Add `black` Dockerfile (#1916)

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

diff --git a/src/black/__init__.py b/src/black/__init__.py
index e37caa98a2c00386afd2acd52a4174e7d556d82c..431ee027270d670086251d6acaba42fc304b0fa4 100644 (file)
--- a/src/black/__init__.py
+++ b/src/black/__init__.py
@@ -42,14 +42,26 @@ from typing import (
     cast,
     TYPE_CHECKING,
 )
     cast,
     TYPE_CHECKING,
 )

-from typing_extensions import Final

 from mypy_extensions import mypyc_attr
 
 from appdirs import user_cache_dir
 from dataclasses import dataclass, field, replace
 import click
 import toml
 from mypy_extensions import mypyc_attr
 
 from appdirs import user_cache_dir
 from dataclasses import dataclass, field, replace
 import click
 import toml

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

 from pathspec import PathSpec
 
 # lib2to3 fork
 from pathspec import PathSpec
 
 # lib2to3 fork


@@ -61,6 +73,11 @@ from blib2to3.pgen2.parse import ParseError
 
 from _black_version import version as __version__
 
 
 from _black_version import version as __version__
 

+if sys.version_info < (3, 8):
+    from typing_extensions import Final
+else:
+    from typing import Final
+

 if TYPE_CHECKING:
     import colorama  # noqa: F401
 
 if TYPE_CHECKING:
     import colorama  # noqa: F401
 


@@ -68,6 +85,7 @@ DEFAULT_LINE_LENGTH = 88
 DEFAULT_EXCLUDES = r"/(\.direnv|\.eggs|\.git|\.hg|\.mypy_cache|\.nox|\.tox|\.venv|\.svn|_build|buck-out|build|dist)/"  # noqa: B950
 DEFAULT_INCLUDES = r"\.pyi?$"
 CACHE_DIR = Path(user_cache_dir("black", version=__version__))
 DEFAULT_EXCLUDES = r"/(\.direnv|\.eggs|\.git|\.hg|\.mypy_cache|\.nox|\.tox|\.venv|\.svn|_build|buck-out|build|dist)/"  # noqa: B950
 DEFAULT_INCLUDES = r"\.pyi?$"
 CACHE_DIR = Path(user_cache_dir("black", version=__version__))

+STDIN_PLACEHOLDER = "__BLACK_STDIN_FILENAME__"

 
 STRING_PREFIX_CHARS: Final = "furbFURB"  # All possible string prefix characters.
 
 
 STRING_PREFIX_CHARS: Final = "furbFURB"  # All possible string prefix characters.
 


@@ -88,7 +106,7 @@ Transformer = Callable[["Line", Collection["Feature"]], Iterator["Line"]]
 Timestamp = float
 FileSize = int
 CacheInfo = Tuple[Timestamp, FileSize]
 Timestamp = float
 FileSize = int
 CacheInfo = Tuple[Timestamp, FileSize]

-Cache = Dict[Path, CacheInfo]
+Cache = Dict[str, CacheInfo]

 out = partial(click.secho, bold=True, err=True)
 err = partial(click.secho, fg="red", err=True)
 
 out = partial(click.secho, bold=True, err=True)
 err = partial(click.secho, fg="red", err=True)
 


@@ -112,6 +130,10 @@ class InvalidInput(ValueError):
     """Raised when input source code fails all parse attempts."""
 
 
     """Raised when input source code fails all parse attempts."""
 
 

+class BracketMatchError(KeyError):
+    """Raised when an opening bracket is unable to be matched to a closing bracket."""
+
+

 T = TypeVar("T")
 E = TypeVar("E", bound=Exception)
 
 T = TypeVar("T")
 E = TypeVar("E", bound=Exception)
 


@@ -174,14 +196,12 @@ class TargetVersion(Enum):
     PY36 = 6
     PY37 = 7
     PY38 = 8
     PY36 = 6
     PY37 = 7
     PY38 = 8

+    PY39 = 9

 
     def is_python2(self) -> bool:
         return self is TargetVersion.PY27
 
 
 
     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
 class Feature(Enum):
     # All string literals are unicode
     UNICODE_LITERALS = 1


@@ -195,6 +215,7 @@ class Feature(Enum):
     ASYNC_KEYWORDS = 7
     ASSIGNMENT_EXPRESSIONS = 8
     POS_ONLY_ARGUMENTS = 9
     ASYNC_KEYWORDS = 7
     ASSIGNMENT_EXPRESSIONS = 8
     POS_ONLY_ARGUMENTS = 9

+    RELAXED_DECORATORS = 10

     FORCE_OPTIONAL_PARENTHESES = 50
 
 
     FORCE_OPTIONAL_PARENTHESES = 50
 
 


@@ -233,6 +254,17 @@ VERSION_TO_FEATURES: Dict[TargetVersion, Set[Feature]] = {
         Feature.ASSIGNMENT_EXPRESSIONS,
         Feature.POS_ONLY_ARGUMENTS,
     },
         Feature.ASSIGNMENT_EXPRESSIONS,
         Feature.POS_ONLY_ARGUMENTS,
     },

+    TargetVersion.PY39: {
+        Feature.UNICODE_LITERALS,
+        Feature.F_STRINGS,
+        Feature.NUMERIC_UNDERSCORES,
+        Feature.TRAILING_COMMA_IN_CALL,
+        Feature.TRAILING_COMMA_IN_DEF,
+        Feature.ASYNC_KEYWORDS,
+        Feature.ASSIGNMENT_EXPRESSIONS,
+        Feature.RELAXED_DECORATORS,
+        Feature.POS_ONLY_ARGUMENTS,
+    },

 }
 
 
 }
 
 


@@ -241,6 +273,7 @@ class Mode:
     target_versions: Set[TargetVersion] = field(default_factory=set)
     line_length: int = DEFAULT_LINE_LENGTH
     string_normalization: bool = True
     target_versions: Set[TargetVersion] = field(default_factory=set)
     line_length: int = DEFAULT_LINE_LENGTH
     string_normalization: bool = True

+    magic_trailing_comma: bool = True

     experimental_string_processing: bool = False
     is_pyi: bool = False
 
     experimental_string_processing: bool = False
     is_pyi: bool = False
 


@@ -273,7 +306,11 @@ def find_pyproject_toml(path_search_start: Iterable[str]) -> Optional[str]:
     """Find the absolute filepath to a pyproject.toml if it exists"""
     path_project_root = find_project_root(path_search_start)
     path_pyproject_toml = path_project_root / "pyproject.toml"
     """Find the absolute filepath to a pyproject.toml if it exists"""
     path_project_root = find_project_root(path_search_start)
     path_pyproject_toml = path_project_root / "pyproject.toml"

-    return str(path_pyproject_toml) if path_pyproject_toml.is_file() else None
+    if path_pyproject_toml.is_file():
+        return str(path_pyproject_toml)
+
+    path_user_pyproject_toml = find_user_pyproject_toml()
+    return str(path_user_pyproject_toml) if path_user_pyproject_toml.is_file() else None

 
 
 def parse_pyproject_toml(path_config: str) -> Dict[str, Any]:
 
 
 def parse_pyproject_toml(path_config: str) -> Dict[str, Any]:


@@ -343,6 +380,17 @@ def target_version_option_callback(
     return [TargetVersion[val.upper()] for val in v]
 
 
     return [TargetVersion[val.upper()] for val in v]
 
 

+def validate_regex(
+    ctx: click.Context,
+    param: click.Parameter,
+    value: Optional[str],
+) -> Optional[Pattern]:
+    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")
+
+

 @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(
 @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(


@@ -378,6 +426,12 @@ def target_version_option_callback(
     is_flag=True,
     help="Don't normalize string quotes or prefixes.",
 )
     is_flag=True,
     help="Don't normalize string quotes or prefixes.",
 )

+@click.option(
+    "-C",
+    "--skip-magic-trailing-comma",
+    is_flag=True,
+    help="Don't use trailing commas as a reason to split lines.",
+)

 @click.option(
     "--experimental-string-processing",
     is_flag=True,
 @click.option(
     "--experimental-string-processing",
     is_flag=True,


@@ -391,8 +445,8 @@ def target_version_option_callback(
     "--check",
     is_flag=True,
     help=(
     "--check",
     is_flag=True,
     help=(

-        "Don't write the files back, just return the status.  Return code 0 means"
-        " nothing would change.  Return code 1 means some files would be reformatted."
+        "Don't write the files back, just return the status. Return code 0 means"
+        " nothing would change. Return code 1 means some files would be reformatted."

         " Return code 123 means there was an internal error."
     ),
 )
         " Return code 123 means there was an internal error."
     ),
 )


@@ -415,11 +469,12 @@ def target_version_option_callback(
     "--include",
     type=str,
     default=DEFAULT_INCLUDES,
     "--include",
     type=str,
     default=DEFAULT_INCLUDES,

+    callback=validate_regex,

     help=(
         "A regular expression that matches files and directories that should be"
     help=(
         "A regular expression that matches files and directories that should be"

-        " included on recursive searches.  An empty value means all files are included"
-        " regardless of the name.  Use forward slashes for directories on all platforms"
-        " (Windows, too).  Exclusions are calculated first, inclusions later."
+        " included on recursive searches. An empty value means all files are included"
+        " regardless of the name. Use forward slashes for directories on all platforms"
+        " (Windows, too). Exclusions are calculated first, inclusions later."

     ),
     show_default=True,
 )
     ),
     show_default=True,
 )


@@ -427,20 +482,40 @@ def target_version_option_callback(
     "--exclude",
     type=str,
     default=DEFAULT_EXCLUDES,
     "--exclude",
     type=str,
     default=DEFAULT_EXCLUDES,

+    callback=validate_regex,

     help=(
         "A regular expression that matches files and directories that should be"
     help=(
         "A regular expression that matches files and directories that should be"

-        " excluded on recursive searches.  An empty value means no paths are excluded."
-        " Use forward slashes for directories on all platforms (Windows, too). "
+        " excluded on recursive searches. An empty value means no paths are excluded."
+        " Use forward slashes for directories on all platforms (Windows, too)."

         " Exclusions are calculated first, inclusions later."
     ),
     show_default=True,
 )
         " Exclusions are calculated first, inclusions later."
     ),
     show_default=True,
 )

+@click.option(
+    "--extend-exclude",
+    type=str,
+    callback=validate_regex,
+    help=(
+        "Like --exclude, but adds additional files and directories on top of the"
+        " excluded ones. (Useful if you simply want to add to the default)"
+    ),
+)

 @click.option(
     "--force-exclude",
     type=str,
 @click.option(
     "--force-exclude",
     type=str,

+    callback=validate_regex,

     help=(
         "Like --exclude, but files and directories matching this regex will be "
     help=(
         "Like --exclude, but files and directories matching this regex will be "

-        "excluded even when they are passed explicitly as arguments"
+        "excluded even when they are passed explicitly as arguments."
+    ),
+)
+@click.option(
+    "--stdin-filename",
+    type=str,
+    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(
     ),
 )
 @click.option(


@@ -458,7 +533,7 @@ def target_version_option_callback(
     is_flag=True,
     help=(
         "Also emit messages to stderr about files that were not changed or were ignored"
     is_flag=True,
     help=(
         "Also emit messages to stderr about files that were not changed or were ignored"

-        " due to --exclude=."
+        " due to exclusion patterns."

