimental-string-processing",
     is_flag=True,
     hidden=True,
     "--experimental-string-processing",
     is_flag=True,
     hidden=True,

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

     help=(
     help=(

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

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


@@ -246,7 +318,8 @@ def validate_regex(
     type=str,
     help=(
         "Require a specific version of Black to be running (useful for unifying results"
     type=str,
     help=(
         "Require a specific version of Black to be running (useful for unifying results"

-        " across many environments e.g. with a pyproject.toml file)."
+        " across many environments e.g. with a pyproject.toml file). It can be"
+        " either a major version number or an exact version."

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


@@ -296,12 +369,23 @@ def validate_regex(
 @click.option(
     "--stdin-filename",
     type=str,
 @click.option(
     "--stdin-filename",
     type=str,

+    is_eager=True,

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

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

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


@@ -320,7 +404,13 @@ def validate_regex(
         " due to exclusion patterns."
     ),
 )
         " due to exclusion patterns."
     ),
 )

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

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


@@ -328,6 +418,7 @@ def validate_regex(
         exists=True, file_okay=True, dir_okay=True, readable=True, allow_dash=True
     ),
     is_eager=True,
         exists=True, file_okay=True, dir_okay=True, readable=True, allow_dash=True
     ),
     is_eager=True,

+    metavar="SRC ...",

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


@@ -344,7 +435,7 @@ def validate_regex(
     help="Read configuration from FILE path.",
 )
 @click.pass_context
     help="Read configuration from FILE path.",
 )
 @click.pass_context

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

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


@@ -354,31 +445,84 @@ def main(
     color: bool,
     fast: bool,
     pyi: bool,
     color: bool,
     fast: bool,
     pyi: bool,

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

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

+    preview: bool,

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

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

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

+    workers: Optional[int],

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

-    if config and verbose:
-        out(f"Using configuration from {config}.", bold=False, fg="blue")
+    ctx.ensure_object(dict)
+
+    if src and code is not None:
+        out(
+            main.get_usage(ctx)
+            + "\n\n'SRC' and 'code' cannot be passed simultaneously."
+        )
+        ctx.exit(1)
+    if not src and code is None:
+        out(main.get_usage(ctx) + "\n\nOne of 'SRC' or 'code' is required.")
+        ctx.exit(1)
+
+    root, method = (
+        find_project_root(src, stdin_filename) if code is None else (None, None)
+    )
+    ctx.obj["root"] = root
+
+    if verbose:
+        if root:
+            out(
+                f"Identified `{root}` as project root containing a {method}.",
+                fg="blue",
+            )
+
+        if config:
+            config_source = ctx.get_parameter_source("config")
+            user_level_config = str(find_user_pyproject_toml())
+            if config == user_level_config:
+                out(
+                    "Using configuration from user-level config at "
+                    f"'{user_level_config}'.",
+                    fg="blue",
+                )
+            elif config_source in (
+                ParameterSource.DEFAULT,
+                ParameterSource.DEFAULT_MAP,
+            ):
+                out("Using configuration from project root.", fg="blue")
+            else:
+                out(f"Using configuration in '{config}'.", fg="blue")
+            if ctx.default_map:
+                for param, value in ctx.default_map.items():
+                    out(f"{param}: {value}")

 
     error_msg = "Oh no! 💥 💔 💥"
 
     error_msg = "Oh no! 💥 💔 💥"

-    if required_version and required_version != __version__:
+    if (
+        required_version
+        and required_version != __version__
+        and required_version != __version__.split(".")[0]
+    ):

         err(
             f"{error_msg} The required version `{required_version}` does not match"
             f" the running version `{__version__}`!"
         )
         ctx.exit(1)
         err(
             f"{error_msg} The required version `{required_version}` does not match"
             f" the running version `{__version__}`!"
         )
         ctx.exit(1)

+    if ipynb and pyi:
+        err("Cannot pass both `pyi` and `ipynb` flags!")
+        ctx.exit(1)

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


@@ -390,9 +534,13 @@ def main(
         target_versions=versions,
         line_length=line_length,
         is_pyi=pyi,
         target_versions=versions,
         line_length=line_length,
         is_pyi=pyi,

