Fix typo (#1931)

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

diff --git a/src/black/__init__.py b/src/black/__init__.py
index df38fd01a9bda205653f757ffabc676faeebe7e8..91f70d96165748741583ce2d7b94007b71111f87 100644 (file)
--- a/src/black/__init__.py
+++ b/src/black/__init__.py
@@ -42,7 +42,6 @@ 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 mypy_extensions import mypyc_attr
 
 from appdirs import user_cache_dir


@@ -61,6 +60,11 @@ 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 +72,7 @@ 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.
 


@@ -112,6 +117,10 @@ 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 +183,12 @@ 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 +202,8 @@ 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

 
 
 VERSION_TO_FEATURES: Dict[TargetVersion, Set[Feature]] = {
 
 
 VERSION_TO_FEATURES: Dict[TargetVersion, Set[Feature]] = {


@@ -232,6 +241,17 @@ 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,
+    },

 }
 
 
 }
 
 


@@ -240,6 +260,7 @@ 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

+    experimental_string_processing: bool = False

     is_pyi: bool = False
 
     def get_cache_key(self) -> str:
     is_pyi: bool = False
 
     def get_cache_key(self) -> str:


@@ -376,6 +397,15 @@ 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(
+    "--experimental-string-processing",
+    is_flag=True,
+    hidden=True,
+    help=(
+        "Experimental option that performs more normalization on string literals."
+        " Currently disabled because it leads to some crashes."
+    ),
+)

 @click.option(
     "--check",
     is_flag=True,
 @click.option(
     "--check",
     is_flag=True,


@@ -429,7 +459,16 @@ def target_version_option_callback(
     type=str,
     help=(
         "Like --exclude, but files and directories matching this regex will be "
     type=str,
     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(


@@ -485,11 +524,13 @@ def main(
     fast: bool,
     pyi: bool,
     skip_string_normalization: bool,
     fast: bool,
     pyi: bool,
     skip_string_normalization: bool,

+    experimental_string_processing: bool,

     quiet: bool,
     verbose: bool,
     include: str,
     exclude: str,
     force_exclude: Optional[str],
     quiet: bool,
     verbose: bool,
     include: str,
     exclude: str,
     force_exclude: Optional[str],

+    stdin_filename: Optional[str],

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


@@ -505,6 +546,7 @@ 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,

+        experimental_string_processing=experimental_string_processing,

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


@@ -521,6 +563,7 @@ def main(
         exclude=exclude,
         force_exclude=force_exclude,
         report=report,
         exclude=exclude,
         force_exclude=force_exclude,
         report=report,

+        stdin_filename=stdin_filename,

     )
 
     path_empty(
     )
 
     path_empty(


@@ -560,6 +603,7 @@ def get_sources(
     exclude: str,
     force_exclude: Optional[str],
     report: "Report",
     exclude: str,
     force_exclude: Optional[str],
     report: "Report",

+    stdin_filename: Optional[str],

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


@@ -583,31 +627,49 @@ def get_sources(
     root = find_project_root(src)
     sources: Set[Path] = set()
     path_empty(src, "No Path provided. Nothing to do 😴", quiet, verbose, ctx)
     root = find_project_root(src)
     sources: Set[Path] = set()
     path_empty(src, "No Path provided. Nothing to do 😴", quiet, verbose, ctx)

-    exclude_regexes = [exclude_regex]
-    if force_exclude_regex is not None:
-        exclude_regexes.append(force_exclude_regex)
+    gitignore = get_gitignore(root)

 
     for s in src:
 
     for s in src:

-        p = Path(s)
-        if p.is_dir():
+        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.
+            if force_exclude_regex:
+                force_exclude_match = force_exclude_regex.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
+
+            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_regex,
             sources.update(
                 gen_python_files(
                     p.iterdir(),
                     root,
                     include_regex,

-                    exclude_regexes,
+                    exclude_regex,
+                    force_exclude_regex,

                     report,
                     report,

-                    get_gitignore(root),
+                    gitignore,

                 )
             )
         elif s == "-":
             sources.add(p)
                 )
             )
         elif s == "-":
             sources.add(p)

-        elif p.is_file():
-            sources.update(
-                gen_python_files(
-                    [p], root, None, exclude_regexes, report, get_gitignore(root)
-                )
-            )

         else:
             err(f"invalid path: {s}")
     return sources
         else:
             err(f"invalid path: {s}")
     return sources


@@ -619,10 +681,9 @@ 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(


@@ -635,12 +696,23 @@ 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()
                 if res_src in cache and cache[res_src] == get_cache_info(res_src):
                 cache = read_cache(mode)
                 res_src = src.resolve()
                 if res_src in cache and cache[res_src] == get_cache_info(res_src):


@@ -655,6 +727,8 @@ def reformat_one(
                 write_cache(cache, [src], mode)
         report.done(src, changed)
     except Exception as exc:
                 write_cache(cache, [src], mode)
         report.done(src, changed)
     except Exception as exc:

+        if report.verbose:
+            traceback.print_exc()

         report.failed(src, str(exc))
 
 
         report.failed(src, str(exc))
 
 


@@ -667,7 +741,7 @@ 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):


@@ -712,7 +786,7 @@ 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):


@@ -723,7 +797,7 @@ 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()


@@ -822,9 +896,9 @@ 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


@@ -834,30 +908,22 @@ 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(


@@ -898,13 +964,13 @@ 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)


@@ -924,11 +990,12 @@ 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:
         ...
 
     A more complex example:
     def f(arg: str = "") -> None:
         ...
 