     ),
 )
 @click.version_option(version=__version__)
     ),
 )
 @click.version_option(version=__version__)


@@ -496,12 +571,15 @@ def main(
     fast: bool,
     pyi: bool,
     skip_string_normalization: bool,
     fast: bool,
     pyi: bool,
     skip_string_normalization: bool,

+    skip_magic_trailing_comma: bool,

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

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

     src: Tuple[str, ...],
     config: Optional[str],
 ) -> None:
     src: Tuple[str, ...],
     config: Optional[str],
 ) -> None:


@@ -517,6 +595,7 @@ def main(
         line_length=line_length,
         is_pyi=pyi,
         string_normalization=not skip_string_normalization,
         line_length=line_length,
         is_pyi=pyi,
         string_normalization=not skip_string_normalization,

+        magic_trailing_comma=not skip_magic_trailing_comma,

         experimental_string_processing=experimental_string_processing,
     )
     if config and verbose:
         experimental_string_processing=experimental_string_processing,
     )
     if config and verbose:


@@ -532,8 +611,10 @@ def main(
         verbose=verbose,
         include=include,
         exclude=exclude,
         verbose=verbose,
         include=include,
         exclude=exclude,

+        extend_exclude=extend_exclude,

         force_exclude=force_exclude,
         report=report,
         force_exclude=force_exclude,
         report=report,

+        stdin_filename=stdin_filename,

     )
 
     path_empty(
     )
 
     path_empty(


@@ -569,29 +650,14 @@ def get_sources(
     src: Tuple[str, ...],
     quiet: bool,
     verbose: bool,
     src: Tuple[str, ...],
     quiet: bool,
     verbose: bool,

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

     report: "Report",
     report: "Report",

+    stdin_filename: Optional[str],

 ) -> Set[Path]:
     """Compute the set of files to be formatted."""
 ) -> Set[Path]:
     """Compute the set of files to be formatted."""

-    try:
-        include_regex = re_compile_maybe_verbose(include)
-    except re.error:
-        err(f"Invalid regular expression for include given: {include!r}")
-        ctx.exit(2)
-    try:
-        exclude_regex = re_compile_maybe_verbose(exclude)
-    except re.error:
-        err(f"Invalid regular expression for exclude given: {exclude!r}")
-        ctx.exit(2)
-    try:
-        force_exclude_regex = (
-            re_compile_maybe_verbose(force_exclude) if force_exclude else None
-        )
-    except re.error:
-        err(f"Invalid regular expression for force_exclude given: {force_exclude!r}")
-        ctx.exit(2)

 
     root = find_project_root(src)
     sources: Set[Path] = set()
 
     root = find_project_root(src)
     sources: Set[Path] = set()


@@ -599,36 +665,46 @@ def get_sources(
     gitignore = get_gitignore(root)
 
     for s in src:
     gitignore = get_gitignore(root)
 
     for s in src:

-        p = Path(s)
-        if p.is_dir():
-            sources.update(
-                gen_python_files(
-                    p.iterdir(),
-                    root,
-                    include_regex,
-                    exclude_regex,
-                    force_exclude_regex,
-                    report,
-                    gitignore,
-                )
-            )
-        elif s == "-":
-            sources.add(p)
-        elif p.is_file():
+        if s == "-" and stdin_filename:
+            p = Path(stdin_filename)
+            is_stdin = True
+        else:
+            p = Path(s)
+            is_stdin = False
+
+        if is_stdin or p.is_file():

             normalized_path = normalize_path_maybe_ignore(p, root, report)
             if normalized_path is None:
                 continue
 
             normalized_path = "/" + normalized_path
             # Hard-exclude any files that matches the `--force-exclude` regex.
             normalized_path = normalize_path_maybe_ignore(p, root, report)
             if normalized_path is None:
                 continue
 
             normalized_path = "/" + normalized_path
             # Hard-exclude any files that matches the `--force-exclude` regex.

-            if force_exclude_regex:
-                force_exclude_match = force_exclude_regex.search(normalized_path)
+            if force_exclude:
+                force_exclude_match = force_exclude.search(normalized_path)

             else:
                 force_exclude_match = None
             if force_exclude_match and force_exclude_match.group(0):
                 report.path_ignored(p, "matches the --force-exclude regular expression")
                 continue
 
             else:
                 force_exclude_match = None
             if force_exclude_match and force_exclude_match.group(0):
                 report.path_ignored(p, "matches the --force-exclude regular expression")
                 continue
 

+            if is_stdin:
+                p = Path(f"{STDIN_PLACEHOLDER}{str(p)}")
+
+            sources.add(p)
+        elif p.is_dir():
+            sources.update(
+                gen_python_files(
+                    p.iterdir(),
+                    root,
+                    include,
+                    exclude,
+                    extend_exclude,
+                    force_exclude,
+                    report,
+                    gitignore,
+                )
+            )
+        elif s == "-":

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


@@ -641,10 +717,9 @@ def path_empty(
     """
     Exit if there is no `src` provided for formatting
     """
     """
     Exit if there is no `src` provided for formatting
     """

-    if len(src) == 0:
-        if verbose or not quiet:
-            out(msg)
-            ctx.exit(0)
+    if not src and (verbose or not quiet):
+        out(msg)
+        ctx.exit(0)

 
 
 def reformat_one(
 
 
 def reformat_one(


@@ -657,15 +732,27 @@ def reformat_one(
     """
     try:
         changed = Changed.NO
     """
     try:
         changed = Changed.NO

-        if not src.is_file() and str(src) == "-":
+
+        if str(src) == "-":
+            is_stdin = True
+        elif str(src).startswith(STDIN_PLACEHOLDER):
+            is_stdin = True
+            # Use the original name again in case we want to print something
+            # to the user
+            src = Path(str(src)[len(STDIN_PLACEHOLDER) :])
+        else:
+            is_stdin = False
+
+        if is_stdin:

             if format_stdin_to_stdout(fast=fast, write_back=write_back, mode=mode):
                 changed = Changed.YES
         else:
             cache: Cache = {}
             if format_stdin_to_stdout(fast=fast, write_back=write_back, mode=mode):
                 changed = Changed.YES
         else:
             cache: Cache = {}

-            if write_back != WriteBack.DIFF:
+            if write_back not in (WriteBack.DIFF, WriteBack.COLOR_DIFF):

                 cache = read_cache(mode)
                 res_src = src.resolve()
                 cache = read_cache(mode)
                 res_src = src.resolve()

-                if res_src in cache and cache[res_src] == get_cache_info(res_src):
+                res_src_s = str(res_src)
+                if res_src_s in cache and cache[res_src_s] == get_cache_info(res_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


@@ -691,7 +778,7 @@ def reformat_many(
     worker_count = os.cpu_count()
     if sys.platform == "win32":
         # Work around https://bugs.python.org/issue26903
     worker_count = os.cpu_count()
     if sys.platform == "win32":
         # Work around https://bugs.python.org/issue26903

-        worker_count = min(worker_count, 61)
+        worker_count = min(worker_count, 60)

     try:
         executor = ProcessPoolExecutor(max_workers=worker_count)
     except (ImportError, OSError):
     try:
         executor = ProcessPoolExecutor(max_workers=worker_count)
     except (ImportError, OSError):


@@ -736,7 +823,7 @@ async def schedule_formatting(
     :func:`format_file_in_place`.
     """
     cache: Cache = {}
     :func:`format_file_in_place`.
     """
     cache: Cache = {}

-    if write_back != WriteBack.DIFF:
+    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):
         cache = read_cache(mode)
         sources, cached = filter_cached(cache, sources)
         for src in sorted(cached):


@@ -747,7 +834,7 @@ async def schedule_formatting(
     cancelled = []
     sources_to_cache = []
     lock = None
     cancelled = []
     sources_to_cache = []
     lock = None

-    if write_back == WriteBack.DIFF:
+    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()
         # For diff output, we need locks to ensure we don't interleave output
         # from different processes.
         manager = Manager()


@@ -823,7 +910,7 @@ def format_file_in_place(
         dst_name = f"{src}\t{now} +0000"
         diff_contents = diff(src_contents, dst_contents, src_name, dst_name)
 
         dst_name = f"{src}\t{now} +0000"
         diff_contents = diff(src_contents, dst_contents, src_name, dst_name)
 

-        if write_back == write_back.COLOR_DIFF:
+        if write_back == WriteBack.COLOR_DIFF:

             diff_contents = color_diff(diff_contents)
 
         with lock or nullcontext():
             diff_contents = color_diff(diff_contents)
 
         with lock or nullcontext():


@@ -846,9 +933,9 @@ def color_diff(contents: str) -> str:
     for i, line in enumerate(lines):
         if line.startswith("+++") or line.startswith("---"):
             line = "\033[1;37m" + line + "\033[0m"  # bold white, reset
     for i, line in enumerate(lines):
         if line.startswith("+++") or line.startswith("---"):
             line = "\033[1;37m" + line + "\033[0m"  # bold white, reset

-        if line.startswith("@@"):
+        elif line.startswith("@@"):

             line = "\033[36m" + line + "\033[0m"  # cyan, reset
             line = "\033[36m" + line + "\033[0m"  # cyan, reset

-        if line.startswith("+"):
+        elif line.startswith("+"):

             line = "\033[32m" + line + "\033[0m"  # green, reset
         elif line.startswith("-"):
             line = "\033[31m" + line + "\033[0m"  # red, reset
             line = "\033[32m" + line + "\033[0m"  # green, reset
         elif line.startswith("-"):
             line = "\033[31m" + line + "\033[0m"  # red, reset


@@ -858,30 +945,22 @@ def color_diff(contents: str) -> str:
 
 def wrap_stream_for_windows(
     f: io.TextIOWrapper,
 
 def wrap_stream_for_windows(
     f: io.TextIOWrapper,

-) -> Union[io.TextIOWrapper, "colorama.AnsiToWin32.AnsiToWin32"]:
+) -> Union[io.TextIOWrapper, "colorama.AnsiToWin32"]:

     """
     """

-    Wrap the stream in colorama's wrap_stream so colors are shown on Windows.
+    Wrap stream with colorama's wrap_stream so colors are shown on Windows.

 
 

-    If `colorama` is not found, then no change is made. If `colorama` does
-    exist, then it handles the logic to determine whether or not to change
-    things.
+    If `colorama` is unavailable, the original stream is returned unmodified.
+    Otherwise, the `wrap_stream()` function determines whether the stream needs
+    to be wrapped for a Windows environment and will accordingly either return
+    an `AnsiToWin32` wrapper or the original stream.

