Don't explode trailers that fit in a single line

[etc/vim.git] / black.py

diff --git a/black.py b/black.py
index 7c487f6e938c1b8113674b10bc91877da0546aa3..7823ae0afe2e809bdb2f5ca208ec9cc0565b4ced 100644 (file)
--- a/black.py
+++ b/black.py
@@ -1,6 +1,5 @@
-#!/usr/bin/env python3
-

 import asyncio
 import asyncio

+import pickle

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


@@ -25,6 +24,7 @@ from typing import (
     List,
     Optional,
     Pattern,
     List,
     Optional,
     Pattern,

+    Sequence,

     Set,
     Tuple,
     Type,
     Set,
     Tuple,
     Type,


@@ -32,6 +32,7 @@ from typing import (
     Union,
 )
 
     Union,
 )
 

+from appdirs import user_cache_dir

 from attr import dataclass, Factory
 import click
 
 from attr import dataclass, Factory
 import click
 


@@ -41,8 +42,10 @@ from blib2to3 import pygram, pytree
 from blib2to3.pgen2 import driver, token
 from blib2to3.pgen2.parse import ParseError
 
 from blib2to3.pgen2 import driver, token
 from blib2to3.pgen2.parse import ParseError
 

-__version__ = "18.4a2"
+
+__version__ = "18.4a6"

 DEFAULT_LINE_LENGTH = 88
 DEFAULT_LINE_LENGTH = 88

+

 # types
 syms = pygram.python_symbols
 FileContent = str
 # types
 syms = pygram.python_symbols
 FileContent = str


@@ -54,6 +57,10 @@ Priority = int
 Index = int
 LN = Union[Leaf, Node]
 SplitFunc = Callable[["Line", bool], Iterator["Line"]]
 Index = int
 LN = Union[Leaf, Node]
 SplitFunc = Callable[["Line", bool], Iterator["Line"]]

+Timestamp = float
+FileSize = int
+CacheInfo = Tuple[Timestamp, FileSize]
+Cache = Dict[Path, 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)
 


@@ -82,11 +89,11 @@ class FormatError(Exception):
         self.consumed = consumed
 
     def trim_prefix(self, leaf: Leaf) -> None:
         self.consumed = consumed
 
     def trim_prefix(self, leaf: Leaf) -> None:

-        leaf.prefix = leaf.prefix[self.consumed:]
+        leaf.prefix = leaf.prefix[self.consumed :]

 
     def leaf_from_consumed(self, leaf: Leaf) -> Leaf:
         """Returns a new Leaf from the consumed part of the prefix."""
 
     def leaf_from_consumed(self, leaf: Leaf) -> Leaf:
         """Returns a new Leaf from the consumed part of the prefix."""

-        unformatted_prefix = leaf.prefix[:self.consumed]
+        unformatted_prefix = leaf.prefix[: self.consumed]

         return Leaf(token.NEWLINE, unformatted_prefix)
 
 
         return Leaf(token.NEWLINE, unformatted_prefix)
 
 


@@ -104,6 +111,12 @@ class WriteBack(Enum):
     DIFF = 2
 
 
     DIFF = 2
 
 

+class Changed(Enum):
+    NO = 0
+    CACHED = 1
+    YES = 2
+
+

 @click.command()
 @click.option(
     "-l",
 @click.command()
 @click.option(
     "-l",


@@ -172,59 +185,83 @@ def main(
             sources.append(Path("-"))
         else:
             err(f"invalid path: {s}")
             sources.append(Path("-"))
         else:
             err(f"invalid path: {s}")

-    if check and diff:
-        exc = click.ClickException("Options --check and --diff are mutually exclusive")
-        exc.exit_code = 2
-        raise exc

 
 

-    if check:
+    if check and not diff:

         write_back = WriteBack.NO
     elif diff:
         write_back = WriteBack.DIFF
     else:
         write_back = WriteBack.YES
         write_back = WriteBack.NO
     elif diff:
         write_back = WriteBack.DIFF
     else:
         write_back = WriteBack.YES

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

     if len(sources) == 0:
     if len(sources) == 0:

+        out("No paths given. Nothing to do 😴")

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

+        return
+

     elif len(sources) == 1:
     elif len(sources) == 1:

-        p = sources[0]
-        report = Report(check=check, quiet=quiet)
-        try:
-            if not p.is_file() and str(p) == "-":
-                changed = format_stdin_to_stdout(
-                    line_length=line_length, fast=fast, write_back=write_back
-                )
-            else:
-                changed = format_file_in_place(
-                    p, line_length=line_length, fast=fast, write_back=write_back
-                )
-            report.done(p, changed)
-        except Exception as exc:
-            report.failed(p, str(exc))
-        ctx.exit(report.return_code)
+        reformat_one(sources[0], line_length, fast, write_back, report)

     else:
         loop = asyncio.get_event_loop()
         executor = ProcessPoolExecutor(max_workers=os.cpu_count())
     else:
         loop = asyncio.get_event_loop()
         executor = ProcessPoolExecutor(max_workers=os.cpu_count())

-        return_code = 1

         try:
         try:

-            return_code = loop.run_until_complete(
+            loop.run_until_complete(

                 schedule_formatting(
                 schedule_formatting(

-                    sources, line_length, write_back, fast, quiet, loop, executor
+                    sources, line_length, fast, write_back, report, loop, executor

                 )
             )
         finally:
             shutdown(loop)
                 )
             )
         finally:
             shutdown(loop)

-            ctx.exit(return_code)
+        if not quiet:
+            out("All done! ✨ 🍰 ✨")
+            click.echo(str(report))
+    ctx.exit(report.return_code)
+
+
+def reformat_one(
+    src: Path, line_length: int, fast: bool, write_back: WriteBack, report: "Report"
+) -> None:
+    """Reformat a single file under `src` without spawning child processes.
+
+    If `quiet` is True, non-error messages are not output. `line_length`,
+    `write_back`, and `fast` options are passed to :func:`format_file_in_place`.
+    """
+    try:
+        changed = Changed.NO
+        if not src.is_file() and str(src) == "-":
+            if format_stdin_to_stdout(
+                line_length=line_length, fast=fast, write_back=write_back
+            ):
+                changed = Changed.YES
+        else:
+            cache: Cache = {}
+            if write_back != WriteBack.DIFF:
+                cache = read_cache(line_length)
+                src = src.resolve()
+                if src in cache and cache[src] == get_cache_info(src):
+                    changed = Changed.CACHED
+            if (
+                changed is not Changed.CACHED
+                and format_file_in_place(
+                    src, line_length=line_length, fast=fast, write_back=write_back
+                )
+            ):
+                changed = Changed.YES
+            if write_back == WriteBack.YES and changed is not Changed.NO:
+                write_cache(cache, [src], line_length)
+        report.done(src, changed)
+    except Exception as exc:
+        report.failed(src, str(exc))

 
 
 async def schedule_formatting(
     sources: List[Path],
     line_length: int,
 
 
 async def schedule_formatting(
     sources: List[Path],
     line_length: int,

-    write_back: WriteBack,

     fast: bool,
     fast: bool,

-    quiet: bool,
+    write_back: WriteBack,
+    report: "Report",

     loop: BaseEventLoop,
     executor: Executor,
     loop: BaseEventLoop,
     executor: Executor,

-) -> int:
+) -> None:

     """Run formatting of `sources` in parallel using the provided `executor`.
 