     A more complex example:

+

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


@@ -973,10 +1040,7 @@ def format_str(src_contents: str, *, mode: Mode) -> FileContent:
         before, after = elt.maybe_empty_lines(current_line)
         dst_contents.append(str(empty_line) * before)
         for line in transform_line(
         before, after = elt.maybe_empty_lines(current_line)
         dst_contents.append(str(empty_line) * before)
         for line in transform_line(

-            current_line,
-            line_length=mode.line_length,
-            normalize_strings=mode.string_normalization,
-            features=split_line_features,
+            current_line, mode=mode, features=split_line_features

         ):
             dst_contents.append(str(line))
     return "".join(dst_contents)
         ):
             dst_contents.append(str(line))
     return "".join(dst_contents)


@@ -1040,7 +1104,7 @@ 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)):


@@ -1263,6 +1327,7 @@ class BracketTracker:
     previous: Optional[Leaf] = None
     _for_loop_depths: List[int] = field(default_factory=list)
     _lambda_argument_depths: List[int] = field(default_factory=list)
     previous: Optional[Leaf] = None
     _for_loop_depths: List[int] = field(default_factory=list)
     _lambda_argument_depths: List[int] = field(default_factory=list)

+    invisible: List[Leaf] = field(default_factory=list)

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


@@ -1286,8 +1351,16 @@ 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
             leaf.opening_bracket = opening_bracket

+            if not leaf.value:
+                self.invisible.append(leaf)

         leaf.bracket_depth = self.depth
         if self.depth == 0:
             delim = is_split_before_delimiter(leaf, self.previous)
         leaf.bracket_depth = self.depth
         if self.depth == 0:
             delim = is_split_before_delimiter(leaf, self.previous)


@@ -1300,6 +1373,8 @@ class BracketTracker:
         if leaf.type in OPENING_BRACKETS:
             self.bracket_match[self.depth, BRACKET[leaf.type]] = leaf
             self.depth += 1
         if leaf.type in OPENING_BRACKETS:
             self.bracket_match[self.depth, BRACKET[leaf.type]] = leaf
             self.depth += 1

+            if not leaf.value:
+                self.invisible.append(leaf)

         self.previous = leaf
         self.maybe_increment_lambda_arguments(leaf)
         self.maybe_increment_for_loop_variable(leaf)
         self.previous = leaf
         self.maybe_increment_lambda_arguments(leaf)
         self.maybe_increment_for_loop_variable(leaf)


@@ -1421,7 +1496,8 @@ 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)

-            self.maybe_remove_trailing_comma(leaf)
+            if self.maybe_should_explode(leaf):
+                self.should_explode = True

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


@@ -1473,69 +1549,6 @@ class Line:
             Leaf(token.DOT, ".") for _ in range(3)
         ]
 
             Leaf(token.DOT, ".") for _ in range(3)
         ]
 

-    @property
-    def is_collection_with_optional_trailing_comma(self) -> bool:
-        """Is this line a collection literal with a trailing comma that's optional?
-
-        Note that the trailing comma in a 1-tuple is not optional.
-        """
-        if not self.leaves or len(self.leaves) < 4:
-            return False
-
-        # Look for and address a trailing colon.
-        if self.leaves[-1].type == token.COLON:
-            closer = self.leaves[-2]
-            close_index = -2
-        else:
-            closer = self.leaves[-1]
-            close_index = -1
-        if closer.type not in CLOSING_BRACKETS or self.inside_brackets:
-            return False
-
-        if closer.type == token.RPAR:
-            # Tuples require an extra check, because if there's only
-            # one element in the tuple removing the comma unmakes the
-            # tuple.
-            #
-            # We also check for parens before looking for the trailing
-            # comma because in some cases (eg assigning a dict
-            # literal) the literal gets wrapped in temporary parens
-            # during parsing. This case is covered by the
-            # collections.py test data.
-            opener = closer.opening_bracket
-            for _open_index, leaf in enumerate(self.leaves):
-                if leaf is opener:
-                    break
-
-            else:
-                # Couldn't find the matching opening paren, play it safe.
-                return False
-
-            commas = 0
-            comma_depth = self.leaves[close_index - 1].bracket_depth
-            for leaf in self.leaves[_open_index + 1 : close_index]:
-                if leaf.bracket_depth == comma_depth and leaf.type == token.COMMA:
-                    commas += 1
-            if commas > 1:
-                # We haven't looked yet for the trailing comma because
-                # we might also have caught noop parens.
-                return self.leaves[close_index - 1].type == token.COMMA
-
-            elif commas == 1:
-                return False  # it's either a one-tuple or didn't have a trailing comma
-
-            if self.leaves[close_index - 1].type in CLOSING_BRACKETS:
-                close_index -= 1
-                closer = self.leaves[close_index]
-                if closer.type == token.RPAR:
-                    # TODO: this is a gut feeling. Will we ever see this?
-                    return False
-
-        if self.leaves[close_index - 1].type != token.COMMA:
-            return False
-
-        return True
-

     @property
     def is_def(self) -> bool:
         """Is this a function definition? (Also returns True for async defs.)"""
     @property
     def is_def(self) -> bool:
         """Is this a function definition? (Also returns True for async defs.)"""