     """
     try:
     """
     try:

-        from colorama import initialise
-
-        # We set `strip=False` so that we can don't have to modify
-        # test_express_diff_with_color.
-        f = initialise.wrap_stream(
-            f, convert=None, strip=False, autoreset=False, wrap=True
-        )
-
-        # wrap_stream returns a `colorama.AnsiToWin32.AnsiToWin32` object
-        # which does not have a `detach()` method. So we fake one.
-        f.detach = lambda *args, **kwargs: None  # type: ignore
+        from colorama.initialise import wrap_stream

     except ImportError:
     except ImportError:

-        pass
-
-    return f
+        return f
+    else:
+        # Set `strip=False` to avoid needing to modify test_express_diff_with_color.
+        return wrap_stream(f, convert=None, strip=False, autoreset=False, wrap=True)

 
 
 def format_stdin_to_stdout(
 
 
 def format_stdin_to_stdout(


@@ -922,13 +1001,13 @@ def format_stdin_to_stdout(
 
 
 def format_file_contents(src_contents: str, *, fast: bool, mode: Mode) -> FileContent:
 
 
 def format_file_contents(src_contents: str, *, fast: bool, mode: Mode) -> FileContent:

-    """Reformat contents a file and return new contents.
+    """Reformat contents of 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.
     `mode` is passed to :func:`format_str`.
     """
 
     If `fast` is False, additionally confirm that the reformatted code is
     valid by calling :func:`assert_equivalent` and :func:`assert_stable` on it.
     `mode` is passed to :func:`format_str`.
     """

-    if src_contents.strip() == "":
+    if not src_contents.strip():

         raise NothingChanged
 
     dst_contents = format_str(src_contents, mode=mode)
         raise NothingChanged
 
     dst_contents = format_str(src_contents, mode=mode)


@@ -948,7 +1027,7 @@ def format_str(src_contents: str, *, mode: Mode) -> FileContent:
     allowed.  Example:
 
     >>> import black
     allowed.  Example:
 
     >>> import black

-    >>> print(black.format_str("def f(arg:str='')->None:...", mode=Mode()))
+    >>> print(black.format_str("def f(arg:str='')->None:...", mode=black.Mode()))

     def f(arg: str = "") -> None:
         ...
 
     def f(arg: str = "") -> None:
         ...
 


@@ -980,13 +1059,12 @@ def format_str(src_contents: str, *, mode: Mode) -> FileContent:
         versions = detect_target_versions(src_node)
     normalize_fmt_off(src_node)
     lines = LineGenerator(
         versions = detect_target_versions(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),
         remove_u_prefix="unicode_literals" in future_imports
         or supports_feature(versions, Feature.UNICODE_LITERALS),

-        is_pyi=mode.is_pyi,
-        normalize_strings=mode.string_normalization,

     )
     elt = EmptyLineTracker(is_pyi=mode.is_pyi)
     )
     elt = EmptyLineTracker(is_pyi=mode.is_pyi)

-    empty_line = Line()
+    empty_line = Line(mode=mode)

     after = 0
     split_line_features = {
         feature
     after = 0
     split_line_features = {
         feature


@@ -1062,7 +1140,7 @@ def get_grammars(target_versions: Set[TargetVersion]) -> List[Grammar]:
 
 def lib2to3_parse(src_txt: str, target_versions: Iterable[TargetVersion] = ()) -> Node:
     """Given a string with source, return the lib2to3 Node."""
 
 def lib2to3_parse(src_txt: str, target_versions: Iterable[TargetVersion] = ()) -> Node:
     """Given a string with source, return the lib2to3 Node."""

-    if src_txt[-1:] != "\n":
+    if not src_txt.endswith("\n"):

         src_txt += "\n"
 
     for grammar in get_grammars(set(target_versions)):
         src_txt += "\n"
 
     for grammar in get_grammars(set(target_versions)):


@@ -1309,7 +1387,13 @@ class BracketTracker:
         self.maybe_decrement_after_lambda_arguments(leaf)
         if leaf.type in CLOSING_BRACKETS:
             self.depth -= 1
         self.maybe_decrement_after_lambda_arguments(leaf)
         if leaf.type in CLOSING_BRACKETS:
             self.depth -= 1

-            opening_bracket = self.bracket_match.pop((self.depth, leaf.type))
+            try:
+                opening_bracket = self.bracket_match.pop((self.depth, leaf.type))
+            except KeyError as e:
+                raise BracketMatchError(
+                    "Unable to match a closing bracket to the following opening"
+                    f" bracket: {leaf}"
+                ) from e

             leaf.opening_bracket = opening_bracket
             if not leaf.value:
                 self.invisible.append(leaf)
             leaf.opening_bracket = opening_bracket
             if not leaf.value:
                 self.invisible.append(leaf)


@@ -1416,13 +1500,15 @@ class BracketTracker:
 class Line:
     """Holds leaves and comments. Can be printed with `str(line)`."""
 
 class Line:
     """Holds leaves and comments. Can be printed with `str(line)`."""
 

+    mode: Mode

     depth: int = 0
     leaves: List[Leaf] = field(default_factory=list)
     # keys ordered like `leaves`
     comments: Dict[LeafID, List[Leaf]] = field(default_factory=dict)
     bracket_tracker: BracketTracker = field(default_factory=BracketTracker)
     inside_brackets: bool = False
     depth: int = 0
     leaves: List[Leaf] = field(default_factory=list)
     # keys ordered like `leaves`
     comments: Dict[LeafID, List[Leaf]] = field(default_factory=dict)
     bracket_tracker: BracketTracker = field(default_factory=BracketTracker)
     inside_brackets: bool = False

-    should_explode: bool = False
+    should_split_rhs: bool = False
+    magic_trailing_comma: Optional[Leaf] = None

 
     def append(self, leaf: Leaf, preformatted: bool = False) -> None:
         """Add a new `leaf` to the end of the line.
 
     def append(self, leaf: Leaf, preformatted: bool = False) -> None:
         """Add a new `leaf` to the end of the line.


@@ -1448,8 +1534,11 @@ class Line:
             )
         if self.inside_brackets or not preformatted:
             self.bracket_tracker.mark(leaf)
             )
         if self.inside_brackets or not preformatted:
             self.bracket_tracker.mark(leaf)

-            if self.maybe_should_explode(leaf):
-                self.should_explode = True
+            if self.mode.magic_trailing_comma:
+                if self.has_magic_trailing_comma(leaf):
+                    self.magic_trailing_comma = leaf
+            elif self.has_magic_trailing_comma(leaf, ensure_removable=True):
+                self.remove_trailing_comma()

         if not self.append_comment(leaf):
             self.leaves.append(leaf)
 
         if not self.append_comment(leaf):
             self.leaves.append(leaf)
 


@@ -1625,22 +1714,31 @@ class Line:
     def contains_multiline_strings(self) -> bool:
         return any(is_multiline_string(leaf) for leaf in self.leaves)
 
     def contains_multiline_strings(self) -> bool:
         return any(is_multiline_string(leaf) for leaf in self.leaves)
 

-    def maybe_should_explode(self, closing: Leaf) -> bool:
-        """Return True if this line should explode (always be split), that is when:
-        - there's a pre-existing trailing comma here; and
-        - it's not a one-tuple.
+    def has_magic_trailing_comma(
+        self, closing: Leaf, ensure_removable: bool = False
+    ) -> bool:
+        """Return True if we have a magic trailing comma, that is when:
+        - there's a trailing comma here
+        - it's not a one-tuple
+        Additionally, if ensure_removable:
+        - it's not from square bracket indexing

         """
         if not (
             closing.type in CLOSING_BRACKETS
             and self.leaves
             and self.leaves[-1].type == token.COMMA
         """
         if not (
             closing.type in CLOSING_BRACKETS
             and self.leaves
             and self.leaves[-1].type == token.COMMA

-            and not self.leaves[-1].was_checked  # pre-existing

         ):
             return False
 
         ):
             return False
 

-        if closing.type in {token.RBRACE, token.RSQB}:
+        if closing.type == token.RBRACE:

             return True
 
             return True
 

+        if closing.type == token.RSQB:
+            if not ensure_removable:
+                return True
+            comma = self.leaves[-1]
+            return bool(comma.parent and comma.parent.type == syms.listmaker)
+

         if self.is_import:
             return True
 
         if self.is_import:
             return True
 


@@ -1718,9 +1816,11 @@ class Line:
 
     def clone(self) -> "Line":
         return Line(
 
     def clone(self) -> "Line":
         return Line(

+            mode=self.mode,

             depth=self.depth,
             inside_brackets=self.inside_brackets,
             depth=self.depth,
             inside_brackets=self.inside_brackets,

-            should_explode=self.should_explode,
+            should_split_rhs=self.should_split_rhs,
+            magic_trailing_comma=self.magic_trailing_comma,

         )
 
     def __str__(self) -> str:
         )
 
     def __str__(self) -> str:


@@ -1826,6 +1926,10 @@ class EmptyLineTracker:
             return 0, 0
 
         if self.previous_line.is_decorator:
             return 0, 0
 
         if self.previous_line.is_decorator:

+            if self.is_pyi and current_line.is_stub_class:
+                # Insert an empty line after a decorated stub class
+                return 0, 1
+

             return 0, 0
 
         if self.previous_line.depth < current_line.depth and (
             return 0, 0
 
         if self.previous_line.depth < current_line.depth and (


@@ -1849,8 +1953,11 @@ class EmptyLineTracker:
                     newlines = 0
                 else:
                     newlines = 1
                     newlines = 0
                 else:
                     newlines = 1

-            elif current_line.is_def and not self.previous_line.is_def:
-                # Blank line between a block of functions and a block of non-functions
+            elif (
+                current_line.is_def or current_line.is_decorator
+            ) and not self.previous_line.is_def:
+                # Blank line between a block of functions (maybe with preceding
+                # decorators) and a block of non-functions

                 newlines = 1
             else:
                 newlines = 0
                 newlines = 1
             else:
                 newlines = 0


@@ -1869,10 +1976,9 @@ class LineGenerator(Visitor[Line]):
     in ways that will no longer stringify to valid Python code on the tree.
     """
 
     in ways that will no longer stringify to valid Python code on the tree.
     """
 

-    is_pyi: bool = False
-    normalize_strings: bool = True
-    current_line: Line = field(default_factory=Line)
+    mode: Mode

     remove_u_prefix: bool = False
     remove_u_prefix: bool = False

+    current_line: Line = field(init=False)

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


@@ -1887,7 +1993,7 @@ class LineGenerator(Visitor[Line]):
             return  # Line is empty, don't emit. Creating a new one unnecessary.
 
         complete_line = self.current_line
             return  # Line is empty, don't emit. Creating a new one unnecessary.
 
         complete_line = self.current_line

-        self.current_line = Line(depth=complete_line.depth + indent)
+        self.current_line = Line(mode=self.mode, depth=complete_line.depth + indent)

         yield complete_line
 
     def visit_default(self, node: LN) -> Iterator[Line]:
         yield complete_line
 
     def visit_default(self, node: LN) -> Iterator[Line]:


@@ -1911,7 +2017,7 @@ class LineGenerator(Visitor[Line]):
                     yield from self.line()
 
             normalize_prefix(node, inside_brackets=any_open_brackets)
                     yield from self.line()
 
             normalize_prefix(node, inside_brackets=any_open_brackets)

-            if self.normalize_strings and node.type == token.STRING:
+            if self.mode.string_normalization and node.type == token.STRING:

                 normalize_string_prefix(node, remove_u_prefix=self.remove_u_prefix)
                 normalize_string_quotes(node)
             if node.type == token.NUMBER:
                 normalize_string_prefix(node, remove_u_prefix=self.remove_u_prefix)
                 normalize_string_quotes(node)
             if node.type == token.NUMBER:


@@ -1963,16 +2069,18 @@ class LineGenerator(Visitor[Line]):
 
     def visit_suite(self, node: Node) -> Iterator[Line]:
         """Visit a suite."""
 
     def visit_suite(self, node: Node) -> Iterator[Line]:
         """Visit a suite."""