     (Use ProcessPoolExecutors for actual parallelism.)
     """Run formatting of `sources` in parallel using the provided `executor`.
 
     (Use ProcessPoolExecutors for actual parallelism.)


@@ -232,42 +269,52 @@ async def schedule_formatting(
     `line_length`, `write_back`, and `fast` options are passed to
     :func:`format_file_in_place`.
     """
     `line_length`, `write_back`, and `fast` options are passed to
     :func:`format_file_in_place`.
     """

-    lock = None
-    if write_back == WriteBack.DIFF:
-        # For diff output, we need locks to ensure we don't interleave output
-        # from different processes.
-        manager = Manager()
-        lock = manager.Lock()
-    tasks = {
-        src: loop.run_in_executor(
-            executor, format_file_in_place, src, line_length, fast, write_back, lock
-        )
-        for src in sources
-    }
-    _task_values = list(tasks.values())
-    loop.add_signal_handler(signal.SIGINT, cancel, _task_values)
-    loop.add_signal_handler(signal.SIGTERM, cancel, _task_values)
-    await asyncio.wait(tasks.values())
+    cache: Cache = {}
+    if write_back != WriteBack.DIFF:
+        cache = read_cache(line_length)
+        sources, cached = filter_cached(cache, sources)
+        for src in cached:
+            report.done(src, Changed.CACHED)

     cancelled = []
     cancelled = []

-    report = Report(check=write_back is WriteBack.NO, quiet=quiet)
-    for src, task in tasks.items():
-        if not task.done():
-            report.failed(src, "timed out, cancelling")
-            task.cancel()
-            cancelled.append(task)
-        elif task.cancelled():
-            cancelled.append(task)
-        elif task.exception():
-            report.failed(src, str(task.exception()))
-        else:
-            report.done(src, task.result())
+    formatted = []
+    if sources:
+        lock = None
+        if write_back == WriteBack.DIFF:
+            # For diff output, we need locks to ensure we don't interleave output
+            # from different processes.
+            manager = Manager()
+            lock = manager.Lock()
+        tasks = {
+            src: loop.run_in_executor(
+                executor, format_file_in_place, src, line_length, fast, write_back, lock
+            )
+            for src in sources
+        }
+        _task_values = list(tasks.values())
+        try:
+            loop.add_signal_handler(signal.SIGINT, cancel, _task_values)
+            loop.add_signal_handler(signal.SIGTERM, cancel, _task_values)
+        except NotImplementedError:
+            # There are no good alternatives for these on Windows
+            pass
+        await asyncio.wait(_task_values)
+        for src, task in tasks.items():
+            if not task.done():
+                report.failed(src, "timed out, cancelling")
+                task.cancel()
+                cancelled.append(task)
+            elif task.cancelled():
+                cancelled.append(task)
+            elif task.exception():
+                report.failed(src, str(task.exception()))
+            else:
+                formatted.append(src)
+                report.done(src, Changed.YES if task.result() else Changed.NO)
+

     if cancelled:
         await asyncio.gather(*cancelled, loop=loop, return_exceptions=True)
     if cancelled:
         await asyncio.gather(*cancelled, loop=loop, return_exceptions=True)

-    elif not quiet:
-        out("All done! ✨ 🍰 ✨")
-    if not quiet:
-        click.echo(str(report))
-    return report.return_code
+    if write_back == WriteBack.YES and formatted:
+        write_cache(cache, formatted, line_length)

 
 
 def format_file_in_place(
 
 
 def format_file_in_place(


@@ -282,6 +329,7 @@ def format_file_in_place(
     If `write_back` is True, write reformatted code back to stdout.
     `line_length` and `fast` options are passed to :func:`format_file_contents`.
     """
     If `write_back` is True, write reformatted code back to stdout.
     `line_length` and `fast` options are passed to :func:`format_file_contents`.
     """

+

     with tokenize.open(src) as src_buffer:
         src_contents = src_buffer.read()
     try:
     with tokenize.open(src) as src_buffer:
         src_contents = src_buffer.read()
     try:


@@ -295,8 +343,8 @@ def format_file_in_place(
         with open(src, "w", encoding=src_buffer.encoding) as f:
             f.write(dst_contents)
     elif write_back == write_back.DIFF:
         with open(src, "w", encoding=src_buffer.encoding) as f:
             f.write(dst_contents)
     elif write_back == write_back.DIFF:

-        src_name = f"{src.name}  (original)"
-        dst_name = f"{src.name}  (formatted)"
+        src_name = f"{src}  (original)"
+        dst_name = f"{src}  (formatted)"

         diff_contents = diff(src_contents, dst_contents, src_name, dst_name)
         if lock:
             lock.acquire()
         diff_contents = diff(src_contents, dst_contents, src_name, dst_name)
         if lock:
             lock.acquire()


@@ -363,9 +411,10 @@ def format_str(src_contents: str, line_length: int) -> FileContent:
     """
     src_node = lib2to3_parse(src_contents)
     dst_contents = ""
     """
     src_node = lib2to3_parse(src_contents)
     dst_contents = ""

-    lines = LineGenerator()
-    elt = EmptyLineTracker()
+    future_imports = get_future_imports(src_node)

     py36 = is_python36(src_node)
     py36 = is_python36(src_node)

+    lines = LineGenerator(remove_u_prefix=py36 or "unicode_literals" in future_imports)
+    elt = EmptyLineTracker()

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


@@ -382,7 +431,6 @@ def format_str(src_contents: str, line_length: int) -> FileContent:
 GRAMMARS = [
     pygram.python_grammar_no_print_statement_no_exec_statement,
     pygram.python_grammar_no_print_statement,
 GRAMMARS = [
     pygram.python_grammar_no_print_statement_no_exec_statement,
     pygram.python_grammar_no_print_statement,

-    pygram.python_grammar_no_exec_statement,

     pygram.python_grammar,
 ]
 
     pygram.python_grammar,
 ]
 