@@ -1660,42 +1673,28 @@ 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_remove_trailing_comma(self, closing: Leaf) -> bool:
-        """Remove trailing comma if there is one and it's safe."""
-        if not (self.leaves and self.leaves[-1].type == token.COMMA):
-            return False
-
-        # We remove trailing commas only in the case of importing a
-        # single name from a module.
+    def maybe_should_explode(self, closing: Leaf) -> bool:
+        """Return True if this line should explode (always be split), that is when:
+        - there's a trailing comma here; and
+        - it's not a one-tuple.
+        """

         if not (
         if not (

-            self.leaves
-            and self.is_import
-            and len(self.leaves) > 4
+            closing.type in CLOSING_BRACKETS
+            and self.leaves

             and self.leaves[-1].type == token.COMMA
             and self.leaves[-1].type == token.COMMA

-            and closing.type in CLOSING_BRACKETS
-            and self.leaves[-4].type == token.NAME
-            and (
-                # regular `from foo import bar,`
-                self.leaves[-4].value == "import"
-                # `from foo import (bar as baz,)
-                or (
-                    len(self.leaves) > 6
-                    and self.leaves[-6].value == "import"
-                    and self.leaves[-3].value == "as"
-                )
-                # `from foo import bar as baz,`
-                or (
-                    len(self.leaves) > 5
-                    and self.leaves[-5].value == "import"
-                    and self.leaves[-3].value == "as"
-                )
-            )
-            and closing.type == token.RPAR

         ):
             return False
 
         ):
             return False
 

-        self.remove_trailing_comma()
-        return True
+        if closing.type in {token.RBRACE, token.RSQB}:
+            return True
+
+        if self.is_import:
+            return True
+
+        if not is_one_tuple_between(closing.opening_bracket, closing, self.leaves):
+            return True
+
+        return False

 
     def append_comment(self, comment: Leaf) -> bool:
         """Add an inline or standalone comment to the line."""
 
     def append_comment(self, comment: Leaf) -> bool:
         """Add an inline or standalone comment to the line."""


@@ -1874,6 +1873,10 @@ 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 (


@@ -1897,8 +1900,11 @@ 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


@@ -2085,14 +2091,20 @@ 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)
 
 
         yield from self.visit_default(leaf)
 


@@ -2211,6 +2223,9 @@ 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


@@ -2638,10 +2653,7 @@ def make_comment(content: str) -> str:
 
 
 def transform_line(
 
 
 def transform_line(

-    line: Line,
-    line_length: int,
-    normalize_strings: bool,
-    features: Collection[Feature] = (),
+    line: Line, mode: Mode, features: Collection[Feature] = ()

 ) -> Iterator[Line]:
     """Transform a `line`, potentially splitting it into many lines.
 
 ) -> Iterator[Line]:
     """Transform a `line`, potentially splitting it into many lines.
 


@@ -2657,7 +2669,7 @@ def transform_line(
 
     def init_st(ST: Type[StringTransformer]) -> StringTransformer:
         """Initialize StringTransformer"""
 
     def init_st(ST: Type[StringTransformer]) -> StringTransformer:
         """Initialize StringTransformer"""

-        return ST(line_length, normalize_strings)
+        return ST(mode.line_length, mode.string_normalization)

 
     string_merge = init_st(StringMerger)
     string_paren_strip = init_st(StringParenStripper)
 
     string_merge = init_st(StringMerger)
     string_paren_strip = init_st(StringParenStripper)


@@ -2668,72 +2680,79 @@ def transform_line(
     if (
         not line.contains_uncollapsable_type_comments()
         and not line.should_explode
     if (
         not line.contains_uncollapsable_type_comments()
         and not line.should_explode

-        and not line.is_collection_with_optional_trailing_comma

         and (
         and (

-            is_line_short_enough(line, line_length=line_length, line_str=line_str)
+            is_line_short_enough(line, line_length=mode.line_length, line_str=line_str)

             or line.contains_unsplittable_type_ignore()
         )
             or line.contains_unsplittable_type_ignore()
         )

-        and not (line.contains_standalone_comments() and line.inside_brackets)
+        and not (line.inside_brackets and line.contains_standalone_comments())

     ):
         # Only apply basic string preprocessing, since lines shouldn't be split here.
     ):
         # Only apply basic string preprocessing, since lines shouldn't be split here.

-        transformers = [string_merge, string_paren_strip]
+        if mode.experimental_string_processing:
+            transformers = [string_merge, string_paren_strip]
+        else:
+            transformers = []

     elif line.is_def:
         transformers = [left_hand_split]
     else:
 
         def rhs(line: Line, features: Collection[Feature]) -> Iterator[Line]:
     elif line.is_def:
         transformers = [left_hand_split]
     else:
 
         def rhs(line: Line, features: Collection[Feature]) -> Iterator[Line]:

-            for omit in generate_trailers_to_omit(line, line_length):
-                lines = list(right_hand_split(line, line_length, features, omit=omit))
-                if is_line_short_enough(lines[0], line_length=line_length):
+            """Wraps calls to `right_hand_split`.
+
+            The calls increasingly `omit` right-hand trailers (bracket pairs with
+            content), meaning the trailers get glued together to split on another
+            bracket pair instead.
+            """
+            for omit in generate_trailers_to_omit(line, mode.line_length):
+                lines = list(
+                    right_hand_split(line, mode.line_length, features, omit=omit)
+                )
+                # Note: this check is only able to figure out if the first line of the
+                # *current* transformation fits in the line length.  This is true only
+                # for simple cases.  All others require running more transforms via
+                # `transform_line()`.  This check doesn't know if those would succeed.
+                if is_line_short_enough(lines[0], line_length=mode.line_length):

                     yield from lines
                     return
 
             # All splits failed, best effort split with no omits.
             # This mostly happens to multiline strings that are by definition
                     yield from lines
                     return
 