+        is_ipynb=ipynb,
+        skip_source_first_line=skip_source_first_line,

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

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

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


@@ -407,18 +555,22 @@ def main(
             content=code, fast=fast, write_back=write_back, mode=mode, report=report
         )
     else:
             content=code, fast=fast, write_back=write_back, mode=mode, report=report
         )
     else:

-        sources = get_sources(
-            ctx=ctx,
-            src=src,
-            quiet=quiet,
-            verbose=verbose,
-            include=include,
-            exclude=exclude,
-            extend_exclude=extend_exclude,
-            force_exclude=force_exclude,
-            report=report,
-            stdin_filename=stdin_filename,
-        )
+        assert root is not None  # root is only None if code is not None
+        try:
+            sources = get_sources(
+                root=root,
+                src=src,
+                quiet=quiet,
+                verbose=verbose,
+                include=include,
+                exclude=exclude,
+                extend_exclude=extend_exclude,
+                force_exclude=force_exclude,
+                report=report,
+                stdin_filename=stdin_filename,
+            )
+        except GitWildMatchPatternError:
+            ctx.exit(1)

 
         path_empty(
             sources,
 
         path_empty(
             sources,


@@ -437,15 +589,20 @@ def main(
                 report=report,
             )
         else:
                 report=report,
             )
         else:

+            from black.concurrency import reformat_many
+

             reformat_many(
                 sources=sources,
                 fast=fast,
                 write_back=write_back,
                 mode=mode,
                 report=report,
             reformat_many(
                 sources=sources,
                 fast=fast,
                 write_back=write_back,
                 mode=mode,
                 report=report,

+                workers=workers,

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

+        if code is None and (verbose or report.change_count or report.failure_count):
+            out()

         out(error_msg if report.return_code else "All done! ✨ 🍰 ✨")
         if code is None:
             click.echo(str(report), err=True)
         out(error_msg if report.return_code else "All done! ✨ 🍰 ✨")
         if code is None:
             click.echo(str(report), err=True)


@@ -454,7 +611,7 @@ def main(
 
 def get_sources(
     *,
 
 def get_sources(
     *,

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

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


@@ -466,16 +623,12 @@ def get_sources(
     stdin_filename: Optional[str],
 ) -> Set[Path]:
     """Compute the set of files to be formatted."""
     stdin_filename: Optional[str],
 ) -> Set[Path]:
     """Compute the set of files to be formatted."""

-
-    root = find_project_root(src)

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

-    path_empty(src, "No Path provided. Nothing to do 😴", quiet, verbose, ctx)

 
 

-    if exclude is None:
-        exclude = re_compile_maybe_verbose(DEFAULT_EXCLUDES)
-        gitignore = get_gitignore(root)
-    else:
-        gitignore = None
+    using_default_exclude = exclude is None
+    exclude = re_compile_maybe_verbose(DEFAULT_EXCLUDES) if exclude is None else exclude
+    gitignore: Optional[Dict[Path, PathSpec]] = None
+    root_gitignore = get_gitignore(root)

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


@@ -486,9 +639,15 @@ def get_sources(
             is_stdin = False
 
         if is_stdin or p.is_file():
             is_stdin = False
 
         if is_stdin or p.is_file():

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

             if normalized_path is None:
             if normalized_path is None:

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

                 continue
                 continue

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

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


@@ -503,8 +662,24 @@ def get_sources(
             if is_stdin:
                 p = Path(f"{STDIN_PLACEHOLDER}{str(p)}")
 
             if is_stdin:
                 p = Path(f"{STDIN_PLACEHOLDER}{str(p)}")
 

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

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

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

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


@@ -515,12 +690,17 @@ def get_sources(
                     force_exclude,
                     report,
                     gitignore,
                     force_exclude,
                     report,
                     gitignore,

+                    verbose=verbose,
+                    quiet=quiet,

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

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

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

+

     return sources
 
 
     return sources
 
 


@@ -560,6 +740,9 @@ def reformat_code(
         report.failed(path, str(exc))
 
 
         report.failed(path, str(exc))
 
 

+# diff-shades depends on being to monkeypatch this function to operate. I know it's
+# not ideal, but this shouldn't cause any issues ... hopefully. ~ichard26
+@mypyc_attr(patchable=True)