@@ -508,19 +556,20 @@ COMPARATORS = {
     token.GREATEREQUAL,
 }
 MATH_OPERATORS = {
     token.GREATEREQUAL,
 }
 MATH_OPERATORS = {

+    token.VBAR,
+    token.CIRCUMFLEX,
+    token.AMPER,
+    token.LEFTSHIFT,
+    token.RIGHTSHIFT,

     token.PLUS,
     token.MINUS,
     token.STAR,
     token.SLASH,
     token.PLUS,
     token.MINUS,
     token.STAR,
     token.SLASH,

-    token.VBAR,
-    token.AMPER,
+    token.DOUBLESLASH,

     token.PERCENT,
     token.PERCENT,

-    token.CIRCUMFLEX,
+    token.AT,

     token.TILDE,
     token.TILDE,

-    token.LEFTSHIFT,
-    token.RIGHTSHIFT,

     token.DOUBLESTAR,
     token.DOUBLESTAR,

-    token.DOUBLESLASH,

 }
 STARS = {token.STAR, token.DOUBLESTAR}
 VARARGS_PARENTS = {
 }
 STARS = {token.STAR, token.DOUBLESTAR}
 VARARGS_PARENTS = {


@@ -536,12 +585,61 @@ UNPACKING_PARENTS = {
     syms.listmaker,
     syms.testlist_gexp,
 }
     syms.listmaker,
     syms.testlist_gexp,
 }

+TEST_DESCENDANTS = {
+    syms.test,
+    syms.lambdef,
+    syms.or_test,
+    syms.and_test,
+    syms.not_test,
+    syms.comparison,
+    syms.star_expr,
+    syms.expr,
+    syms.xor_expr,
+    syms.and_expr,
+    syms.shift_expr,
+    syms.arith_expr,
+    syms.trailer,
+    syms.term,
+    syms.power,
+}
+ASSIGNMENTS = {
+    "=",
+    "+=",
+    "-=",
+    "*=",
+    "@=",
+    "/=",
+    "%=",
+    "&=",
+    "|=",
+    "^=",
+    "<<=",
+    ">>=",
+    "**=",
+    "//=",
+}

 COMPREHENSION_PRIORITY = 20
 COMPREHENSION_PRIORITY = 20

-COMMA_PRIORITY = 10
-LOGIC_PRIORITY = 5
-STRING_PRIORITY = 4
-COMPARATOR_PRIORITY = 3
-MATH_PRIORITY = 1
+COMMA_PRIORITY = 18
+TERNARY_PRIORITY = 16
+LOGIC_PRIORITY = 14
+STRING_PRIORITY = 12
+COMPARATOR_PRIORITY = 10
+MATH_PRIORITIES = {
+    token.VBAR: 8,
+    token.CIRCUMFLEX: 7,
+    token.AMPER: 6,
+    token.LEFTSHIFT: 5,
+    token.RIGHTSHIFT: 5,
+    token.PLUS: 4,
+    token.MINUS: 4,
+    token.STAR: 3,
+    token.SLASH: 3,
+    token.DOUBLESLASH: 3,
+    token.PERCENT: 3,
+    token.AT: 3,
+    token.TILDE: 2,
+    token.DOUBLESTAR: 1,
+}

 
 
 @dataclass
 
 
 @dataclass


@@ -552,6 +650,8 @@ class BracketTracker:
     bracket_match: Dict[Tuple[Depth, NodeType], Leaf] = Factory(dict)
     delimiters: Dict[LeafID, Priority] = Factory(dict)
     previous: Optional[Leaf] = None
     bracket_match: Dict[Tuple[Depth, NodeType], Leaf] = Factory(dict)
     delimiters: Dict[LeafID, Priority] = Factory(dict)
     previous: Optional[Leaf] = None

+    _for_loop_variable: int = 0
+    _lambda_arguments: int = 0

 
     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.


@@ -571,6 +671,8 @@ class BracketTracker:
         if leaf.type == token.COMMENT:
             return
 
         if leaf.type == token.COMMENT:
             return
 

+        self.maybe_decrement_after_for_loop_variable(leaf)
+        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))
         if leaf.type in CLOSING_BRACKETS:
             self.depth -= 1
             opening_bracket = self.bracket_match.pop((self.depth, leaf.type))


@@ -588,6 +690,8 @@ class BracketTracker:
             self.bracket_match[self.depth, BRACKET[leaf.type]] = leaf
             self.depth += 1
         self.previous = leaf
             self.bracket_match[self.depth, BRACKET[leaf.type]] = leaf
             self.depth += 1
         self.previous = leaf

+        self.maybe_increment_lambda_arguments(leaf)
+        self.maybe_increment_for_loop_variable(leaf)

 
     def any_open_brackets(self) -> bool:
         """Return True if there is an yet unmatched open bracket on the line."""
 
     def any_open_brackets(self) -> bool:
         """Return True if there is an yet unmatched open bracket on the line."""


@@ -596,11 +700,59 @@ class BracketTracker:
     def max_delimiter_priority(self, exclude: Iterable[LeafID] = ()) -> int:
         """Return the highest priority of a delimiter found on the line.
 
     def max_delimiter_priority(self, exclude: Iterable[LeafID] = ()) -> int:
         """Return the highest priority of a delimiter found on the line.
 

-        Values are consistent with what `is_delimiter()` returns.
+        Values are consistent with what `is_split_*_delimiter()` return.

         Raises ValueError on no delimiters.
         """
         return max(v for k, v in self.delimiters.items() if k not in exclude)
 
         Raises ValueError on no delimiters.
         """
         return max(v for k, v in self.delimiters.items() if k not in exclude)
 

+    def maybe_increment_for_loop_variable(self, leaf: Leaf) -> bool:
+        """In a for loop, or comprehension, the variables are often unpacks.
+
+        To avoid splitting on the comma in this situation, increase the depth of
+        tokens between `for` and `in`.
+        """
+        if leaf.type == token.NAME and leaf.value == "for":
+            self.depth += 1
+            self._for_loop_variable += 1
+            return True
+
+        return False
+
+    def maybe_decrement_after_for_loop_variable(self, leaf: Leaf) -> bool:
+        """See `maybe_increment_for_loop_variable` above for explanation."""
+        if self._for_loop_variable and leaf.type == token.NAME and leaf.value == "in":
+            self.depth -= 1
+            self._for_loop_variable -= 1
+            return True
+
+        return False
+
+    def maybe_increment_lambda_arguments(self, leaf: Leaf) -> bool:
+        """In a lambda expression, there might be more than one argument.
+
+        To avoid splitting on the comma in this situation, increase the depth of
+        tokens between `lambda` and `:`.
+        """
+        if leaf.type == token.NAME and leaf.value == "lambda":
+            self.depth += 1
+            self._lambda_arguments += 1
+            return True
+
+        return False
+
+    def maybe_decrement_after_lambda_arguments(self, leaf: Leaf) -> bool:
+        """See `maybe_increment_lambda_arguments` above for explanation."""
+        if self._lambda_arguments and leaf.type == token.COLON:
+            self.depth -= 1
+            self._lambda_arguments -= 1
+            return True
+
+        return False
+
+    def get_open_lsqb(self) -> Optional[Leaf]:
+        """Return the most recent opening square bracket (if any)."""
+        return self.bracket_match.get((self.depth - 1, token.RSQB))
+