             # 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.
-            # line_length=1 here was historically a bug that somehow became a feature.
-            # See #762 and #781 for the full story.
-            yield from right_hand_split(line, line_length=1, features=features)
-
-        if line.inside_brackets:
-            transformers = [
-                string_merge,
-                string_paren_strip,
-                delimiter_split,
-                standalone_comment_split,
-                string_split,
-                string_paren_wrap,
-                rhs,
-            ]
+            # reported as not fitting a single line, as well as lines that contain
+            # trailing commas (those have to be exploded).
+            yield from right_hand_split(
+                line, line_length=mode.line_length, features=features
+            )
+
+        if mode.experimental_string_processing:
+            if line.inside_brackets:
+                transformers = [
+                    string_merge,
+                    string_paren_strip,
+                    string_split,
+                    delimiter_split,
+                    standalone_comment_split,
+                    string_paren_wrap,
+                    rhs,
+                ]
+            else:
+                transformers = [
+                    string_merge,
+                    string_paren_strip,
+                    string_split,
+                    string_paren_wrap,
+                    rhs,
+                ]

         else:
         else:

-            transformers = [
-                string_merge,
-                string_paren_strip,
-                string_split,
-                string_paren_wrap,
-                rhs,
-            ]
+            if line.inside_brackets:
+                transformers = [delimiter_split, standalone_comment_split, rhs]
+            else:
+                transformers = [rhs]

 
     for transform in transformers:
         # We are accumulating lines in `result` because we might want to abort
         # mission and return the original line in the end, or attempt a different
         # split altogether.
 
     for transform in transformers:
         # We are accumulating lines in `result` because we might want to abort
         # mission and return the original line in the end, or attempt a different
         # split altogether.

-        result: List[Line] = []

         try:
         try:

-            for transformed_line in transform(line, features):
-                if str(transformed_line).strip("\n") == line_str:
-                    raise CannotTransform(
-                        "Line transformer returned an unchanged result"
-                    )
-
-                result.extend(
-                    transform_line(
-                        transformed_line,
-                        line_length=line_length,
-                        normalize_strings=normalize_strings,
-                        features=features,
-                    )
-                )
+            result = run_transformer(line, transform, mode, features, line_str=line_str)

         except CannotTransform:
             continue
         else:
         except CannotTransform:
             continue
         else:


@@ -2774,6 +2793,7 @@ class StringTransformer(ABC):
 
     line_length: int
     normalize_strings: bool
 
     line_length: int
     normalize_strings: bool

+    __name__ = "StringTransformer"

 
     @abstractmethod
     def do_match(self, line: Line) -> TMatchResult:
 
     @abstractmethod
     def do_match(self, line: Line) -> TMatchResult:


@@ -2928,11 +2948,8 @@ 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.
 


@@ -3020,7 +3037,7 @@ class StringMerger(CustomSplitMapMixin, StringTransformer):
             )
 
         new_line = line.clone()
             )
 
         new_line = line.clone()

-        new_line.comments = line.comments
+        new_line.comments = line.comments.copy()

         append_leaves(new_line, line, LL)
 
         new_string_leaf = new_line.leaves[string_idx]
         append_leaves(new_line, line, LL)
 
         new_string_leaf = new_line.leaves[string_idx]


@@ -3181,6 +3198,7 @@ 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.


@@ -3189,6 +3207,26 @@ 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


@@ -3237,8 +3275,11 @@ 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 RPAR is NOT followed by an attribute access (i.e. a dot).
+            - 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.

 
     Transformations:
         The parentheses mentioned in the 'Requirements' section are stripped.
 
     Transformations:
         The parentheses mentioned in the 'Requirements' section are stripped.


@@ -3259,6 +3300,14 @@ 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)


@@ -3281,14 +3330,51 @@ class StringParenStripper(StringTransformer):
             string_parser = StringParser()
             next_idx = string_parser.parse(LL, string_idx)
 
             string_parser = StringParser()
             next_idx = string_parser.parse(LL, string_idx)
 

+            # if the leaves in the parsed string include a PERCENT, we need to
+            # make sure the initial LPAR is NOT preceded by an operator with
+            # higher or equal precedence to PERCENT
+            if is_valid_index(idx - 2):
+                # mypy can't quite follow unless we name this
+                before_lpar = LL[idx - 2]
+                if token.PERCENT in {leaf.type for leaf in LL[idx - 1 : next_idx]} and (
+                    (
+                        before_lpar.type
+                        in {
+                            token.STAR,
+                            token.AT,
+                            token.SLASH,
+                            token.DOUBLESLASH,
+                            token.PERCENT,
+                            token.TILDE,
+                            token.DOUBLESTAR,
+                            token.AWAIT,
+                            token.LSQB,
+                            token.LPAR,
+                        }
+                    )
+                    or (
+                        # only unary PLUS/MINUS
+                        before_lpar.parent
+                        and before_lpar.parent.type == syms.factor
+                        and (before_lpar.type in {token.PLUS, token.MINUS})
+                    )
+                ):
+                    continue
+

             # Should be followed by a non-empty RPAR...
             if (
                 is_valid_index(next_idx)
                 and LL[next_idx].type == token.RPAR
                 and not is_empty_rpar(LL[next_idx])
             ):
             # Should be followed by a non-empty RPAR...
             if (
                 is_valid_index(next_idx)
                 and LL[next_idx].type == token.RPAR
                 and not is_empty_rpar(LL[next_idx])
             ):