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


@@ -584,15 +767,14 @@ def reformat_one(
         if is_stdin:
             if src.suffix == ".pyi":
                 mode = replace(mode, is_pyi=True)
         if is_stdin:
             if src.suffix == ".pyi":
                 mode = replace(mode, is_pyi=True)

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

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

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

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

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

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


@@ -601,7 +783,7 @@ def reformat_one(
             if (write_back is WriteBack.YES and changed is not Changed.CACHED) or (
                 write_back is WriteBack.CHECK and changed is Changed.NO
             ):
             if (write_back is WriteBack.YES and changed is not Changed.CACHED) or (
                 write_back is WriteBack.CHECK and changed is Changed.NO
             ):

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

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


@@ -609,114 +791,6 @@ def reformat_one(
         report.failed(src, str(exc))
 
 
         report.failed(src, str(exc))
 
 

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

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


@@ -732,23 +806,37 @@ def format_file_in_place(
     """
     if src.suffix == ".pyi":
         mode = replace(mode, is_pyi=True)
     """
     if src.suffix == ".pyi":
         mode = replace(mode, is_pyi=True)

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

 
 

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

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

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

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

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

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

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

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


@@ -782,7 +870,7 @@ def format_stdin_to_stdout(
     write a diff to stdout. The `mode` argument is passed to
     :func:`format_file_contents`.
     """
     write a diff to stdout. The `mode` argument is passed to
     :func:`format_file_contents`.
     """

-    then = datetime.utcnow()
+    then = datetime.now(timezone.utc)

 
     if content is None:
         src, encoding, newline = decode_bytes(sys.stdin.buffer.read())
 
     if content is None:
         src, encoding, newline = decode_bytes(sys.stdin.buffer.read())


@@ -803,12 +891,13 @@ def format_stdin_to_stdout(
         )
         if write_back == WriteBack.YES:
             # Make sure there's a newline after the content
         )
         if write_back == WriteBack.YES:
             # Make sure there's a newline after the content

-            dst += "" if dst[-1] == "\n" else "\n"
+            if dst and dst[-1] != "\n":
+                dst += "\n"

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

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

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


@@ -817,6 +906,19 @@ def format_stdin_to_stdout(
         f.detach()
 
 
         f.detach()
 
 

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

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


@@ -824,30 +926,127 @@ def format_file_contents(src_contents: str, *, fast: bool, mode: Mode) -> FileCo
     valid by calling :func:`assert_equivalent` and :func:`assert_stable` on it.
     `mode` is passed to :func:`format_str`.
     """
     valid by calling :func:`assert_equivalent` and :func:`assert_stable` on it.
     `mode` is passed to :func:`format_str`.
     """

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

         raise NothingChanged
 
         raise NothingChanged
 

-    dst_contents = format_str(src_contents, mode=mode)
-    if src_contents == dst_contents:
+    if not fast and not mode.is_ipynb:
+        # Jupyter notebooks will already have been checked above.
+        check_stability_and_equivalence(src_contents, dst_contents, mode=mode)
+    return dst_contents
+
+
+def validate_cell(src: str, mode: Mode) -> None:
+    """Check that cell does not already contain TransformerManager transformations,
+    or non-Python cell magics, which might cause tokenizer_rt to break because of
+    indentations.
+
+    If a cell contains ``!ls``, then it'll be transformed to
+    ``get_ipython().system('ls')``. However, if the cell originally contained
+    ``get_ipython().system('ls')``, then it would get transformed in the same way:
+
+        >>> TransformerManager().transform_cell("get_ipython().system('ls')")
+        "get_ipython().system('ls')\n"
+        >>> TransformerManager().transform_cell("!ls")
+        "get_ipython().system('ls')\n"
+
+    Due to the impossibility of safely roundtripping in such situations, cells
+    containing transformed magics will be ignored.
+    """
+    if any(transformed_magic in src for transformed_magic in TRANSFORMED_MAGICS):
+        raise NothingChanged
+    if (
+        src[:2] == "%%"
+        and src.split()[0][2:] not in PYTHON_CELL_MAGICS | mode.python_cell_magics
+    ):

         raise NothingChanged
 
         raise NothingChanged
 

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

     if not fast:
     if not fast:

-        assert_equivalent(src_contents, dst_contents)
-
-        # Forced second pass to work around optional trailing commas (becoming
-        # forced trailing commas on pass 2) interacting differently with optional
-        # parentheses.  Admittedly ugly.
-        dst_contents_pass2 = format_str(dst_contents, mode=mode)
-        if dst_contents != dst_contents_pass2:
-            dst_contents = dst_contents_pass2
-            assert_equivalent(src_contents, dst_contents, pass_num=2)
-            assert_stable(src_contents, dst_contents, mode=mode)
-        # Note: no need to explicitly call `assert_stable` if `dst_contents` was
-        # the same as `dst_contents_pass2`.
-    return dst_contents
+        check_stability_and_equivalence(masked_src, masked_dst, mode=mode)
+    dst_without_trailing_semicolon = unmask_cell(masked_dst, replacements)
+    dst = put_trailing_semicolon_back(
+        dst_without_trailing_semicolon, has_trailing_semicolon
+    )
+    dst = dst.rstrip("\n")
+    if dst == src:
+        raise NothingChanged from None
+    return dst
+
+
+def validate_metadata(nb: MutableMapping[str, Any]) -> None:
+    """If notebook is marked as non-Python, don't format it.
+
+    All notebook metadata fields are optional, see
+    https://nbformat.readthedocs.io/en/latest/format_description.html. So
+    if a notebook has empty metadata, we will try to parse it anyway.
+    """
+    language = nb.get("metadata", {}).get("language_info", {}).get("name", None)
+    if language is not None and language != "python":
+        raise NothingChanged from None

 
 
 
 

-def format_str(src_contents: str, *, mode: Mode) -> FileContent:
+def format_ipynb_string(src_contents: str, *, fast: bool, mode: Mode) -> FileContent:
+    """Format Jupyter notebook.
+
+    Operate cell-by-cell, only on code cells, only for Python notebooks.
+    If the ``.ipynb`` originally had a trailing newline, it'll be preserved.
+    """
+    if not src_contents:
+        raise NothingChanged
+
+    trailing_newline = src_contents[-1] == "\n"
+    modified = False
+    nb = json.loads(src_contents)
+    validate_metadata(nb)
+    for cell in nb["cells"]:
+        if cell.get("cell_type", None) == "code":
+            try:
+                src = "".join(cell["source"])
+                dst = format_cell(src, fast=fast, mode=mode)
+            except NothingChanged:
+                pass
+            else:
+                cell["source"] = dst.splitlines(keepends=True)
+                modified = True
+    if modified:
+        dst_contents = json.dumps(nb, indent=1, ensure_ascii=False)
+        if trailing_newline:
+            dst_contents = dst_contents + "\n"
+        return dst_contents
+    else:
+        raise NothingChanged
+
+
+def format_str(src_contents: str, *, mode: Mode) -> str:

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


@@ -877,35 +1076,57 @@ def format_str(src_contents: str, *, mode: Mode) -> FileContent:
         hey
 
     """
         hey
 
     """

+    dst_contents = _format_str_once(src_contents, mode=mode)
+    # Forced second pass to work around optional trailing commas (becoming
+    # forced trailing commas on pass 2) interacting differently with optional
+    # parentheses.  Admittedly ugly.
+    if src_contents != dst_contents:
+        return _format_str_once(dst_contents, mode=mode)
+    return dst_contents
+
+
+def _format_str_once(src_contents: str, *, mode: Mode) -> str:

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

-    dst_contents = []
-    future_imports = get_future_imports(src_node)
+    dst_blocks: List[LinesBlock] = []

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

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

     normalize_fmt_off(src_node)
     normalize_fmt_off(src_node)

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

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

+    block: Optional[LinesBlock] = None

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

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

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

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

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


@@ -926,30 +1147,55 @@ def decode_bytes(src: bytes) -> Tuple[FileContent, Encoding, NewLine]:
         return tiow.read(), encoding, newline
 
 
         return tiow.read(), encoding, newline
 
 

-def get_features_used(node: Node) -> Set[Feature]:
+def get_features_used(  # noqa: C901
+    node: Node, *, future_imports: Optional[Set[str]] = None
+) -> Set[Feature]:

     """Return a set of (relatively) new Python features used in this file.
 
     Currently looking for:
     - f-strings;
     """Return a set of (relatively) new Python features used in this file.
 