 
 @dataclass
 class Line:
 
 @dataclass
 class Line:


@@ -611,8 +763,6 @@ class Line:
     comments: List[Tuple[Index, Leaf]] = Factory(list)
     bracket_tracker: BracketTracker = Factory(BracketTracker)
     inside_brackets: bool = False
     comments: List[Tuple[Index, Leaf]] = Factory(list)
     bracket_tracker: BracketTracker = Factory(BracketTracker)
     inside_brackets: bool = False

-    has_for: bool = False
-    _for_loop_variable: bool = False

 
     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.


@@ -628,16 +778,17 @@ class Line:
         if not has_value:
             return
 
         if not has_value:
             return
 

+        if token.COLON == leaf.type and self.is_class_paren_empty:
+            del self.leaves[-2:]

         if self.leaves and not preformatted:
             # Note: at this point leaf.prefix should be empty except for
             # imports, for which we only preserve newlines.
         if self.leaves and not preformatted:
             # Note: at this point leaf.prefix should be empty except for
             # imports, for which we only preserve newlines.

-            leaf.prefix += whitespace(leaf)
+            leaf.prefix += whitespace(
+                leaf, complex_subscript=self.is_complex_subscript(leaf)
+            )

         if self.inside_brackets or not preformatted:
         if self.inside_brackets or not preformatted:

-            self.maybe_decrement_after_for_loop_variable(leaf)

             self.bracket_tracker.mark(leaf)
             self.maybe_remove_trailing_comma(leaf)
             self.bracket_tracker.mark(leaf)
             self.maybe_remove_trailing_comma(leaf)

-            self.maybe_increment_for_loop_variable(leaf)
-

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


@@ -725,6 +876,22 @@ class Line:
             and self.leaves[0].value == "yield"
         )
 
             and self.leaves[0].value == "yield"
         )
 

+    @property
+    def is_class_paren_empty(self) -> bool:
+        """Is this a class with no base classes but using parentheses?
+
+        Those are unnecessary and should be removed.
+        """
+        return (
+            bool(self)
+            and len(self.leaves) == 4
+            and self.is_class
+            and self.leaves[2].type == token.LPAR
+            and self.leaves[2].value == "("
+            and self.leaves[3].type == token.RPAR
+            and self.leaves[3].value == ")"
+        )
+

     def contains_standalone_comments(self, depth_limit: int = sys.maxsize) -> bool:
         """If so, needs to be split before emitting."""
         for leaf in self.leaves:
     def contains_standalone_comments(self, depth_limit: int = sys.maxsize) -> bool:
         """If so, needs to be split before emitting."""
         for leaf in self.leaves:


@@ -753,9 +920,14 @@ class Line:
                 self.remove_trailing_comma()
                 return True
 
                 self.remove_trailing_comma()
                 return True
 

-        # For parens let's check if it's safe to remove the comma.  If the
-        # trailing one is the only one, we might mistakenly change a tuple
-        # into a different type by removing the comma.
+        # For parens let's check if it's safe to remove the comma.
+        # Imports are always safe.
+        if self.is_import:
+            self.remove_trailing_comma()
+            return True
+
+        # Otheriwsse, if the trailing one is the only one, we might mistakenly
+        # change a tuple into a different type by removing the comma.

         depth = closing.bracket_depth + 1
         commas = 0
         opening = closing.opening_bracket
         depth = closing.bracket_depth + 1
         commas = 0
         opening = closing.opening_bracket


@@ -766,7 +938,7 @@ class Line:
         else:
             return False
 
         else:
             return False
 

-        for leaf in self.leaves[_opening_index + 1:]:
+        for leaf in self.leaves[_opening_index + 1 :]:

             if leaf is closing:
                 break
 
             if leaf is closing:
                 break
 


@@ -783,29 +955,6 @@ class Line:
 
         return False
 
 
         return False
 

-    def maybe_increment_for_loop_variable(self, leaf: Leaf) -> bool:
-        """In a for loop, or comprehension, the variables are often unpacks.
-
-        To avoid splitting on the comma in this situation, increase the depth of
-        tokens between `for` and `in`.
-        """
-        if leaf.type == token.NAME and leaf.value == "for":
-            self.has_for = True
-            self.bracket_tracker.depth += 1
-            self._for_loop_variable = True
-            return True
-
-        return False
-
-    def maybe_decrement_after_for_loop_variable(self, leaf: Leaf) -> bool:
-        """See `maybe_increment_for_loop_variable` above for explanation."""
-        if self._for_loop_variable and leaf.type == token.NAME and leaf.value == "in":
-            self.bracket_tracker.depth -= 1
-            self._for_loop_variable = False
-            return True
-
-        return False
-

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


@@ -828,17 +977,21 @@ class Line:
             self.comments.append((after, comment))
             return True
 
             self.comments.append((after, comment))
             return True
 

-    def comments_after(self, leaf: Leaf) -> Iterator[Leaf]:
-        """Generate comments that should appear directly after `leaf`."""
-        for _leaf_index, _leaf in enumerate(self.leaves):
-            if leaf is _leaf:
-                break
+    def comments_after(self, leaf: Leaf, _index: int = -1) -> Iterator[Leaf]:
+        """Generate comments that should appear directly after `leaf`.