-                # That RPAR should NOT be followed by a '.' symbol.
-                if is_valid_index(next_idx + 1) and LL[next_idx + 1].type == token.DOT:
+                # That RPAR should NOT be followed by anything with higher
+                # precedence than PERCENT
+                if is_valid_index(next_idx + 1) and LL[next_idx + 1].type in {
+                    token.DOUBLESTAR,
+                    token.LSQB,
+                    token.LPAR,
+                    token.DOT,
+                }:

                     continue
 
                 return Ok(string_idx)
                     continue
 
                 return Ok(string_idx)


@@ -3306,11 +3392,17 @@ 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()


@@ -3318,7 +3410,7 @@ class StringParenStripper(StringTransformer):
         new_line.append(string_leaf)
 
         append_leaves(
         new_line.append(string_leaf)
 
         append_leaves(

-            new_line, line, LL[string_idx + 1 : rpar_idx] + LL[rpar_idx + 1 :],
+            new_line, line, LL[string_idx + 1 : rpar_idx] + LL[rpar_idx + 1 :]

         )
 
         LL[rpar_idx].remove()
         )
 
         LL[rpar_idx].remove()


@@ -3473,9 +3565,12 @@ 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]


@@ -3541,7 +3636,8 @@ 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


@@ -3561,13 +3657,14 @@ 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:


@@ -3881,11 +3978,23 @@ 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


@@ -3991,12 +4100,13 @@ 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


@@ -4195,7 +4305,7 @@ 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


@@ -4580,7 +4690,9 @@ 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.


@@ -4596,7 +4708,7 @@ 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)


@@ -4753,8 +4865,7 @@ def right_hand_split(
     tail = bracket_split_build_line(tail_leaves, line, opening_bracket)
     bracket_split_succeeded_or_raise(head, body, tail)
     if (
     tail = bracket_split_build_line(tail_leaves, line, opening_bracket)
     bracket_split_succeeded_or_raise(head, body, tail)
     if (

-        # the body shouldn't be exploded
-        not body.should_explode
+        Feature.FORCE_OPTIONAL_PARENTHESES not in features

         # the opening bracket is an optional paren
         and opening_bracket.type == token.LPAR
         and not opening_bracket.value
         # the opening bracket is an optional paren
         and opening_bracket.type == token.LPAR
         and not opening_bracket.value


@@ -4767,7 +4878,7 @@ def right_hand_split(
         # there are no standalone comments in the body
         and not body.contains_standalone_comments(0)
         # and we can actually remove the parens
         # there are no standalone comments in the body
         and not body.contains_standalone_comments(0)
         # and we can actually remove the parens

-        and can_omit_invisible_parens(body, line_length)
+        and can_omit_invisible_parens(body, line_length, omit_on_explode=omit)

     ):
         omit = {id(closing_bracket), *omit}
         try:
     ):
         omit = {id(closing_bracket), *omit}
         try:


@@ -4853,7 +4964,8 @@ def bracket_split_build_line(
                         continue
 
                     if leaves[i].type != token.COMMA:
                         continue
 
                     if leaves[i].type != token.COMMA:

-                        leaves.insert(i + 1, Leaf(token.COMMA, ","))
+                        new_comma = Leaf(token.COMMA, ",")
+                        leaves.insert(i + 1, new_comma)

                     break
 
     # Populate the line
                     break
 
     # Populate the line


@@ -4861,8 +4973,8 @@ 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:
-        result.should_explode = should_explode(result, opening_bracket)
+    if is_body and should_split_body_explode(result, opening_bracket):
+        result.should_explode = True

     return result
 
 
     return result
 
 


@@ -4947,7 +5059,8 @@ def delimiter_split(line: Line, features: Collection[Feature] = ()) -> Iterator[
             and current_line.leaves[-1].type != token.COMMA
             and current_line.leaves[-1].type != STANDALONE_COMMENT
         ):
             and current_line.leaves[-1].type != token.COMMA
             and current_line.leaves[-1].type != STANDALONE_COMMENT
         ):

-            current_line.append(Leaf(token.COMMA, ","))
+            new_comma = Leaf(token.COMMA, ",")
+            current_line.append(new_comma)

         yield current_line
 
 
         yield current_line
 
 


@@ -5125,31 +5238,52 @@ 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:


@@ -5189,9 +5323,9 @@ def normalize_invisible_parens(node: Node, parens_after: Set[str]) -> None:
 
         if check_lpar:
             if is_walrus_assignment(child):
 
         if check_lpar:
             if is_walrus_assignment(child):

-                continue
+                pass

 
 

-            if child.type == syms.atom:
+            elif child.type == syms.atom:

                 if maybe_make_parens_invisible_in_atom(child, parent=node):
                     wrap_in_parentheses(node, child, visible=False)
             elif is_one_tuple(child):
                 if maybe_make_parens_invisible_in_atom(child, parent=node):
                     wrap_in_parentheses(node, child, visible=False)
             elif is_one_tuple(child):


@@ -5451,6 +5585,49 @@ 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:


@@ -5569,24 +5746,63 @@ def ensure_visible(leaf: Leaf) -> None:
         leaf.value = ")"
 
 
         leaf.value = ")"
 
 

-def should_explode(line: Line, opening_bracket: Leaf) -> bool:
-    """Should `line` immediately be split with `delimiter_split()` after RHS?"""
+def should_split_body_explode(line: Line, opening_bracket: Leaf) -> bool:
+    """Should `line` be immediately split with `delimiter_split()` after RHS?"""