-        if self.is_pyi and is_stub_suite(node):
+        if self.mode.is_pyi and is_stub_suite(node):

             yield from self.visit(node.children[2])
         else:
             yield from self.visit_default(node)
 
     def visit_simple_stmt(self, node: Node) -> Iterator[Line]:
         """Visit a statement without nested statements."""
             yield from self.visit(node.children[2])
         else:
             yield from self.visit_default(node)
 
     def visit_simple_stmt(self, node: Node) -> Iterator[Line]:
         """Visit a statement without nested statements."""

+        if first_child_is_arith(node):
+            wrap_in_parentheses(node, node.children[0], visible=False)

         is_suite_like = node.parent and node.parent.type in STATEMENT
         if is_suite_like:
         is_suite_like = node.parent and node.parent.type in STATEMENT
         if is_suite_like:

-            if self.is_pyi and is_stub_body(node):
+            if self.mode.is_pyi and is_stub_body(node):

                 yield from self.visit_default(node)
             else:
                 yield from self.line(+1)
                 yield from self.visit_default(node)
             else:
                 yield from self.line(+1)


@@ -1980,7 +2088,11 @@ class LineGenerator(Visitor[Line]):
                 yield from self.line(-1)
 
         else:
                 yield from self.line(-1)
 
         else:

-            if not self.is_pyi or not node.parent or not is_stub_suite(node.parent):
+            if (
+                not self.mode.is_pyi
+                or not node.parent
+                or not is_stub_suite(node.parent)
+            ):

                 yield from self.line()
             yield from self.visit_default(node)
 
                 yield from self.line()
             yield from self.visit_default(node)
 


@@ -2037,19 +2149,27 @@ class LineGenerator(Visitor[Line]):
         yield from self.visit_default(node)
 
     def visit_STRING(self, leaf: Leaf) -> Iterator[Line]:
         yield from self.visit_default(node)
 
     def visit_STRING(self, leaf: Leaf) -> Iterator[Line]:

-        # Check if it's a docstring
-        if prev_siblings_are(
-            leaf.parent, [None, token.NEWLINE, token.INDENT, syms.simple_stmt]
-        ) and is_multiline_string(leaf):
-            prefix = "    " * self.current_line.depth
-            docstring = fix_docstring(leaf.value[3:-3], prefix)
-            leaf.value = leaf.value[0:3] + docstring + leaf.value[-3:]
-            normalize_string_quotes(leaf)
+        if is_docstring(leaf) and "\\\n" not in leaf.value:
+            # We're ignoring docstrings with backslash newline escapes because changing
+            # indentation of those changes the AST representation of the code.
+            prefix = get_string_prefix(leaf.value)
+            lead_len = len(prefix) + 3
+            tail_len = -3
+            indent = " " * 4 * self.current_line.depth
+            docstring = fix_docstring(leaf.value[lead_len:tail_len], indent)
+            if docstring:
+                if leaf.value[lead_len - 1] == docstring[0]:
+                    docstring = " " + docstring
+                if leaf.value[tail_len + 1] == docstring[-1]:
+                    docstring = docstring + " "
+            leaf.value = leaf.value[0:lead_len] + docstring + leaf.value[tail_len:]

 
         yield from self.visit_default(leaf)
 
     def __post_init__(self) -> None:
         """You are in a twisty little maze of passages."""
 
         yield from self.visit_default(leaf)
 
     def __post_init__(self) -> None:
         """You are in a twisty little maze of passages."""

+        self.current_line = Line(mode=self.mode)
+

         v = self.visit_stmt
         Ø: Set[str] = set()
         self.visit_assert_stmt = partial(v, keywords={"assert"}, parens={"assert", ","})
         v = self.visit_stmt
         Ø: Set[str] = set()
         self.visit_assert_stmt = partial(v, keywords={"assert"}, parens={"assert", ","})


@@ -2163,6 +2283,9 @@ def whitespace(leaf: Leaf, *, complex_subscript: bool) -> str:  # noqa: C901
         ):
             # Python 2 print chevron
             return NO
         ):
             # Python 2 print chevron
             return NO

+        elif prevp.type == token.AT and p.parent and p.parent.type == syms.decorator:
+            # no space in decorators
+            return NO

 
     elif prev.type in OPENING_BRACKETS:
         return NO
 
     elif prev.type in OPENING_BRACKETS:
         return NO


@@ -2489,6 +2612,8 @@ def is_split_before_delimiter(leaf: Leaf, previous: Optional[Leaf] = None) -> Pr
 
 
 FMT_OFF = {"# fmt: off", "# fmt:off", "# yapf: disable"}
 
 
 FMT_OFF = {"# fmt: off", "# fmt:off", "# yapf: disable"}

+FMT_SKIP = {"# fmt: skip", "# fmt:skip"}
+FMT_PASS = {*FMT_OFF, *FMT_SKIP}

 FMT_ON = {"# fmt: on", "# fmt:on", "# yapf: enable"}
 
 
 FMT_ON = {"# fmt: on", "# fmt:on", "# yapf: enable"}
 
 


@@ -2543,7 +2668,7 @@ def list_comments(prefix: str, *, is_endmarker: bool) -> List[ProtoComment]:
     consumed = 0
     nlines = 0
     ignored_lines = 0
     consumed = 0
     nlines = 0
     ignored_lines = 0

-    for index, line in enumerate(prefix.split("\n")):
+    for index, line in enumerate(re.split("\r?\n", prefix)):

         consumed += len(line) + 1  # adding the length of the split '\n'
         line = line.lstrip()
         if not line:
         consumed += len(line) + 1  # adding the length of the split '\n'
         line = line.lstrip()
         if not line:


@@ -2616,7 +2741,8 @@ def transform_line(
     transformers: List[Transformer]
     if (
         not line.contains_uncollapsable_type_comments()
     transformers: List[Transformer]
     if (
         not line.contains_uncollapsable_type_comments()

-        and not line.should_explode
+        and not line.should_split_rhs
+        and not line.magic_trailing_comma

         and (
             is_line_short_enough(line, line_length=mode.line_length, line_str=line_str)
             or line.contains_unsplittable_type_ignore()
         and (
             is_line_short_enough(line, line_length=mode.line_length, line_str=line_str)
             or line.contains_unsplittable_type_ignore()


@@ -2654,7 +2780,7 @@ def transform_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, as well as lines that contain
             # All splits failed, best effort split with no omits.
             # This mostly happens to multiline strings that are by definition
             # reported as not fitting a single line, as well as lines that contain

-            # pre-existing trailing commas (those have to be exploded).
+            # trailing commas (those have to be exploded).

             yield from right_hand_split(
                 line, line_length=mode.line_length, features=features
             )
             yield from right_hand_split(
                 line, line_length=mode.line_length, features=features
             )


@@ -2664,9 +2790,9 @@ def transform_line(
                 transformers = [
                     string_merge,
                     string_paren_strip,
                 transformers = [
                     string_merge,
                     string_paren_strip,

+                    string_split,

                     delimiter_split,
                     standalone_comment_split,
                     delimiter_split,
                     standalone_comment_split,

-                    string_split,

                     string_paren_wrap,
                     rhs,
                 ]
                     string_paren_wrap,
                     rhs,
                 ]


@@ -2885,11 +3011,8 @@ class StringMerger(CustomSplitMapMixin, StringTransformer):
     """StringTransformer that merges strings together.
 
     Requirements:
     """StringTransformer that merges strings together.
 
     Requirements:

-        (A) The line contains adjacent strings such that at most one substring
-        has inline comments AND none of those inline comments are pragmas AND
-        the set of all substring prefixes is either of length 1 or equal to
-        {"", "f"} AND none of the substrings are raw strings (i.e. are prefixed
-        with 'r').
+        (A) The line contains adjacent strings such that ALL of the validation checks
+        listed in StringMerger.__validate_msg(...)'s docstring pass.

             OR
         (B) The line contains a string which uses line continuation backslashes.
 
             OR
         (B) The line contains a string which uses line continuation backslashes.
 


@@ -3138,6 +3261,7 @@ class StringMerger(CustomSplitMapMixin, StringTransformer):
             * Ok(None), if ALL validation checks (listed below) pass.
                 OR
             * Err(CannotTransform), if any of the following are true:
             * Ok(None), if ALL validation checks (listed below) pass.
                 OR
             * Err(CannotTransform), if any of the following are true:

+                - The target string group does not contain ANY stand-alone comments.

                 - The target string is not in a string group (i.e. it has no
                   adjacent strings).
                 - The string group has more than one inline comment.
                 - The target string is not in a string group (i.e. it has no
                   adjacent strings).
                 - The string group has more than one inline comment.


@@ -3146,6 +3270,26 @@ class StringMerger(CustomSplitMapMixin, StringTransformer):
                   length greater than one and is not equal to {"", "f"}.
                 - The string group consists of raw strings.
         """
                   length greater than one and is not equal to {"", "f"}.
                 - The string group consists of raw strings.
         """

+        # We first check for "inner" stand-alone comments (i.e. stand-alone
+        # comments that have a string leaf before them AND after them).
+        for inc in [1, -1]:
+            i = string_idx
+            found_sa_comment = False
+            is_valid_index = is_valid_index_factory(line.leaves)
+            while is_valid_index(i) and line.leaves[i].type in [
+                token.STRING,
+                STANDALONE_COMMENT,
+            ]:
+                if line.leaves[i].type == STANDALONE_COMMENT:
+                    found_sa_comment = True
+                elif found_sa_comment:
+                    return TErr(
+                        "StringMerger does NOT merge string groups which contain "
+                        "stand-alone comments."
+                    )
+
+                i += inc
+

         num_of_inline_string_comments = 0
         set_of_prefixes = set()
         num_of_strings = 0
         num_of_inline_string_comments = 0
         set_of_prefixes = set()
         num_of_strings = 0


@@ -3194,7 +3338,8 @@ class StringParenStripper(StringTransformer):
 
     Requirements:
         The line contains a string which is surrounded by parentheses and:
 
     Requirements:
         The line contains a string which is surrounded by parentheses and:

-            - The target string is NOT the only argument to a function call).
+            - The target string is NOT the only argument to a function call.
+            - The target string is NOT a "pointless" string.

             - If the target string contains a PERCENT, the brackets are not
               preceeded or followed by an operator with higher precedence than
               PERCENT.
             - If the target string contains a PERCENT, the brackets are not
               preceeded or followed by an operator with higher precedence than
               PERCENT.