 
 

-        else:
-            return
+        Provide a non-negative leaf `_index` to speed up the function.
+        """
+        if _index == -1:
+            for _index, _leaf in enumerate(self.leaves):
+                if leaf is _leaf:
+                    break
+
+            else:
+                return

 
         for index, comment_after in self.comments:
 
         for index, comment_after in self.comments:

-            if _leaf_index == index:
+            if _index == index:

                 yield comment_after
 
     def remove_trailing_comma(self) -> None:
                 yield comment_after
 
     def remove_trailing_comma(self) -> None:


@@ -850,6 +1003,25 @@ class Line:
                 self.comments[i] = (comma_index - 1, comment)
         self.leaves.pop()
 
                 self.comments[i] = (comma_index - 1, comment)
         self.leaves.pop()
 

+    def is_complex_subscript(self, leaf: Leaf) -> bool:
+        """Return True iff `leaf` is part of a slice with non-trivial exprs."""
+        open_lsqb = (
+            leaf if leaf.type == token.LSQB else self.bracket_tracker.get_open_lsqb()
+        )
+        if open_lsqb is None:
+            return False
+
+        subscript_start = open_lsqb.next_sibling
+        if (
+            isinstance(subscript_start, Node)
+            and subscript_start.type == syms.subscriptlist
+        ):
+            subscript_start = child_towards(subscript_start, leaf)
+        return (
+            subscript_start is not None
+            and any(n.type in TEST_DESCENDANTS for n in subscript_start.pre_order())
+        )
+

     def __str__(self) -> str:
         """Render the line."""
         if not self:
     def __str__(self) -> str:
         """Render the line."""
         if not self:


@@ -972,8 +1144,14 @@ class EmptyLineTracker:
                 # Don't insert empty lines before the first line in the file.
                 return 0, 0
 
                 # Don't insert empty lines before the first line in the file.
                 return 0, 0
 

-            if self.previous_line and self.previous_line.is_decorator:
-                # Don't insert empty lines between decorators.
+            if self.previous_line.is_decorator:
+                return 0, 0
+
+            if (
+                self.previous_line.is_comment
+                and self.previous_line.depth == current_line.depth
+                and before == 0
+            ):

                 return 0, 0
 
             newlines = 2
                 return 0, 0
 
             newlines = 2


@@ -981,9 +1159,6 @@ class EmptyLineTracker:
                 newlines -= 1
             return newlines, 0
 
                 newlines -= 1
             return newlines, 0
 

-        if current_line.is_flow_control:
-            return before, 1
-

         if (
             self.previous_line
             and self.previous_line.is_import
         if (
             self.previous_line
             and self.previous_line.is_import


@@ -992,13 +1167,6 @@ class EmptyLineTracker:
         ):
             return (before or 1), 0
 
         ):
             return (before or 1), 0
 

-        if (
-            self.previous_line
-            and self.previous_line.is_yield
-            and (not current_line.is_yield or depth != self.previous_line.depth)
-        ):
-            return (before or 1), 0
-

         return before, 0
 
 
         return before, 0
 
 


@@ -1010,6 +1178,7 @@ class LineGenerator(Visitor[Line]):
     in ways that will no longer stringify to valid Python code on the tree.
     """
     current_line: Line = Factory(Line)
     in ways that will no longer stringify to valid Python code on the tree.
     """
     current_line: Line = Factory(Line)

+    remove_u_prefix: bool = False

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


@@ -1077,6 +1246,7 @@ class LineGenerator(Visitor[Line]):
             else:
                 normalize_prefix(node, inside_brackets=any_open_brackets)
                 if node.type == token.STRING:
             else:
                 normalize_prefix(node, inside_brackets=any_open_brackets)
                 if node.type == token.STRING:

+                    normalize_string_prefix(node, remove_u_prefix=self.remove_u_prefix)

                     normalize_string_quotes(node)
                 if node.type not in WHITESPACE:
                     self.current_line.append(node)
                     normalize_string_quotes(node)
                 if node.type not in WHITESPACE:
                     self.current_line.append(node)


@@ -1090,7 +1260,16 @@ class LineGenerator(Visitor[Line]):
 
     def visit_DEDENT(self, node: Node) -> Iterator[Line]:
         """Decrease indentation level, maybe yield a line."""
 
     def visit_DEDENT(self, node: Node) -> Iterator[Line]:
         """Decrease indentation level, maybe yield a line."""

-        # DEDENT has no value. Additionally, in blib2to3 it never holds comments.
+        # The current line might still wait for trailing comments.  At DEDENT time
+        # there won't be any (they would be prefixes on the preceding NEWLINE).
+        # Emit the line then.
+        yield from self.line()
+
+        # While DEDENT has no value, its prefix may contain standalone comments
+        # that belong to the current indentation level.  Get 'em.
+        yield from self.visit_default(node)
+
+        # Finally, emit the dedent.

         yield from self.line(-1)
 
     def visit_stmt(
         yield from self.line(-1)
 
     def visit_stmt(


@@ -1099,14 +1278,13 @@ class LineGenerator(Visitor[Line]):
         """Visit a statement.
 
         This implementation is shared for `if`, `while`, `for`, `try`, `except`,
         """Visit a statement.
 
         This implementation is shared for `if`, `while`, `for`, `try`, `except`,

-        `def`, `with`, `class`, and `assert`.
+        `def`, `with`, `class`, `assert` and assignments.

 
         The relevant Python language `keywords` for a given statement will be
         NAME leaves within it. This methods puts those on a separate line.
 
 
         The relevant Python language `keywords` for a given statement will be
         NAME leaves within it. This methods puts those on a separate line.
 

-        `parens` holds pairs of nodes where invisible parentheses should be put.
-        Keys hold nodes after which opening parentheses should be put, values
-        hold nodes before which closing parentheses should be put.
+        `parens` holds a set of string leaf values immeditely after which
+        invisible parens should be put.

         """
         normalize_invisible_parens(node, parens_after=parens)
         for child in node.children:
         """
         normalize_invisible_parens(node, parens_after=parens)
         for child in node.children:


@@ -1206,7 +1384,9 @@ class LineGenerator(Visitor[Line]):
         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", ","})

-        self.visit_if_stmt = partial(v, keywords={"if", "else", "elif"}, parens={"if"})
+        self.visit_if_stmt = partial(
+            v, keywords={"if", "else", "elif"}, parens={"if", "elif"}
+        )

         self.visit_while_stmt = partial(v, keywords={"while", "else"}, parens={"while"})
         self.visit_for_stmt = partial(v, keywords={"for", "else"}, parens={"for", "in"})
         self.visit_try_stmt = partial(
         self.visit_while_stmt = partial(v, keywords={"while", "else"}, parens={"while"})
         self.visit_for_stmt = partial(v, keywords={"for", "else"}, parens={"for", "in"})
         self.visit_try_stmt = partial(


@@ -1216,6 +1396,8 @@ class LineGenerator(Visitor[Line]):
         self.visit_with_stmt = partial(v, keywords={"with"}, parens=Ø)
         self.visit_funcdef = partial(v, keywords={"def"}, parens=Ø)
         self.visit_classdef = partial(v, keywords={"class"}, parens=Ø)
         self.visit_with_stmt = partial(v, keywords={"with"}, parens=Ø)
         self.visit_funcdef = partial(v, keywords={"def"}, parens=Ø)
         self.visit_classdef = partial(v, keywords={"class"}, parens=Ø)