 
 

-    if not (
-        opening_bracket.parent
-        and opening_bracket.parent.type in {syms.atom, syms.import_from}
-        and opening_bracket.value in "[{("
-    ):
+    if not (opening_bracket.parent and opening_bracket.value in "[{("):

         return False
 
         return False
 

+    # We're essentially checking if the body is delimited by commas and there's more
+    # 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()
+    trailing_comma = False

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

-        exclude = {id(last_leaf)} if last_leaf.type == token.COMMA else set()
+        if last_leaf.type == token.COMMA:
+            trailing_comma = True
+            exclude.add(id(last_leaf))

         max_priority = line.bracket_tracker.max_delimiter_priority(exclude=exclude)
     except (IndexError, ValueError):
         return False
 
         max_priority = line.bracket_tracker.max_delimiter_priority(exclude=exclude)
     except (IndexError, ValueError):
         return False
 

-    return max_priority == COMMA_PRIORITY
+    return max_priority == COMMA_PRIORITY and (
+        trailing_comma
+        # always explode imports
+        or opening_bracket.parent.type in {syms.atom, syms.import_from}
+    )
+
+
+def is_one_tuple_between(opening: Leaf, closing: Leaf, leaves: List[Leaf]) -> bool:
+    """Return True if content between `opening` and `closing` looks like a one-tuple."""
+    if opening.type != token.LPAR and closing.type != token.RPAR:
+        return False
+
+    depth = closing.bracket_depth + 1
+    for _opening_index, leaf in enumerate(leaves):
+        if leaf is opening:
+            break
+
+    else:
+        raise LookupError("Opening paren not found in `leaves`")
+
+    commas = 0
+    _opening_index += 1
+    for leaf in leaves[_opening_index:]:
+        if leaf is closing:
+            break
+
+        bracket_depth = leaf.bracket_depth
+        if bracket_depth == depth and leaf.type == token.COMMA:
+            commas += 1
+            if leaf.parent and leaf.parent.type in {
+                syms.arglist,
+                syms.typedargslist,
+            }:
+                commas += 1
+                break
+
+    return commas < 2

 
 
 def get_features_used(node: Node) -> Set[Feature]:
 
 
 def get_features_used(node: Node) -> Set[Feature]:


@@ -5597,6 +5813,8 @@ 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():


@@ -5616,6 +5834,12 @@ 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


@@ -5653,11 +5877,13 @@ def generate_trailers_to_omit(line: Line, line_length: int) -> Iterator[Set[Leaf
     a preceding closing bracket fits in one line.
 
     Yielded sets are cumulative (contain results of previous yields, too).  First
     a preceding closing bracket fits in one line.
 
     Yielded sets are cumulative (contain results of previous yields, too).  First

-    set is empty.
+    set is empty, unless the line should explode, in which case bracket pairs until
+    the one that needs to explode are omitted.

     """
 
     omit: Set[LeafID] = set()
     """
 
     omit: Set[LeafID] = set()

-    yield omit
+    if not line.should_explode:
+        yield omit

 
     length = 4 * line.depth
     opening_bracket: Optional[Leaf] = None
 
     length = 4 * line.depth
     opening_bracket: Optional[Leaf] = None


@@ -5676,9 +5902,23 @@ def generate_trailers_to_omit(line: Line, line_length: int) -> Iterator[Set[Leaf
             if leaf is opening_bracket:
                 opening_bracket = None
             elif leaf.type in CLOSING_BRACKETS:
             if leaf is opening_bracket:
                 opening_bracket = None
             elif leaf.type in CLOSING_BRACKETS:

+                prev = line.leaves[index - 1] if index > 0 else None
+                if (
+                    line.should_explode
+                    and prev
+                    and prev.type == token.COMMA
+                    and not is_one_tuple_between(
+                        leaf.opening_bracket, leaf, line.leaves
+                    )
+                ):
+                    # Never omit bracket pairs with trailing commas.
+                    # We need to explode on those.
+                    break
+

                 inner_brackets.add(id(leaf))
         elif leaf.type in CLOSING_BRACKETS:
                 inner_brackets.add(id(leaf))
         elif leaf.type in CLOSING_BRACKETS:

-            if index > 0 and line.leaves[index - 1].type in OPENING_BRACKETS:
+            prev = line.leaves[index - 1] if index > 0 else None
+            if prev and prev.type in OPENING_BRACKETS:

                 # Empty brackets would fail a split so treat them as "inner"
                 # brackets (e.g. only add them to the `omit` set if another
                 # pair of brackets was good enough.
                 # Empty brackets would fail a split so treat them as "inner"
                 # brackets (e.g. only add them to the `omit` set if another
                 # pair of brackets was good enough.


@@ -5691,6 +5931,16 @@ def generate_trailers_to_omit(line: Line, line_length: int) -> Iterator[Set[Leaf
                 inner_brackets.clear()
                 yield omit
 
                 inner_brackets.clear()
                 yield omit
 

+            if (
+                line.should_explode
+                and prev
+                and prev.type == token.COMMA
+                and not is_one_tuple_between(leaf.opening_bracket, leaf, line.leaves)
+            ):
+                # Never omit bracket pairs with trailing commas.
+                # We need to explode on those.
+                break
+

             if leaf.value:
                 opening_bracket = leaf.opening_bracket
                 closing_bracket = leaf
             if leaf.value:
                 opening_bracket = leaf.opening_bracket
                 closing_bracket = leaf


@@ -5757,16 +6007,41 @@ def get_gitignore(root: Path) -> PathSpec:
     return PathSpec.from_lines("gitwildmatch", lines)
 
 
     return PathSpec.from_lines("gitwildmatch", lines)
 
 

+def normalize_path_maybe_ignore(
+    path: Path, root: Path, report: "Report"
+) -> Optional[str]:
+    """Normalize `path`. May return `None` if `path` was ignored.
+
+    `report` is where "path ignored" output goes.
+    """
+    try:
+        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 ValueError:
+        if path.is_symlink():
+            report.path_ignored(path, f"is a symbolic link that points outside {root}")
+            return None
+
+        raise
+
+    return normalized_path
+
+

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

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

-    `exclude` regex, but are included by the `include` regex.
+    `exclude_regex` 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.
 