     Currently looking for:
     - f-strings;

+    - self-documenting expressions in f-strings (f"{x=}");

     - underscores in numeric literals;
     - trailing commas after * or ** in function signatures and calls;
     - positional only arguments in function signatures and lambdas;
     - assignment expression;
     - relaxed decorator syntax;
     - underscores in numeric literals;
     - trailing commas after * or ** in function signatures and calls;
     - positional only arguments in function signatures and lambdas;
     - assignment expression;
     - relaxed decorator syntax;

+    - usage of __future__ flags (annotations);
+    - print / exec statements;
+    - parenthesized context managers;
+    - match statements;
+    - except* clause;
+    - variadic generics;

     """
     features: Set[Feature] = set()
     """
     features: Set[Feature] = set()

+    if future_imports:
+        features |= {
+            FUTURE_FLAG_TO_FEATURE[future_import]
+            for future_import in future_imports
+            if future_import in FUTURE_FLAG_TO_FEATURE
+        }
+

     for n in node.pre_order():
     for n in node.pre_order():

-        if n.type == token.STRING:
-            value_head = n.value[:2]  # type: ignore
+        if is_string_token(n):
+            value_head = n.value[:2]

             if value_head in {'f"', 'F"', "f'", "F'", "rf", "fr", "RF", "FR"}:
                 features.add(Feature.F_STRINGS)
             if value_head in {'f"', 'F"', "f'", "F'", "rf", "fr", "RF", "FR"}:
                 features.add(Feature.F_STRINGS)

-
-        elif n.type == token.NUMBER:
-            if "_" in n.value:  # type: ignore
+                if Feature.DEBUG_F_STRINGS not in features:
+                    for span_beg, span_end in iter_fexpr_spans(n.value):
+                        if n.value[span_beg : span_end - 1].rstrip().endswith("="):
+                            features.add(Feature.DEBUG_F_STRINGS)
+                            break
+
+        elif is_number_token(n):
+            if "_" in n.value:

                 features.add(Feature.NUMERIC_UNDERSCORES)
 
         elif n.type == token.SLASH:
                 features.add(Feature.NUMERIC_UNDERSCORES)
 
         elif n.type == token.SLASH:

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

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


@@ -980,12 +1226,68 @@ def get_features_used(node: Node) -> Set[Feature]:
                         if argch.type in STARS:
                             features.add(feature)
 
                         if argch.type in STARS:
                             features.add(feature)
 

+        elif (
+            n.type in {syms.return_stmt, syms.yield_expr}
+            and len(n.children) >= 2
+            and n.children[1].type == syms.testlist_star_expr
+            and any(child.type == syms.star_expr for child in n.children[1].children)
+        ):
+            features.add(Feature.UNPACKING_ON_FLOW)
+
+        elif (
+            n.type == syms.annassign
+            and len(n.children) >= 4
+            and n.children[3].type == syms.testlist_star_expr
+        ):
+            features.add(Feature.ANN_ASSIGN_EXTENDED_RHS)
+
+        elif (
+            n.type == syms.with_stmt
+            and len(n.children) > 2
+            and n.children[1].type == syms.atom
+        ):
+            atom_children = n.children[1].children
+            if (
+                len(atom_children) == 3
+                and atom_children[0].type == token.LPAR
+                and atom_children[1].type == syms.testlist_gexp
+                and atom_children[2].type == token.RPAR
+            ):
+                features.add(Feature.PARENTHESIZED_CONTEXT_MANAGERS)
+
+        elif n.type == syms.match_stmt:
+            features.add(Feature.PATTERN_MATCHING)
+
+        elif (
+            n.type == syms.except_clause
+            and len(n.children) >= 2
+            and n.children[1].type == token.STAR
+        ):
+            features.add(Feature.EXCEPT_STAR)
+
+        elif n.type in {syms.subscriptlist, syms.trailer} and any(
+            child.type == syms.star_expr for child in n.children
+        ):
+            features.add(Feature.VARIADIC_GENERICS)
+
+        elif (
+            n.type == syms.tname_star
+            and len(n.children) == 3
+            and n.children[2].type == syms.star_expr
+        ):
+            features.add(Feature.VARIADIC_GENERICS)
+
+        elif n.type in (syms.type_stmt, syms.typeparams):
+            features.add(Feature.TYPE_PARAMS)
+

     return features
 
 
     return features
 
 

-def detect_target_versions(node: Node) -> Set[TargetVersion]:
+def detect_target_versions(
+    node: Node, *, future_imports: Optional[Set[str]] = None
+) -> Set[TargetVersion]:

     """Detect the version to target based on the nodes used."""
     """Detect the version to target based on the nodes used."""