+        self.visit_expr_stmt = partial(v, keywords=Ø, parens=ASSIGNMENTS)
+        self.visit_return_stmt = partial(v, keywords={"return"}, parens={"return"})

         self.visit_async_funcdef = self.visit_async_stmt
         self.visit_decorated = self.visit_decorators
 
         self.visit_async_funcdef = self.visit_async_stmt
         self.visit_decorated = self.visit_decorators
 


@@ -1228,8 +1410,12 @@ BRACKETS = OPENING_BRACKETS | CLOSING_BRACKETS
 ALWAYS_NO_SPACE = CLOSING_BRACKETS | {token.COMMA, STANDALONE_COMMENT}
 
 
 ALWAYS_NO_SPACE = CLOSING_BRACKETS | {token.COMMA, STANDALONE_COMMENT}
 
 

-def whitespace(leaf: Leaf) -> str:  # noqa C901
-    """Return whitespace prefix if needed for the given `leaf`."""
+def whitespace(leaf: Leaf, *, complex_subscript: bool) -> str:  # noqa C901
+    """Return whitespace prefix if needed for the given `leaf`.
+
+    `complex_subscript` signals whether the given leaf is part of a subscription
+    which has non-trivial arguments, like arithmetic expressions or function calls.
+    """

     NO = ""
     SPACE = " "
     DOUBLESPACE = "  "
     NO = ""
     SPACE = " "
     DOUBLESPACE = "  "


@@ -1243,7 +1429,10 @@ def whitespace(leaf: Leaf) -> str:  # noqa C901
         return DOUBLESPACE
 
     assert p is not None, f"INTERNAL ERROR: hand-made leaf without parent: {leaf!r}"
         return DOUBLESPACE
 
     assert p is not None, f"INTERNAL ERROR: hand-made leaf without parent: {leaf!r}"

-    if t == token.COLON and p.type not in {syms.subscript, syms.subscriptlist}:
+    if (
+        t == token.COLON
+        and p.type not in {syms.subscript, syms.subscriptlist, syms.sliceop}
+    ):

         return NO
 
     prev = leaf.prev_sibling
         return NO
 
     prev = leaf.prev_sibling


@@ -1253,7 +1442,13 @@ def whitespace(leaf: Leaf) -> str:  # noqa C901
             return NO
 
         if t == token.COLON:
             return NO
 
         if t == token.COLON:

-            return SPACE if prevp.type == token.COMMA else NO
+            if prevp.type == token.COLON:
+                return NO
+
+            elif prevp.type != token.COMMA and not complex_subscript:
+                return NO
+
+            return SPACE

 
         if prevp.type == token.EQUAL:
             if prevp.parent:
 
         if prevp.type == token.EQUAL:
             if prevp.parent:


@@ -1274,7 +1469,7 @@ def whitespace(leaf: Leaf) -> str:  # noqa C901
 
         elif prevp.type == token.COLON:
             if prevp.parent and prevp.parent.type in {syms.subscript, syms.sliceop}:
 
         elif prevp.type == token.COLON:
             if prevp.parent and prevp.parent.type in {syms.subscript, syms.sliceop}:

-                return NO
+                return SPACE if complex_subscript else NO

 
         elif (
             prevp.parent
 
         elif (
             prevp.parent


@@ -1299,17 +1494,11 @@ def whitespace(leaf: Leaf) -> str:  # noqa C901
 
     if p.type in {syms.parameters, syms.arglist}:
         # untyped function signatures or calls
 
     if p.type in {syms.parameters, syms.arglist}:
         # untyped function signatures or calls

-        if t == token.RPAR:
-            return NO
-

         if not prev or prev.type != token.COMMA:
             return NO
 
     elif p.type == syms.varargslist:
         # lambdas
         if not prev or prev.type != token.COMMA:
             return NO
 
     elif p.type == syms.varargslist:
         # lambdas

-        if t == token.RPAR:
-            return NO
-

         if prev and prev.type != token.COMMA:
             return NO
 
         if prev and prev.type != token.COMMA:
             return NO
 


@@ -1364,7 +1553,7 @@ def whitespace(leaf: Leaf) -> str:  # noqa C901
             if not prevp or prevp.type == token.LPAR:
                 return NO
 
             if not prevp or prevp.type == token.LPAR:
                 return NO
 

-        elif prev.type in {token.EQUAL, token.STAR, token.DOUBLESTAR}:
+        elif prev.type in {token.EQUAL} | STARS:

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


@@ -1386,7 +1575,7 @@ def whitespace(leaf: Leaf) -> str:  # noqa C901
         if prev and prev.type == token.LPAR:
             return NO
 
         if prev and prev.type == token.LPAR:
             return NO
 

-    elif p.type == syms.subscript:
+    elif p.type in {syms.subscript, syms.sliceop}:

         # indexing
         if not prev:
             assert p.parent is not None, "subscripts are always parented"
         # indexing
         if not prev:
             assert p.parent is not None, "subscripts are always parented"


@@ -1395,7 +1584,7 @@ def whitespace(leaf: Leaf) -> str:  # noqa C901
 
             return NO
 
 
             return NO
 

-        else:
+        elif not complex_subscript:

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


@@ -1403,21 +1592,9 @@ def whitespace(leaf: Leaf) -> str:  # noqa C901
             # dots, but not the first one.
             return NO
 
             # dots, but not the first one.
             return NO
 

-    elif (
-        p.type == syms.listmaker
-        or p.type == syms.testlist_gexp
-        or p.type == syms.subscriptlist
-    ):
-        # list interior, including unpacking
-        if not prev:
-            return NO
-

     elif p.type == syms.dictsetmaker:
     elif p.type == syms.dictsetmaker:

-        # dict and set interior, including unpacking
-        if not prev:
-            return NO
-
-        if prev.type == token.DOUBLESTAR:
+        # dict unpacking
+        if prev and prev.type == token.DOUBLESTAR:

             return NO
 
     elif p.type in {syms.factor, syms.star_expr}:
             return NO
 
     elif p.type in {syms.factor, syms.star_expr}:


@@ -1477,6 +1654,14 @@ def preceding_leaf(node: Optional[LN]) -> Optional[Leaf]:
     return None
 
 
     return None
 
 

+def child_towards(ancestor: Node, descendant: LN) -> Optional[LN]:
+    """Return the child of `ancestor` that contains `descendant`."""
+    node: Optional[LN] = descendant
+    while node and node.parent != ancestor:
+        node = node.parent
+    return node
+
+

 def is_split_after_delimiter(leaf: Leaf, previous: Leaf = None) -> int:
     """Return the priority of the `leaf` delimiter, given a line break after it.
 
 def is_split_after_delimiter(leaf: Leaf, previous: Leaf = None) -> int:
     """Return the priority of the `leaf` delimiter, given a line break after it.
 