@@ -3218,6 +3363,14 @@ class StringParenStripper(StringTransformer):
             if leaf.type != token.STRING:
                 continue
 
             if leaf.type != token.STRING:
                 continue
 

+            # If this is a "pointless" string...
+            if (
+                leaf.parent
+                and leaf.parent.parent
+                and leaf.parent.parent.type == syms.simple_stmt
+            ):
+                continue
+

             # Should be preceded by a non-empty LPAR...
             if (
                 not is_valid_index(idx - 1)
             # Should be preceded by a non-empty LPAR...
             if (
                 not is_valid_index(idx - 1)


@@ -3302,10 +3455,17 @@ class StringParenStripper(StringTransformer):
                 yield TErr(
                     "Will not strip parentheses which have comments attached to them."
                 )
                 yield TErr(
                     "Will not strip parentheses which have comments attached to them."
                 )

+                return

 
         new_line = line.clone()
         new_line.comments = line.comments.copy()
 
         new_line = line.clone()
         new_line.comments = line.comments.copy()

-        append_leaves(new_line, line, LL[: string_idx - 1])
+        try:
+            append_leaves(new_line, line, LL[: string_idx - 1])
+        except BracketMatchError:
+            # HACK: I believe there is currently a bug somewhere in
+            # right_hand_split() that is causing brackets to not be tracked
+            # properly by a shared BracketTracker.
+            append_leaves(new_line, line, LL[: string_idx - 1], preformatted=True)

 
         string_leaf = Leaf(token.STRING, LL[string_idx].value)
         LL[string_idx - 1].remove()
 
         string_leaf = Leaf(token.STRING, LL[string_idx].value)
         LL[string_idx - 1].remove()


@@ -3468,9 +3628,12 @@ class BaseStringSplitter(StringTransformer):
                 # WMA4 a single space.
                 offset += 1
 
                 # WMA4 a single space.
                 offset += 1
 

-                # WMA4 the lengths of any leaves that came before that space.
-                for leaf in LL[: p_idx + 1]:
+                # WMA4 the lengths of any leaves that came before that space,
+                # but after any closing bracket before that space.
+                for leaf in reversed(LL[: p_idx + 1]):

                     offset += len(str(leaf))
                     offset += len(str(leaf))

+                    if leaf.type in CLOSING_BRACKETS:
+                        break

 
         if is_valid_index(string_idx + 1):
             N = LL[string_idx + 1]
 
         if is_valid_index(string_idx + 1):
             N = LL[string_idx + 1]


@@ -3536,7 +3699,8 @@ class StringSplitter(CustomSplitMapMixin, BaseStringSplitter):
         MIN_SUBSTR_SIZE characters.
 
         The string will ONLY be split on spaces (i.e. each new substring should
         MIN_SUBSTR_SIZE characters.
 
         The string will ONLY be split on spaces (i.e. each new substring should

-        start with a space).
+        start with a space). Note that the string will NOT be split on a space
+        which is escaped with a backslash.

 
         If the string is an f-string, it will NOT be split in the middle of an
         f-expression (e.g. in f"FooBar: {foo() if x else bar()}", {foo() if x
 
         If the string is an f-string, it will NOT be split in the middle of an
         f-expression (e.g. in f"FooBar: {foo() if x else bar()}", {foo() if x


@@ -3556,13 +3720,14 @@ class StringSplitter(CustomSplitMapMixin, BaseStringSplitter):
     MIN_SUBSTR_SIZE = 6
     # Matches an "f-expression" (e.g. {var}) that might be found in an f-string.
     RE_FEXPR = r"""
     MIN_SUBSTR_SIZE = 6
     # Matches an "f-expression" (e.g. {var}) that might be found in an f-string.
     RE_FEXPR = r"""

-    (?<!\{)\{
+    (?<!\{) (?:\{\{)* \{ (?!\{)

         (?:
             [^\{\}]
             | \{\{
             | \}\}
         (?:
             [^\{\}]
             | \{\{
             | \}\}

+            | (?R)

         )+?
         )+?

-    (?<!\})(?:\}\})*\}(?!\})
+    (?<!\}) \} (?:\}\})* (?!\})

     """
 
     def do_splitter_match(self, line: Line) -> TMatchResult:
     """
 
     def do_splitter_match(self, line: Line) -> TMatchResult:


@@ -3876,11 +4041,23 @@ class StringSplitter(CustomSplitMapMixin, BaseStringSplitter):
                 section of this classes' docstring would be be met by returning @i.
             """
             is_space = string[i] == " "
                 section of this classes' docstring would be be met by returning @i.
             """
             is_space = string[i] == " "

+
+            is_not_escaped = True
+            j = i - 1
+            while is_valid_index(j) and string[j] == "\\":
+                is_not_escaped = not is_not_escaped
+                j -= 1
+

             is_big_enough = (
                 len(string[i:]) >= self.MIN_SUBSTR_SIZE
                 and len(string[:i]) >= self.MIN_SUBSTR_SIZE
             )
             is_big_enough = (
                 len(string[i:]) >= self.MIN_SUBSTR_SIZE
                 and len(string[:i]) >= self.MIN_SUBSTR_SIZE
             )

-            return is_space and is_big_enough and not breaks_fstring_expression(i)
+            return (
+                is_space
+                and is_not_escaped
+                and is_big_enough
+                and not breaks_fstring_expression(i)
+            )

 
         # First, we check all indices BELOW @max_break_idx.
         break_idx = max_break_idx
 
         # First, we check all indices BELOW @max_break_idx.
         break_idx = max_break_idx


@@ -3986,12 +4163,13 @@ class StringParenWrapper(CustomSplitMapMixin, BaseStringSplitter):
     def do_splitter_match(self, line: Line) -> TMatchResult:
         LL = line.leaves
 
     def do_splitter_match(self, line: Line) -> TMatchResult:
         LL = line.leaves
 

-        string_idx = None
-        string_idx = string_idx or self._return_match(LL)
-        string_idx = string_idx or self._else_match(LL)
-        string_idx = string_idx or self._assert_match(LL)
-        string_idx = string_idx or self._assign_match(LL)
-        string_idx = string_idx or self._dict_match(LL)
+        string_idx = (
+            self._return_match(LL)
+            or self._else_match(LL)
+            or self._assert_match(LL)
+            or self._assign_match(LL)
+            or self._dict_match(LL)
+        )

 
         if string_idx is not None:
             string_value = line.leaves[string_idx].value
 
         if string_idx is not None:
             string_value = line.leaves[string_idx].value


@@ -4190,7 +4368,7 @@ class StringParenWrapper(CustomSplitMapMixin, BaseStringSplitter):
         is_valid_index = is_valid_index_factory(LL)
         insert_str_child = insert_str_child_factory(LL[string_idx])
 
         is_valid_index = is_valid_index_factory(LL)
         insert_str_child = insert_str_child_factory(LL[string_idx])
 

-        comma_idx = len(LL) - 1
+        comma_idx = -1

         ends_with_comma = False
         if LL[comma_idx].type == token.COMMA:
             ends_with_comma = True
         ends_with_comma = False
         if LL[comma_idx].type == token.COMMA:
             ends_with_comma = True


@@ -4235,9 +4413,11 @@ class StringParenWrapper(CustomSplitMapMixin, BaseStringSplitter):
         # `StringSplitter` will break it down further if necessary.
         string_value = LL[string_idx].value
         string_line = Line(
         # `StringSplitter` will break it down further if necessary.
         string_value = LL[string_idx].value
         string_line = Line(

+            mode=line.mode,

             depth=line.depth + 1,
             inside_brackets=True,
             depth=line.depth + 1,
             inside_brackets=True,

-            should_explode=line.should_explode,
+            should_split_rhs=line.should_split_rhs,
+            magic_trailing_comma=line.magic_trailing_comma,

         )
         string_leaf = Leaf(token.STRING, string_value)
         insert_str_child(string_leaf)
         )
         string_leaf = Leaf(token.STRING, string_value)
         insert_str_child(string_leaf)


@@ -4575,7 +4755,9 @@ def line_to_string(line: Line) -> str:
     return str(line).strip("\n")
 
 
     return str(line).strip("\n")
 
 

-def append_leaves(new_line: Line, old_line: Line, leaves: List[Leaf]) -> None:
+def append_leaves(
+    new_line: Line, old_line: Line, leaves: List[Leaf], preformatted: bool = False
+) -> None:

     """
     Append leaves (taken from @old_line) to @new_line, making sure to fix the
     underlying Node structure where appropriate.
     """
     Append leaves (taken from @old_line) to @new_line, making sure to fix the
     underlying Node structure where appropriate.


@@ -4591,7 +4773,7 @@ def append_leaves(new_line: Line, old_line: Line, leaves: List[Leaf]) -> None:
     for old_leaf in leaves:
         new_leaf = Leaf(old_leaf.type, old_leaf.value)
         replace_child(old_leaf, new_leaf)
     for old_leaf in leaves:
         new_leaf = Leaf(old_leaf.type, old_leaf.value)
         replace_child(old_leaf, new_leaf)

-        new_line.append(new_leaf)
+        new_line.append(new_leaf, preformatted=preformatted)

 
         for comment_leaf in old_line.comments_after(old_leaf):
             new_line.append(comment_leaf, preformatted=True)
 
         for comment_leaf in old_line.comments_after(old_leaf):
             new_line.append(comment_leaf, preformatted=True)


@@ -4826,7 +5008,7 @@ def bracket_split_build_line(
     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.
     """
     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)
+    result = Line(mode=original.mode, depth=original.depth)

     if is_body:
         result.inside_brackets = True
         result.depth += 1
     if is_body:
         result.inside_brackets = True
         result.depth += 1


@@ -4848,7 +5030,6 @@ def bracket_split_build_line(
 
                     if leaves[i].type != token.COMMA:
                         new_comma = Leaf(token.COMMA, ",")
 
                     if leaves[i].type != token.COMMA:
                         new_comma = Leaf(token.COMMA, ",")

-                        new_comma.was_checked = True

                         leaves.insert(i + 1, new_comma)
                     break
 
                         leaves.insert(i + 1, new_comma)
                     break
 


@@ -4857,8 +5038,8 @@ def bracket_split_build_line(
         result.append(leaf, preformatted=True)
         for comment_after in original.comments_after(leaf):
             result.append(comment_after, preformatted=True)
         result.append(leaf, preformatted=True)
         for comment_after in original.comments_after(leaf):
             result.append(comment_after, preformatted=True)

-    if is_body and should_split_body_explode(result, opening_bracket):
-        result.should_explode = True
+    if is_body and should_split_line(result, opening_bracket):
+        result.should_split_rhs = True

     return result
 
 
     return result
 
 


@@ -4899,7 +5080,9 @@ def delimiter_split(line: Line, features: Collection[Feature] = ()) -> Iterator[
         if bt.delimiter_count_with_priority(delimiter_priority) == 1:
             raise CannotSplit("Splitting a single attribute from its owner looks wrong")
 
         if bt.delimiter_count_with_priority(delimiter_priority) == 1:
             raise CannotSplit("Splitting a single attribute from its owner looks wrong")
 