-    features = get_features_used(node)
+    features = get_features_used(node, future_imports=future_imports)

     return {
         version for version in TargetVersion if features <= VERSION_TO_FEATURES[version]
     }
     return {
         version for version in TargetVersion if features <= VERSION_TO_FEATURES[version]
     }


@@ -1041,22 +1343,24 @@ def get_future_imports(node: Node) -> Set[str]:
     return imports
 
 
     return imports
 
 

-def assert_equivalent(src: str, dst: str, *, pass_num: int = 1) -> None:
+def assert_equivalent(src: str, dst: str) -> None:

     """Raise AssertionError if `src` and `dst` aren't equivalent."""
     try:
         src_ast = parse_ast(src)
     except Exception as exc:
         raise AssertionError(
     """Raise AssertionError if `src` and `dst` aren't equivalent."""
     try:
         src_ast = parse_ast(src)
     except Exception as exc:
         raise AssertionError(

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

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

-            f"INTERNAL ERROR: Black produced invalid code on pass {pass_num}: {exc}. "
+            f"INTERNAL ERROR: Black produced invalid code: {exc}. "

             "Please report a bug on https://github.com/psf/black/issues.  "
             f"This invalid output might be helpful: {log}"
         ) from None
             "Please report a bug on https://github.com/psf/black/issues.  "
             f"This invalid output might be helpful: {log}"
         ) from None


@@ -1067,14 +1371,17 @@ def assert_equivalent(src: str, dst: str, *, pass_num: int = 1) -> None:
         log = dump_to_file(diff(src_ast_str, dst_ast_str, "src", "dst"))
         raise AssertionError(
             "INTERNAL ERROR: Black produced code that is not equivalent to the"
         log = dump_to_file(diff(src_ast_str, dst_ast_str, "src", "dst"))
         raise AssertionError(
             "INTERNAL ERROR: Black produced code that is not equivalent to the"

-            f" source on pass {pass_num}.  Please report a bug on "
+            " source.  Please report a bug on "

             f"https://github.com/psf/black/issues.  This diff might be helpful: {log}"
         ) from None
 
 
 def assert_stable(src: str, dst: str, mode: Mode) -> None:
     """Raise AssertionError if `dst` reformats differently the second time."""
             f"https://github.com/psf/black/issues.  This diff might be helpful: {log}"
         ) from None
 
 
 def assert_stable(src: str, dst: str, mode: Mode) -> None:
     """Raise AssertionError if `dst` reformats differently the second time."""

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

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


@@ -1097,34 +1404,14 @@ def nullcontext() -> Iterator[None]:
     yield
 
 
     yield
 
 

-def patch_click() -> None:
-    """Make Click not crash on Python 3.6 with LANG=C.
-
-    On certain misconfigured environments, Python 3 selects the ASCII encoding as the
-    default which restricts paths that it can access during the lifetime of the
-    application.  Click refuses to work in this scenario by raising a RuntimeError.
-
-    In case of Black the likelihood that non-ASCII characters are going to be used in
-    file paths is minimal since it's Python source code.  Moreover, this crash was
-    spurious on Python 3.7 thanks to PEP 538 and PEP 540.
-    """
-    try:
-        from click import core
-        from click import _unicodefun  # type: ignore
-    except ModuleNotFoundError:
-        return
-
-    for module in (core, _unicodefun):
-        if hasattr(module, "_verify_python3_env"):
-            module._verify_python3_env = lambda: None  # type: ignore
-        if hasattr(module, "_verify_python_env"):
-            module._verify_python_env = lambda: None  # type: ignore
+def patched_main() -> None:
+    # PyInstaller patches multiprocessing to need freeze_support() even in non-Windows
+    # environments so just assume we always need to call it if frozen.
+    if getattr(sys, "frozen", False):
+        from multiprocessing import freeze_support

 
 

+        freeze_support()

 
 

-def patched_main() -> None:
-    maybe_install_uvloop()
-    freeze_support()
-    patch_click()

     main()
 
 
     main()