@@ -1509,7 +1694,7 @@ def is_split_before_delimiter(leaf: Leaf, previous: Leaf = None) -> int:
         and leaf.parent
         and leaf.parent.type not in {syms.factor, syms.star_expr}
     ):
         and leaf.parent
         and leaf.parent.type not in {syms.factor, syms.star_expr}
     ):

-        return MATH_PRIORITY
+        return MATH_PRIORITIES[leaf.type]

 
     if leaf.type in COMPARATORS:
         return COMPARATOR_PRIORITY
 
     if leaf.type in COMPARATORS:
         return COMPARATOR_PRIORITY


@@ -1537,23 +1722,20 @@ def is_split_before_delimiter(leaf: Leaf, previous: Leaf = None) -> int:
     ):
         return COMPREHENSION_PRIORITY
 
     ):
         return COMPREHENSION_PRIORITY
 

+    if (
+        leaf.type == token.NAME
+        and leaf.value in {"if", "else"}
+        and leaf.parent
+        and leaf.parent.type == syms.test
+    ):
+        return TERNARY_PRIORITY
+

     if leaf.type == token.NAME and leaf.value in LOGIC_OPERATORS and leaf.parent:
         return LOGIC_PRIORITY
 
     return 0
 
 
     if leaf.type == token.NAME and leaf.value in LOGIC_OPERATORS and leaf.parent:
         return LOGIC_PRIORITY
 
     return 0
 
 

-def is_delimiter(leaf: Leaf, previous: Leaf = None) -> int:
-    """Return the priority of the `leaf` delimiter. Return 0 if not delimiter.
-
-    Higher numbers are higher priority.
-    """
-    return max(
-        is_split_before_delimiter(leaf, previous),
-        is_split_after_delimiter(leaf, previous),
-    )
-
-

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


@@ -1648,21 +1830,31 @@ def split_line(
         return
 
     line_str = str(line).strip("\n")
         return
 
     line_str = str(line).strip("\n")

-    if (
-        len(line_str) <= line_length
-        and "\n" not in line_str  # multiline strings
-        and not line.contains_standalone_comments()
-    ):
+    if is_line_short_enough(line, line_length=line_length, line_str=line_str):

         yield line
         return
 
     split_funcs: List[SplitFunc]
     if line.is_def:
         split_funcs = [left_hand_split]
         yield line
         return
 
     split_funcs: List[SplitFunc]
     if line.is_def:
         split_funcs = [left_hand_split]

-    elif line.inside_brackets:
-        split_funcs = [delimiter_split, standalone_comment_split, right_hand_split]
+    elif line.is_import:
+        split_funcs = [explode_split]

     else:
     else:

-        split_funcs = [right_hand_split]
+
+        def rhs(line: Line, py36: bool = False) -> Iterator[Line]:
+            for omit in generate_trailers_to_omit(line, line_length):
+                lines = list(right_hand_split(line, py36, omit=omit))
+                if is_line_short_enough(lines[0], line_length=line_length):
+                    yield from lines
+                    return
+
+            # All splits failed, best effort split with no omits.
+            yield from right_hand_split(line, py36)
+
+        if line.inside_brackets:
+            split_funcs = [delimiter_split, standalone_comment_split, rhs]
+        else:
+            split_funcs = [rhs]

     for split_func in split_funcs:
         # 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
     for split_func in split_funcs:
         # 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


@@ -1691,7 +1883,8 @@ def left_hand_split(line: Line, py36: bool = False) -> Iterator[Line]:
     """Split line into many lines, starting with the first matching bracket pair.
 
     Note: this usually looks weird, only use this for function definitions.
     """Split line into many lines, starting with the first matching bracket pair.
 
     Note: this usually looks weird, only use this for function definitions.

-    Prefer RHS otherwise.
+    Prefer RHS otherwise.  This is why this function is not symmetrical with
+    :func:`right_hand_split` which also handles optional parentheses.

     """
     head = Line(depth=line.depth)
     body = Line(depth=line.depth + 1, inside_brackets=True)
     """
     head = Line(depth=line.depth)
     body = Line(depth=line.depth + 1, inside_brackets=True)


@@ -1731,7 +1924,12 @@ def left_hand_split(line: Line, py36: bool = False) -> Iterator[Line]:
 def right_hand_split(
     line: Line, py36: bool = False, omit: Collection[LeafID] = ()
 ) -> Iterator[Line]:
 def right_hand_split(
     line: Line, py36: bool = False, omit: Collection[LeafID] = ()
 ) -> Iterator[Line]:

-    """Split line into many lines, starting with the last matching bracket pair."""
+    """Split line into many lines, starting with the last matching bracket pair.
+
+    If the split was by optional parentheses, attempt splitting without them, too.
+    `omit` is a collection of closing bracket IDs that shouldn't be considered for
+    this split.
+    """

     head = Line(depth=line.depth)
     body = Line(depth=line.depth + 1, inside_brackets=True)
     tail = Line(depth=line.depth)
     head = Line(depth=line.depth)
     body = Line(depth=line.depth + 1, inside_brackets=True)
     tail = Line(depth=line.depth)


@@ -1770,20 +1968,25 @@ def right_hand_split(
     bracket_split_succeeded_or_raise(head, body, tail)
     assert opening_bracket and closing_bracket
     if (
     bracket_split_succeeded_or_raise(head, body, tail)
     assert opening_bracket and closing_bracket
     if (

+        # the opening bracket is an optional paren

         opening_bracket.type == token.LPAR
         and not opening_bracket.value
         opening_bracket.type == token.LPAR
         and not opening_bracket.value

+        # the closing bracket is an optional paren

         and closing_bracket.type == token.RPAR
         and not closing_bracket.value
         and closing_bracket.type == token.RPAR
         and not closing_bracket.value

+        # there are no delimiters or standalone comments in the body
+        and not body.bracket_tracker.delimiters
+        and not line.contains_standalone_comments(0)
+        # and it's not an import (optional parens are the only thing we can split
+        # on in this case; attempting a split without them is a waste of time)
+        and not line.is_import

     ):
     ):

-        # These parens were optional. If there aren't any delimiters or standalone
-        # comments in the body, they were unnecessary and another split without
-        # them should be attempted.
-        if not (
-            body.bracket_tracker.delimiters or line.contains_standalone_comments(0)
-        ):
-            omit = {id(closing_bracket), *omit}
+        omit = {id(closing_bracket), *omit}
+        try:

             yield from right_hand_split(line, py36=py36, omit=omit)
             return
             yield from right_hand_split(line, py36=py36, omit=omit)
             return