-    current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)
+    current_line = Line(
+        mode=line.mode, depth=line.depth, inside_brackets=line.inside_brackets
+    )

     lowest_depth = sys.maxsize
     trailing_comma_safe = True
 
     lowest_depth = sys.maxsize
     trailing_comma_safe = True
 


@@ -4911,7 +5094,9 @@ def delimiter_split(line: Line, features: Collection[Feature] = ()) -> Iterator[
         except ValueError:
             yield current_line
 
         except ValueError:
             yield current_line
 

-            current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)
+            current_line = Line(
+                mode=line.mode, depth=line.depth, inside_brackets=line.inside_brackets
+            )

             current_line.append(leaf)
 
     for leaf in line.leaves:
             current_line.append(leaf)
 
     for leaf in line.leaves:


@@ -4935,7 +5120,9 @@ def delimiter_split(line: Line, features: Collection[Feature] = ()) -> Iterator[
         if leaf_priority == delimiter_priority:
             yield current_line
 
         if leaf_priority == delimiter_priority:
             yield current_line
 

-            current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)
+            current_line = Line(
+                mode=line.mode, depth=line.depth, inside_brackets=line.inside_brackets
+            )

     if current_line:
         if (
             trailing_comma_safe
     if current_line:
         if (
             trailing_comma_safe


@@ -4944,7 +5131,6 @@ def delimiter_split(line: Line, features: Collection[Feature] = ()) -> Iterator[
             and current_line.leaves[-1].type != STANDALONE_COMMENT
         ):
             new_comma = Leaf(token.COMMA, ",")
             and current_line.leaves[-1].type != STANDALONE_COMMENT
         ):
             new_comma = Leaf(token.COMMA, ",")

-            new_comma.was_checked = True

             current_line.append(new_comma)
         yield current_line
 
             current_line.append(new_comma)
         yield current_line
 


@@ -4957,7 +5143,9 @@ def standalone_comment_split(
     if not line.contains_standalone_comments(0):
         raise CannotSplit("Line does not have any standalone comments")
 
     if not line.contains_standalone_comments(0):
         raise CannotSplit("Line does not have any standalone comments")
 

-    current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)
+    current_line = Line(
+        mode=line.mode, depth=line.depth, inside_brackets=line.inside_brackets
+    )

 
     def append_to_line(leaf: Leaf) -> Iterator[Line]:
         """Append `leaf` to current line or to new line if appending impossible."""
 
     def append_to_line(leaf: Leaf) -> Iterator[Line]:
         """Append `leaf` to current line or to new line if appending impossible."""


@@ -4967,7 +5155,9 @@ def standalone_comment_split(
         except ValueError:
             yield current_line
 
         except ValueError:
             yield current_line
 

-            current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)
+            current_line = Line(
+                line.mode, depth=line.depth, inside_brackets=line.inside_brackets
+            )

             current_line.append(leaf)
 
     for leaf in line.leaves:
             current_line.append(leaf)
 
     for leaf in line.leaves:


@@ -5123,31 +5313,52 @@ def normalize_numeric_literal(leaf: Leaf) -> None:
         # Leave octal and binary literals alone.
         pass
     elif text.startswith("0x"):
         # Leave octal and binary literals alone.
         pass
     elif text.startswith("0x"):

-        # Change hex literals to upper case.
-        before, after = text[:2], text[2:]
-        text = f"{before}{after.upper()}"
+        text = format_hex(text)

     elif "e" in text:
     elif "e" in text:

-        before, after = text.split("e")
-        sign = ""
-        if after.startswith("-"):
-            after = after[1:]
-            sign = "-"
-        elif after.startswith("+"):
-            after = after[1:]
-        before = format_float_or_int_string(before)
-        text = f"{before}e{sign}{after}"
+        text = format_scientific_notation(text)

     elif text.endswith(("j", "l")):
     elif text.endswith(("j", "l")):

-        number = text[:-1]
-        suffix = text[-1]
-        # Capitalize in "2L" because "l" looks too similar to "1".
-        if suffix == "l":
-            suffix = "L"
-        text = f"{format_float_or_int_string(number)}{suffix}"
+        text = format_long_or_complex_number(text)

     else:
         text = format_float_or_int_string(text)
     leaf.value = text
 
 
     else:
         text = format_float_or_int_string(text)
     leaf.value = text
 
 

+def format_hex(text: str) -> str:
+    """
+    Formats a hexadecimal string like "0x12b3"
+
+    Uses lowercase because of similarity between "B" and "8", which
+    can cause security issues.
+    see: https://github.com/psf/black/issues/1692
+    """
+
+    before, after = text[:2], text[2:]
+    return f"{before}{after.lower()}"
+
+
+def format_scientific_notation(text: str) -> str:
+    """Formats a numeric string utilizing scentific notation"""
+    before, after = text.split("e")
+    sign = ""
+    if after.startswith("-"):
+        after = after[1:]
+        sign = "-"
+    elif after.startswith("+"):
+        after = after[1:]
+    before = format_float_or_int_string(before)
+    return f"{before}e{sign}{after}"
+
+
+def format_long_or_complex_number(text: str) -> str:
+    """Formats a long or complex string like `10L` or `10j`"""
+    number = text[:-1]
+    suffix = text[-1]
+    # Capitalize in "2L" because "l" looks too similar to "1".
+    if suffix == "l":
+        suffix = "L"
+    return f"{format_float_or_int_string(number)}{suffix}"
+
+

 def format_float_or_int_string(text: str) -> str:
     """Formats a float string like "1.0"."""
     if "." not in text:
 def format_float_or_int_string(text: str) -> str:
     """Formats a float string like "1.0"."""
     if "." not in text:


@@ -5186,9 +5397,6 @@ def normalize_invisible_parens(node: Node, parens_after: Set[str]) -> None:
             check_lpar = True
 
         if check_lpar:
             check_lpar = True
 
         if check_lpar:

-            if is_walrus_assignment(child):
-                continue
-

             if child.type == syms.atom:
                 if maybe_make_parens_invisible_in_atom(child, parent=node):
                     wrap_in_parentheses(node, child, visible=False)
             if child.type == syms.atom:
                 if maybe_make_parens_invisible_in_atom(child, parent=node):
                     wrap_in_parentheses(node, child, visible=False)


@@ -5228,58 +5436,80 @@ def convert_one_fmt_off_pair(node: Node) -> bool:
     for leaf in node.leaves():
         previous_consumed = 0
         for comment in list_comments(leaf.prefix, is_endmarker=False):
     for leaf in node.leaves():
         previous_consumed = 0
         for comment in list_comments(leaf.prefix, is_endmarker=False):

-            if comment.value in FMT_OFF:
-                # We only want standalone comments. If there's no previous leaf or
-                # the previous leaf is indentation, it's a standalone comment in
-                # disguise.
-                if comment.type != STANDALONE_COMMENT:
-                    prev = preceding_leaf(leaf)
-                    if prev and prev.type not in WHITESPACE:
+            if comment.value not in FMT_PASS:
+                previous_consumed = comment.consumed
+                continue
+            # We only want standalone comments. If there's no previous leaf or
+            # the previous leaf is indentation, it's a standalone comment in
+            # disguise.
+            if comment.value in FMT_PASS and comment.type != STANDALONE_COMMENT:
+                prev = preceding_leaf(leaf)
+                if prev:
+                    if comment.value in FMT_OFF and prev.type not in WHITESPACE:
+                        continue
+                    if comment.value in FMT_SKIP and prev.type in WHITESPACE:

                         continue
 
                         continue
 

-                ignored_nodes = list(generate_ignored_nodes(leaf))
-                if not ignored_nodes:
-                    continue
-
-                first = ignored_nodes[0]  # Can be a container node with the `leaf`.
-                parent = first.parent
-                prefix = first.prefix
-                first.prefix = prefix[comment.consumed :]
-                hidden_value = (
-                    comment.value + "\n" + "".join(str(n) for n in ignored_nodes)
-                )
-                if hidden_value.endswith("\n"):
-                    # That happens when one of the `ignored_nodes` ended with a NEWLINE
-                    # leaf (possibly followed by a DEDENT).
-                    hidden_value = hidden_value[:-1]
-                first_idx: Optional[int] = None
-                for ignored in ignored_nodes:
-                    index = ignored.remove()
-                    if first_idx is None:
-                        first_idx = index
-                assert parent is not None, "INTERNAL ERROR: fmt: on/off handling (1)"
-                assert first_idx is not None, "INTERNAL ERROR: fmt: on/off handling (2)"
-                parent.insert_child(
-                    first_idx,
-                    Leaf(
-                        STANDALONE_COMMENT,
-                        hidden_value,
-                        prefix=prefix[:previous_consumed] + "\n" * comment.newlines,
-                    ),
-                )
-                return True
+            ignored_nodes = list(generate_ignored_nodes(leaf, comment))
+            if not ignored_nodes:
+                continue

 
 

-            previous_consumed = comment.consumed
+            first = ignored_nodes[0]  # Can be a container node with the `leaf`.
+            parent = first.parent
+            prefix = first.prefix
+            first.prefix = prefix[comment.consumed :]
+            hidden_value = "".join(str(n) for n in ignored_nodes)
+            if comment.value in FMT_OFF:
+                hidden_value = comment.value + "\n" + hidden_value
+            if comment.value in FMT_SKIP:
+                hidden_value += "  " + comment.value
+            if hidden_value.endswith("\n"):
+                # That happens when one of the `ignored_nodes` ended with a NEWLINE
+                # leaf (possibly followed by a DEDENT).
+                hidden_value = hidden_value[:-1]
+            first_idx: Optional[int] = None
+            for ignored in ignored_nodes:
+                index = ignored.remove()
+                if first_idx is None:
+                    first_idx = index
+            assert parent is not None, "INTERNAL ERROR: fmt: on/off handling (1)"
+            assert first_idx is not None, "INTERNAL ERROR: fmt: on/off handling (2)"
+            parent.insert_child(
+                first_idx,
+                Leaf(
+                    STANDALONE_COMMENT,
+                    hidden_value,
+                    prefix=prefix[:previous_consumed] + "\n" * comment.newlines,
+                ),
+            )
+            return True

 
     return False
 
 
 
     return False
 
 

-def generate_ignored_nodes(leaf: Leaf) -> Iterator[LN]:
+def generate_ignored_nodes(leaf: Leaf, comment: ProtoComment) -> Iterator[LN]:

     """Starting from the container of `leaf`, generate all leaves until `# fmt: on`.
 
     """Starting from the container of `leaf`, generate all leaves until `# fmt: on`.
 

+    If comment is skip, returns leaf only.

     Stops at the end of the block.
     """
     container: Optional[LN] = container_of(leaf)
     Stops at the end of the block.
     """
     container: Optional[LN] = container_of(leaf)

+    if comment.value in FMT_SKIP:
+        prev_sibling = leaf.prev_sibling
+        if comment.value in leaf.prefix and prev_sibling is not None:
+            leaf.prefix = leaf.prefix.replace(comment.value, "")
+            siblings = [prev_sibling]
+            while (
+                "\n" not in prev_sibling.prefix
+                and prev_sibling.prev_sibling is not None
+            ):
+                prev_sibling = prev_sibling.prev_sibling
+                siblings.insert(0, prev_sibling)
+            for sibling in siblings:
+                yield sibling
+        elif leaf.parent is not None:
+            yield leaf.parent
+        return

     while container is not None and container.type != token.ENDMARKER:
         if is_fmt_on(container):
             return
     while container is not None and container.type != token.ENDMARKER:
         if is_fmt_on(container):
             return


@@ -5339,6 +5569,7 @@ def maybe_make_parens_invisible_in_atom(node: LN, parent: LN) -> bool:
     Returns whether the node should itself be wrapped in invisible parentheses.
 