@@ -5774,43 +6049,41 @@ def gen_python_files(
     """
     assert root.is_absolute(), f"INTERNAL ERROR: `root` must be absolute but is {root}"
     for child in paths:
     """
     assert root.is_absolute(), f"INTERNAL ERROR: `root` must be absolute but is {root}"
     for child in paths:

-        # Then ignore with `exclude` option.
-        try:
-            normalized_path = child.resolve().relative_to(root).as_posix()
-        except OSError as e:
-            report.path_ignored(child, f"cannot be read because {e}")
+        normalized_path = normalize_path_maybe_ignore(child, root, report)
+        if normalized_path is None:

             continue
             continue

-        except ValueError:
-            if child.is_symlink():
-                report.path_ignored(
-                    child, f"is a symbolic link that points outside {root}"
-                )
-                continue
-
-            raise

 
         # First ignore files matching .gitignore
         if gitignore.match_file(normalized_path):
             report.path_ignored(child, "matches the .gitignore file content")
             continue
 
 
         # First ignore files matching .gitignore
         if gitignore.match_file(normalized_path):
             report.path_ignored(child, "matches the .gitignore file content")
             continue
 

+        # Then ignore with `--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 += "/"
 

-        is_excluded = False
-        for exclude in exclude_regexes:
-            exclude_match = exclude.search(normalized_path) if exclude else None
-            if exclude_match and exclude_match.group(0):
-                report.path_ignored(child, "matches the --exclude regular expression")
-                is_excluded = True
-                break
-        if is_excluded:
+        exclude_match = exclude.search(normalized_path) if exclude else None
+        if exclude_match and exclude_match.group(0):
+            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):
+            report.path_ignored(child, "matches the --force-exclude regular expression")

             continue
 
         if child.is_dir():
             yield from gen_python_files(
             continue
 
         if child.is_dir():
             yield from gen_python_files(

-                child.iterdir(), root, include, exclude_regexes, report, gitignore
+                child.iterdir(),
+                root,
+                include,
+                exclude,
+                force_exclude,
+                report,
+                gitignore,

             )
 
         elif child.is_file():
             )
 
         elif child.is_file():


@@ -6030,7 +6303,7 @@ def _stringify_ast(
                 and field == "value"
                 and isinstance(value, str)
             ):
                 and field == "value"
                 and isinstance(value, str)
             ):

-                normalized = re.sub(r" *\n[ \t]+", "\n ", value).strip()
+                normalized = re.sub(r" *\n[ \t]*", "\n", value).strip()

             else:
                 normalized = value
             yield f"{'  ' * (depth+2)}{normalized!r},  # {value.__class__.__name__}"
             else:
                 normalized = value
             yield f"{'  ' * (depth+2)}{normalized!r},  # {value.__class__.__name__}"


@@ -6074,6 +6347,7 @@ 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"),
         )


@@ -6250,7 +6524,11 @@ def can_be_split(line: Line) -> bool:
     return True
 
 
     return True
 
 

-def can_omit_invisible_parens(line: Line, line_length: int) -> bool:
+def can_omit_invisible_parens(
+    line: Line,
+    line_length: int,
+    omit_on_explode: Collection[LeafID] = (),
+) -> bool:

     """Does `line` have a shape safe to reformat without optional parens around it?
 
     Returns True for only a subset of potentially nice looking formattings but
     """Does `line` have a shape safe to reformat without optional parens around it?
 
     Returns True for only a subset of potentially nice looking formattings but


@@ -6273,37 +6551,27 @@ def can_omit_invisible_parens(line: Line, line_length: int) -> bool:
 
     assert len(line.leaves) >= 2, "Stranded delimiter"
 
 
     assert len(line.leaves) >= 2, "Stranded delimiter"
 

-    first = line.leaves[0]
-    second = line.leaves[1]
-    penultimate = line.leaves[-2]
-    last = line.leaves[-1]
-

     # With a single delimiter, omit if the expression starts or ends with
     # a bracket.
     # With a single delimiter, omit if the expression starts or ends with
     # a bracket.

+    first = line.leaves[0]
+    second = line.leaves[1]

     if first.type in OPENING_BRACKETS and second.type not in CLOSING_BRACKETS:
     if first.type in OPENING_BRACKETS and second.type not in CLOSING_BRACKETS:

-        remainder = False
-        length = 4 * line.depth
-        for _index, leaf, leaf_length in enumerate_with_length(line):
-            if leaf.type in CLOSING_BRACKETS and leaf.opening_bracket is first:
-                remainder = True
-            if remainder:
-                length += leaf_length
-                if length > line_length:
-                    break
-
-                if leaf.type in OPENING_BRACKETS:
-                    # There are brackets we can further split on.
-                    remainder = False
-
-        else:
-            # checked the entire string and line length wasn't exceeded
-            if len(line.leaves) == _index + 1:
-                return True
+        if _can_omit_opening_paren(line, first=first, line_length=line_length):
+            return True

 
         # Note: we are not returning False here because a line might have *both*
         # a leading opening bracket and a trailing closing bracket.  If the
         # opening bracket doesn't match our rule, maybe the closing will.
 