     """
     Returns whether the node should itself be wrapped in invisible parentheses.
 
     """

+

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


@@ -5348,6 +5579,10 @@ def maybe_make_parens_invisible_in_atom(node: LN, parent: LN) -> bool:
     ):
         return False
 
     ):
         return False
 

+    if is_walrus_assignment(node):
+        if parent.type in [syms.annassign, syms.expr_stmt]:
+            return False
+

     first = node.children[0]
     last = node.children[-1]
     if first.type == token.LPAR and last.type == token.RPAR:
     first = node.children[0]
     last = node.children[-1]
     if first.type == token.LPAR and last.type == token.RPAR:


@@ -5409,6 +5644,17 @@ def unwrap_singleton_parenthesis(node: LN) -> Optional[LN]:
     return wrapped
 
 
     return wrapped
 
 

+def first_child_is_arith(node: Node) -> bool:
+    """Whether first child is an arithmetic or a binary arithmetic expression"""
+    expr_types = {
+        syms.arith_expr,
+        syms.shift_expr,
+        syms.xor_expr,
+        syms.and_expr,
+    }
+    return bool(node.children and node.children[0].type in expr_types)
+
+

 def wrap_in_parentheses(parent: Node, child: LN, *, visible: bool = True) -> None:
     """Wrap `child` in parentheses.
 
 def wrap_in_parentheses(parent: Node, child: LN, *, visible: bool = True) -> None:
     """Wrap `child` in parentheses.
 


@@ -5449,6 +5695,49 @@ def is_walrus_assignment(node: LN) -> bool:
     return inner is not None and inner.type == syms.namedexpr_test
 
 
     return inner is not None and inner.type == syms.namedexpr_test
 
 

+def is_simple_decorator_trailer(node: LN, last: bool = False) -> bool:
+    """Return True iff `node` is a trailer valid in a simple decorator"""
+    return node.type == syms.trailer and (
+        (
+            len(node.children) == 2
+            and node.children[0].type == token.DOT
+            and node.children[1].type == token.NAME
+        )
+        # last trailer can be arguments
+        or (
+            last
+            and len(node.children) == 3
+            and node.children[0].type == token.LPAR
+            # and node.children[1].type == syms.argument
+            and node.children[2].type == token.RPAR
+        )
+    )
+
+
+def is_simple_decorator_expression(node: LN) -> bool:
+    """Return True iff `node` could be a 'dotted name' decorator
+
+    This function takes the node of the 'namedexpr_test' of the new decorator
+    grammar and test if it would be valid under the old decorator grammar.
+
+    The old grammar was: decorator: @ dotted_name [arguments] NEWLINE
+    The new grammar is : decorator: @ namedexpr_test NEWLINE
+    """
+    if node.type == token.NAME:
+        return True
+    if node.type == syms.power:
+        if node.children:
+            return (
+                node.children[0].type == token.NAME
+                and all(map(is_simple_decorator_trailer, node.children[1:-1]))
+                and (
+                    len(node.children) < 2
+                    or is_simple_decorator_trailer(node.children[-1], last=True)
+                )
+            )
+    return False
+
+

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


@@ -5567,7 +5856,7 @@ def ensure_visible(leaf: Leaf) -> None:
         leaf.value = ")"
 
 
         leaf.value = ")"
 
 

-def should_split_body_explode(line: Line, opening_bracket: Leaf) -> bool:
+def should_split_line(line: Line, opening_bracket: Leaf) -> bool:

     """Should `line` be immediately split with `delimiter_split()` after RHS?"""
 
     if not (opening_bracket.parent and opening_bracket.value in "[{("):
     """Should `line` be immediately split with `delimiter_split()` after RHS?"""
 
     if not (opening_bracket.parent and opening_bracket.value in "[{("):


@@ -5577,20 +5866,20 @@ def should_split_body_explode(line: Line, opening_bracket: Leaf) -> bool:
     # than one of them (we're excluding the trailing comma and if the delimiter priority
     # is still commas, that means there's more).
     exclude = set()
     # than one of them (we're excluding the trailing comma and if the delimiter priority
     # is still commas, that means there's more).
     exclude = set()

-    pre_existing_trailing_comma = False
+    trailing_comma = False

     try:
         last_leaf = line.leaves[-1]
         if last_leaf.type == token.COMMA:
     try:
         last_leaf = line.leaves[-1]
         if last_leaf.type == token.COMMA:

-            pre_existing_trailing_comma = not last_leaf.was_checked
+            trailing_comma = True

             exclude.add(id(last_leaf))
         max_priority = line.bracket_tracker.max_delimiter_priority(exclude=exclude)
     except (IndexError, ValueError):
         return False
 
     return max_priority == COMMA_PRIORITY and (
             exclude.add(id(last_leaf))
         max_priority = line.bracket_tracker.max_delimiter_priority(exclude=exclude)
     except (IndexError, ValueError):
         return False
 
     return max_priority == COMMA_PRIORITY and (

+        (line.mode.magic_trailing_comma and trailing_comma)

         # always explode imports
         # always explode imports

-        opening_bracket.parent.type in {syms.atom, syms.import_from}
-        or pre_existing_trailing_comma
+        or opening_bracket.parent.type in {syms.atom, syms.import_from}

     )
 
 
     )
 
 


@@ -5634,6 +5923,8 @@ def get_features_used(node: Node) -> Set[Feature]:
     - underscores in numeric literals;
     - trailing commas after * or ** in function signatures and calls;
     - positional only arguments in function signatures and lambdas;
     - underscores in numeric literals;
     - trailing commas after * or ** in function signatures and calls;
     - positional only arguments in function signatures and lambdas;

+    - assignment expression;
+    - relaxed decorator syntax;

     """
     features: Set[Feature] = set()
     for n in node.pre_order():
     """
     features: Set[Feature] = set()
     for n in node.pre_order():


@@ -5653,6 +5944,12 @@ def get_features_used(node: Node) -> Set[Feature]:
         elif n.type == token.COLONEQUAL:
             features.add(Feature.ASSIGNMENT_EXPRESSIONS)
 
         elif n.type == token.COLONEQUAL:
             features.add(Feature.ASSIGNMENT_EXPRESSIONS)
 

+        elif n.type == syms.decorator:
+            if len(n.children) > 1 and not is_simple_decorator_expression(
+                n.children[1]
+            ):
+                features.add(Feature.RELAXED_DECORATORS)
+

         elif (
             n.type in {syms.typedargslist, syms.arglist}
             and n.children
         elif (
             n.type in {syms.typedargslist, syms.arglist}
             and n.children


@@ -5695,7 +5992,7 @@ def generate_trailers_to_omit(line: Line, line_length: int) -> Iterator[Set[Leaf
     """
 
     omit: Set[LeafID] = set()
     """
 
     omit: Set[LeafID] = set()

-    if not line.should_explode:
+    if not line.magic_trailing_comma:

         yield omit
 
     length = 4 * line.depth
         yield omit
 
     length = 4 * line.depth


@@ -5717,15 +6014,13 @@ def generate_trailers_to_omit(line: Line, line_length: int) -> Iterator[Set[Leaf
             elif leaf.type in CLOSING_BRACKETS:
                 prev = line.leaves[index - 1] if index > 0 else None
                 if (
             elif leaf.type in CLOSING_BRACKETS:
                 prev = line.leaves[index - 1] if index > 0 else None
                 if (

-                    line.should_explode
-                    and prev
+                    prev

                     and prev.type == token.COMMA
                     and prev.type == token.COMMA

-                    and not prev.was_checked

                     and not is_one_tuple_between(
                         leaf.opening_bracket, leaf, line.leaves
                     )
                 ):
                     and not is_one_tuple_between(
                         leaf.opening_bracket, leaf, line.leaves
                     )
                 ):

-                    # Never omit bracket pairs with pre-existing trailing commas.
+                    # Never omit bracket pairs with trailing commas.

                     # We need to explode on those.
                     break
 
                     # We need to explode on those.
                     break
 


@@ -5746,13 +6041,11 @@ def generate_trailers_to_omit(line: Line, line_length: int) -> Iterator[Set[Leaf
                 yield omit
 
             if (
                 yield omit
 
             if (

-                line.should_explode
-                and prev
+                prev

                 and prev.type == token.COMMA
                 and prev.type == token.COMMA

-                and not prev.was_checked

                 and not is_one_tuple_between(leaf.opening_bracket, leaf, line.leaves)
             ):
                 and not is_one_tuple_between(leaf.opening_bracket, leaf, line.leaves)
             ):

-                # Never omit bracket pairs with pre-existing trailing commas.
+                # Never omit bracket pairs with trailing commas.

                 # We need to explode on those.
                 break
 
                 # We need to explode on those.
                 break
 


@@ -5830,7 +6123,8 @@ def normalize_path_maybe_ignore(
     `report` is where "path ignored" output goes.
     """
     try:
     `report` is where "path ignored" output goes.
     """
     try:

-        normalized_path = path.resolve().relative_to(root).as_posix()
+        abspath = path if path.is_absolute() else Path.cwd() / path
+        normalized_path = abspath.resolve().relative_to(root).as_posix()

     except OSError as e:
         report.path_ignored(path, f"cannot be read because {e}")
         return None
     except OSError as e:
         report.path_ignored(path, f"cannot be read because {e}")
         return None


@@ -5845,17 +6139,27 @@ def normalize_path_maybe_ignore(
     return normalized_path
 
 
     return normalized_path
 
 

+def path_is_excluded(
+    normalized_path: str,
+    pattern: Optional[Pattern[str]],
+) -> bool:
+    match = pattern.search(normalized_path) if pattern else None
+    return bool(match and match.group(0))
+
+

 def gen_python_files(
     paths: Iterable[Path],
     root: Path,
     include: Optional[Pattern[str]],
     exclude: Pattern[str],
 def gen_python_files(
     paths: Iterable[Path],
     root: Path,
     include: Optional[Pattern[str]],
     exclude: Pattern[str],

+    extend_exclude: Optional[Pattern[str]],

     force_exclude: Optional[Pattern[str]],
     report: "Report",
     gitignore: PathSpec,
 ) -> Iterator[Path]:
     """Generate all files under `path` whose paths are not excluded by the
     force_exclude: Optional[Pattern[str]],
     report: "Report",
     gitignore: PathSpec,
 ) -> Iterator[Path]:
     """Generate all files under `path` whose paths are not excluded by the

-    `exclude_regex` or `force_exclude` regexes, but are included by the `include` regex.
+    `exclude_regex`, `extend_exclude`, or `force_exclude` regexes,
+    but are included by the `include` regex.

 
     Symbolic links pointing outside of the `root` directory are ignored.
 
 
     Symbolic links pointing outside of the `root` directory are ignored.
 