 
         # Note: we are not returning False here because a line might have *both*
         # a leading opening bracket and a trailing closing bracket.  If the
         # opening bracket doesn't match our rule, maybe the closing will.
 

+    penultimate = line.leaves[-2]
+    last = line.leaves[-1]
+    if line.should_explode:
+        try:
+            penultimate, last = last_two_except(line.leaves, omit=omit_on_explode)
+        except LookupError:
+            # Turns out we'd omit everything.  We cannot skip the optional parentheses.
+            return False
+

     if (
         last.type == token.RPAR
         or last.type == token.RBRACE
     if (
         last.type == token.RPAR
         or last.type == token.RBRACE


@@ -6324,21 +6592,120 @@ def can_omit_invisible_parens(line: Line, line_length: int) -> bool:
             # unnecessary.
             return True
 
             # unnecessary.
             return True
 

-        length = 4 * line.depth
-        seen_other_brackets = False
-        for _index, leaf, leaf_length in enumerate_with_length(line):
+        if line.should_explode and penultimate.type == token.COMMA:
+            # The rightmost non-omitted bracket pair is the one we want to explode on.
+            return True
+
+        if _can_omit_closing_paren(line, last=last, line_length=line_length):
+            return True
+
+    return False
+
+
+def _can_omit_opening_paren(line: Line, *, first: Leaf, line_length: int) -> bool:
+    """See `can_omit_invisible_parens`."""
+    remainder = False
+    length = 4 * line.depth
+    _index = -1
+    for _index, leaf, leaf_length in enumerate_with_length(line):
+        if leaf.type in CLOSING_BRACKETS and leaf.opening_bracket is first:
+            remainder = True
+        if remainder:

             length += leaf_length
             length += leaf_length

-            if leaf is last.opening_bracket:
-                if seen_other_brackets or length <= line_length:
-                    return True
+            if length > line_length:
+                break

 
 

-            elif leaf.type in OPENING_BRACKETS:
+            if leaf.type in OPENING_BRACKETS:

                 # There are brackets we can further split on.
                 # There are brackets we can further split on.

-                seen_other_brackets = True
+                remainder = False
+
+    else:
+        # checked the entire string and line length wasn't exceeded
+        if len(line.leaves) == _index + 1:
+            return True

 
     return False
 
 
 
     return False
 
 

+def _can_omit_closing_paren(line: Line, *, last: Leaf, line_length: int) -> bool:
+    """See `can_omit_invisible_parens`."""
+    length = 4 * line.depth
+    seen_other_brackets = False
+    for _index, leaf, leaf_length in enumerate_with_length(line):
+        length += leaf_length
+        if leaf is last.opening_bracket:
+            if seen_other_brackets or length <= line_length:
+                return True
+
+        elif leaf.type in OPENING_BRACKETS:
+            # There are brackets we can further split on.
+            seen_other_brackets = True
+
+    return False
+
+
+def last_two_except(leaves: List[Leaf], omit: Collection[LeafID]) -> Tuple[Leaf, Leaf]:
+    """Return (penultimate, last) leaves skipping brackets in `omit` and contents."""
+    stop_after = None
+    last = None
+    for leaf in reversed(leaves):
+        if stop_after:
+            if leaf is stop_after:
+                stop_after = None
+            continue
+
+        if last:
+            return leaf, last
+
+        if id(leaf) in omit:
+            stop_after = leaf.opening_bracket
+        else:
+            last = leaf
+    else:
+        raise LookupError("Last two leaves were also skipped")
+
+
+def run_transformer(
+    line: Line,
+    transform: Transformer,
+    mode: Mode,
+    features: Collection[Feature],
+    *,
+    line_str: str = "",
+) -> List[Line]:
+    if not line_str:
+        line_str = line_to_string(line)
+    result: List[Line] = []
+    for transformed_line in transform(line, features):
+        if str(transformed_line).strip("\n") == line_str:
+            raise CannotTransform("Line transformer returned an unchanged result")
+
+        result.extend(transform_line(transformed_line, mode=mode, features=features))
+
+    if not (
+        transform.__name__ == "rhs"
+        and line.bracket_tracker.invisible
+        and not any(bracket.value for bracket in line.bracket_tracker.invisible)
+        and not line.contains_multiline_strings()
+        and not result[0].contains_uncollapsable_type_comments()
+        and not result[0].contains_unsplittable_type_ignore()
+        and not is_line_short_enough(result[0], line_length=mode.line_length)
+    ):
+        return result
+
+    line_copy = line.clone()
+    append_leaves(line_copy, line, line.leaves)
+    features_fop = set(features) | {Feature.FORCE_OPTIONAL_PARENTHESES}
+    second_opinion = run_transformer(
+        line_copy, transform, mode, features_fop, line_str=line_str
+    )
+    if all(
+        is_line_short_enough(ln, line_length=mode.line_length) for ln in second_opinion
+    ):
+        result = second_opinion
+    return result
+
+

 def get_cache_file(mode: Mode) -> Path:
     return CACHE_DIR / f"cache.{mode.get_cache_key()}.pickle"
 
 def get_cache_file(mode: Mode) -> Path:
     return CACHE_DIR / f"cache.{mode.get_cache_key()}.pickle"
 


@@ -6424,13 +6791,53 @@ 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:]:


@@ -6447,7 +6854,6 @@ 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)