@@ -5872,20 +6176,22 @@ def gen_python_files(
             report.path_ignored(child, "matches the .gitignore file content")
             continue
 
             report.path_ignored(child, "matches the .gitignore file content")
             continue
 

-        # Then ignore with `--exclude` and `--force-exclude` options.
+        # Then ignore with `--exclude` `--extend-exclude` and `--force-exclude` options.

         normalized_path = "/" + normalized_path
         if child.is_dir():
             normalized_path += "/"
 
         normalized_path = "/" + normalized_path
         if child.is_dir():
             normalized_path += "/"
 

-        exclude_match = exclude.search(normalized_path) if exclude else None
-        if exclude_match and exclude_match.group(0):
+        if path_is_excluded(normalized_path, exclude):

             report.path_ignored(child, "matches the --exclude regular expression")
             continue
 
             report.path_ignored(child, "matches the --exclude regular expression")
             continue
 

-        force_exclude_match = (
-            force_exclude.search(normalized_path) if force_exclude else None
-        )
-        if force_exclude_match and force_exclude_match.group(0):
+        if path_is_excluded(normalized_path, extend_exclude):
+            report.path_ignored(
+                child, "matches the --extend-exclude regular expression"
+            )
+            continue
+
+        if path_is_excluded(normalized_path, force_exclude):

             report.path_ignored(child, "matches the --force-exclude regular expression")
             continue
 
             report.path_ignored(child, "matches the --force-exclude regular expression")
             continue
 


@@ -5895,6 +6201,7 @@ def gen_python_files(
                 root,
                 include,
                 exclude,
                 root,
                 include,
                 exclude,

+                extend_exclude,

                 force_exclude,
                 report,
                 gitignore,
                 force_exclude,
                 report,
                 gitignore,


@@ -5945,6 +6252,22 @@ def find_project_root(srcs: Iterable[str]) -> Path:
     return directory
 
 
     return directory
 
 

+@lru_cache()
+def find_user_pyproject_toml() -> Path:
+    r"""Return the path to the top-level user configuration for black.
+
+    This looks for ~\.black on Windows and ~/.config/black on Linux and other
+    Unix systems.
+    """
+    if sys.platform == "win32":
+        # Windows
+        user_config_path = Path.home() / ".black"
+    else:
+        config_root = os.environ.get("XDG_CONFIG_HOME", "~/.config")
+        user_config_path = Path(config_root).expanduser() / "black"
+    return user_config_path.resolve()
+
+

 @dataclass
 class Report:
     """Provides a reformatting counter. Can be rendered with `str(report)`."""
 @dataclass
 class Report:
     """Provides a reformatting counter. Can be rendered with `str(report)`."""


@@ -6046,7 +6369,12 @@ def parse_ast(src: str) -> Union[ast.AST, ast3.AST, ast27.AST]:
                 return ast3.parse(src, filename, feature_version=feature_version)
             except SyntaxError:
                 continue
                 return ast3.parse(src, filename, feature_version=feature_version)
             except SyntaxError:
                 continue

-
+    if ast27.__name__ == "ast":
+        raise SyntaxError(
+            "The requested source code has invalid Python 3 syntax.\n"
+            "If you are trying to format Python 2 files please reinstall Black"
+            " with the 'python2' extra: `python3 -m pip install black[python2]`."
+        )

     return ast27.parse(src)
 
 
     return ast27.parse(src)
 
 


@@ -6161,6 +6489,7 @@ def assert_stable(src: str, dst: str, mode: Mode) -> None:
     newdst = format_str(dst, mode=mode)
     if dst != newdst:
         log = dump_to_file(
     newdst = format_str(dst, mode=mode)
     if dst != newdst:
         log = dump_to_file(

+            str(mode),

             diff(src, dst, "source", "first pass"),
             diff(dst, newdst, "first pass", "second pass"),
         )
             diff(src, dst, "source", "first pass"),
             diff(dst, newdst, "first pass", "second pass"),
         )


@@ -6172,14 +6501,14 @@ def assert_stable(src: str, dst: str, mode: Mode) -> None:
 
 
 @mypyc_attr(patchable=True)
 
 
 @mypyc_attr(patchable=True)

-def dump_to_file(*output: str) -> str:
+def dump_to_file(*output: str, ensure_final_newline: bool = True) -> str:

     """Dump `output` to a temporary file. Return path to the file."""
     with tempfile.NamedTemporaryFile(
         mode="w", prefix="blk_", suffix=".log", delete=False, encoding="utf8"
     ) as f:
         for lines in output:
             f.write(lines)
     """Dump `output` to a temporary file. Return path to the file."""
     with tempfile.NamedTemporaryFile(
         mode="w", prefix="blk_", suffix=".log", delete=False, encoding="utf8"
     ) as f:
         for lines in output:
             f.write(lines)

-            if lines and lines[-1] != "\n":
+            if ensure_final_newline and lines and lines[-1] != "\n":

                 f.write("\n")
     return f.name
 
                 f.write("\n")
     return f.name
 


@@ -6197,11 +6526,20 @@ 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
 
     """Return a unified diff string between strings `a` and `b`."""
     import difflib
 

-    a_lines = [line + "\n" for line in a.splitlines()]
-    b_lines = [line + "\n" for line in b.splitlines()]
-    return "".join(
-        difflib.unified_diff(a_lines, b_lines, fromfile=a_name, tofile=b_name, n=5)
-    )
+    a_lines = [line for line in a.splitlines(keepends=True)]
+    b_lines = [line for line in b.splitlines(keepends=True)]
+    diff_lines = []
+    for line in difflib.unified_diff(
+        a_lines, b_lines, fromfile=a_name, tofile=b_name, n=5
+    ):
+        # Work around https://bugs.python.org/issue2142
+        # See https://www.gnu.org/software/diffutils/manual/html_node/Incomplete-Lines.html
+        if line[-1] == "\n":
+            diff_lines.append(line)
+        else:
+            diff_lines.append(line + "\n")
+            diff_lines.append("\\ No newline at end of file\n")
+    return "".join(diff_lines)

 
 
 def cancel(tasks: Iterable["asyncio.Task[Any]"]) -> None:
 
 
 def cancel(tasks: Iterable["asyncio.Task[Any]"]) -> None:


@@ -6378,7 +6716,7 @@ def can_omit_invisible_parens(
 
     penultimate = line.leaves[-2]
     last = line.leaves[-1]
 
     penultimate = line.leaves[-2]
     last = line.leaves[-1]

-    if line.should_explode:
+    if line.magic_trailing_comma:

         try:
             penultimate, last = last_two_except(line.leaves, omit=omit_on_explode)
         except LookupError:
         try:
             penultimate, last = last_two_except(line.leaves, omit=omit_on_explode)
         except LookupError:


@@ -6405,11 +6743,7 @@ def can_omit_invisible_parens(
             # unnecessary.
             return True
 
             # unnecessary.
             return True
 

-        if (
-            line.should_explode
-            and penultimate.type == token.COMMA
-            and not penultimate.was_checked
-        ):
+        if line.magic_trailing_comma and penultimate.type == token.COMMA:

             # The rightmost non-omitted bracket pair is the one we want to explode on.
             return True
 
             # The rightmost non-omitted bracket pair is the one we want to explode on.
             return True
 


@@ -6559,8 +6893,8 @@ def filter_cached(cache: Cache, sources: Iterable[Path]) -> Tuple[Set[Path], Set
     """
     todo, done = set(), set()
     for src in sources:
     """
     todo, done = set(), set()
     for src in sources:

-        src = src.resolve()
-        if cache.get(src) != get_cache_info(src):
+        res_src = src.resolve()
+        if cache.get(str(res_src)) != get_cache_info(res_src):

             todo.add(src)
         else:
             done.add(src)
             todo.add(src)
         else:
             done.add(src)


@@ -6572,7 +6906,10 @@ def write_cache(cache: Cache, sources: Iterable[Path], mode: Mode) -> None:
     cache_file = get_cache_file(mode)
     try:
         CACHE_DIR.mkdir(parents=True, exist_ok=True)
     cache_file = get_cache_file(mode)
     try:
         CACHE_DIR.mkdir(parents=True, exist_ok=True)

-        new_cache = {**cache, **{src.resolve(): get_cache_info(src) for src in sources}}
+        new_cache = {
+            **cache,
+            **{str(src.resolve()): get_cache_info(src) for src in sources},
+        }

         with tempfile.NamedTemporaryFile(dir=str(cache_file.parent), delete=False) as f:
             pickle.dump(new_cache, f, protocol=4)
         os.replace(f.name, cache_file)
         with tempfile.NamedTemporaryFile(dir=str(cache_file.parent), delete=False) as f:
             pickle.dump(new_cache, f, protocol=4)
         os.replace(f.name, cache_file)


@@ -6608,13 +6945,53 @@ def patched_main() -> None:
     main()
 
 
     main()
 
 

+def is_docstring(leaf: Leaf) -> bool:
+    if not is_multiline_string(leaf):
+        # For the purposes of docstring re-indentation, we don't need to do anything
+        # with single-line docstrings.
+        return False
+
+    if prev_siblings_are(
+        leaf.parent, [None, token.NEWLINE, token.INDENT, syms.simple_stmt]
+    ):
+        return True
+
+    # Multiline docstring on the same line as the `def`.
+    if prev_siblings_are(leaf.parent, [syms.parameters, token.COLON, syms.simple_stmt]):
+        # `syms.parameters` is only used in funcdefs and async_funcdefs in the Python
+        # grammar. We're safe to return True without further checks.
+        return True
+
+    return False
+
+
+def lines_with_leading_tabs_expanded(s: str) -> List[str]:
+    """
+    Splits string into lines and expands only leading tabs (following the normal
+    Python rules)
+    """
+    lines = []
+    for line in s.splitlines():
+        # Find the index of the first non-whitespace character after a string of
+        # whitespace that includes at least one tab
+        match = re.match(r"\s*\t+\s*(\S)", line)
+        if match:
+            first_non_whitespace_idx = match.start(1)
+
+            lines.append(
+                line[:first_non_whitespace_idx].expandtabs()
+                + line[first_non_whitespace_idx:]
+            )
+        else:
+            lines.append(line)
+    return lines
+
+

 def fix_docstring(docstring: str, prefix: str) -> str:
     # https://www.python.org/dev/peps/pep-0257/#handling-docstring-indentation
     if not docstring:
         return ""
 def fix_docstring(docstring: str, prefix: str) -> str:
     # https://www.python.org/dev/peps/pep-0257/#handling-docstring-indentation
     if not docstring:
         return ""

-    # Convert tabs to spaces (following the normal Python rules)
-    # and split into a list of lines:
-    lines = docstring.expandtabs().splitlines()
+    lines = lines_with_leading_tabs_expanded(docstring)

     # Determine minimum indentation (first line doesn't count):
     indent = sys.maxsize
     for line in lines[1:]:
     # Determine minimum indentation (first line doesn't count):
     indent = sys.maxsize
     for line in lines[1:]:


@@ -6631,7 +7008,6 @@ def fix_docstring(docstring: str, prefix: str) -> str:
                 trimmed.append(prefix + stripped_line)
             else:
                 trimmed.append("")
                 trimmed.append(prefix + stripped_line)
             else:
                 trimmed.append("")

-    # Return a single string:

     return "\n".join(trimmed)
 
 
     return "\n".join(trimmed)