+        except CannotSplit:
+            pass

 
     ensure_visible(opening_bracket)
     ensure_visible(closing_bracket)
 
     ensure_visible(opening_bracket)
     ensure_visible(closing_bracket)


@@ -1868,10 +2071,10 @@ def delimiter_split(line: Line, py36: bool = False) -> Iterator[Line]:
             current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)
             current_line.append(leaf)
 
             current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)
             current_line.append(leaf)
 

-    for leaf in line.leaves:
+    for index, leaf in enumerate(line.leaves):

         yield from append_to_line(leaf)
 
         yield from append_to_line(leaf)
 

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

             yield from append_to_line(comment_after)
 
         lowest_depth = min(lowest_depth, leaf.bracket_depth)
             yield from append_to_line(comment_after)
 
         lowest_depth = min(lowest_depth, leaf.bracket_depth)


@@ -1915,16 +2118,36 @@ def standalone_comment_split(line: Line, py36: bool = False) -> Iterator[Line]:
             current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)
             current_line.append(leaf)
 
             current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)
             current_line.append(leaf)
 

-    for leaf in line.leaves:
+    for index, leaf in enumerate(line.leaves):

         yield from append_to_line(leaf)
 
         yield from append_to_line(leaf)
 

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

             yield from append_to_line(comment_after)
 
     if current_line:
         yield current_line
 
 
             yield from append_to_line(comment_after)
 
     if current_line:
         yield current_line
 
 

+def explode_split(
+    line: Line, py36: bool = False, omit: Collection[LeafID] = ()
+) -> Iterator[Line]:
+    """Split by rightmost bracket and immediately split contents by a delimiter."""
+    new_lines = list(right_hand_split(line, py36, omit))
+    if len(new_lines) != 3:
+        yield from new_lines
+        return
+
+    yield new_lines[0]
+
+    try:
+        yield from delimiter_split(new_lines[1], py36)
+
+    except CannotSplit:
+        yield new_lines[1]
+
+    yield new_lines[2]
+
+

 def is_import(leaf: Leaf) -> bool:
     """Return True if the given leaf starts an import statement."""
     p = leaf.parent
 def is_import(leaf: Leaf) -> bool:
     """Return True if the given leaf starts an import statement."""
     p = leaf.parent


@@ -1957,6 +2180,22 @@ def normalize_prefix(leaf: Leaf, *, inside_brackets: bool) -> None:
     leaf.prefix = ""
 
 
     leaf.prefix = ""
 
 

+def normalize_string_prefix(leaf: Leaf, remove_u_prefix: bool = False) -> None:
+    """Make all string prefixes lowercase.
+
+    If remove_u_prefix is given, also removes any u prefix from the string.
+
+    Note: Mutates its argument.
+    """
+    match = re.match(r"^([furbFURB]*)(.*)$", leaf.value, re.DOTALL)
+    assert match is not None, f"failed to match string {leaf.value!r}"
+    orig_prefix = match.group(1)
+    new_prefix = orig_prefix.lower()
+    if remove_u_prefix:
+        new_prefix = new_prefix.replace("u", "")
+    leaf.value = f"{new_prefix}{match.group(2)}"
+
+

 def normalize_string_quotes(leaf: Leaf) -> None:
     """Prefer double quotes but only if it doesn't cause more escaping.
 
 def normalize_string_quotes(leaf: Leaf) -> None:
     """Prefer double quotes but only if it doesn't cause more escaping.
 


@@ -1986,7 +2225,7 @@ def normalize_string_quotes(leaf: Leaf) -> None:
     unescaped_new_quote = re.compile(rf"(([^\\]|^)(\\\\)*){new_quote}")
     escaped_new_quote = re.compile(rf"([^\\]|^)\\(\\\\)*{new_quote}")
     escaped_orig_quote = re.compile(rf"([^\\]|^)\\(\\\\)*{orig_quote}")
     unescaped_new_quote = re.compile(rf"(([^\\]|^)(\\\\)*){new_quote}")
     escaped_new_quote = re.compile(rf"([^\\]|^)\\(\\\\)*{new_quote}")
     escaped_orig_quote = re.compile(rf"([^\\]|^)\\(\\\\)*{orig_quote}")

-    body = leaf.value[first_quote_pos + len(orig_quote):-len(orig_quote)]
+    body = leaf.value[first_quote_pos + len(orig_quote) : -len(orig_quote)]

     if "r" in prefix.casefold():
         if unescaped_new_quote.search(body):
             # There's at least one unescaped new_quote in this raw string
     if "r" in prefix.casefold():
         if unescaped_new_quote.search(body):
             # There's at least one unescaped new_quote in this raw string


@@ -2021,6 +2260,9 @@ def normalize_string_quotes(leaf: Leaf) -> None:
 def normalize_invisible_parens(node: Node, parens_after: Set[str]) -> None:
     """Make existing optional parentheses invisible or create new ones.
 
 def normalize_invisible_parens(node: Node, parens_after: Set[str]) -> None:
     """Make existing optional parentheses invisible or create new ones.
 

+    `parens_after` is a set of string leaf values immeditely after which parens
+    should be put.
+

     Standardizes on visible parentheses for single-element tuples, and keeps
     existing visible parentheses for other tuples and generator expressions.
     """
     Standardizes on visible parentheses for single-element tuples, and keeps
     existing visible parentheses for other tuples and generator expressions.
     """


@@ -2028,17 +2270,7 @@ def normalize_invisible_parens(node: Node, parens_after: Set[str]) -> None:
     for child in list(node.children):
         if check_lpar:
             if child.type == syms.atom:
     for child in list(node.children):
         if check_lpar:
             if child.type == syms.atom:

-                if not (
-                    is_empty_tuple(child)
-                    or is_one_tuple(child)
-                    or max_delimiter_priority_in_atom(child) >= COMMA_PRIORITY
-                ):
-                    first = child.children[0]
-                    last = child.children[-1]
-                    if first.type == token.LPAR and last.type == token.RPAR:
-                        # make parentheses invisible
-                        first.value = ""  # type: ignore
-                        last.value = ""  # type: ignore
+                maybe_make_parens_invisible_in_atom(child)

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


@@ -2055,6 +2287,30 @@ def normalize_invisible_parens(node: Node, parens_after: Set[str]) -> None:
         check_lpar = isinstance(child, Leaf) and child.value in parens_after
 
 
         check_lpar = isinstance(child, Leaf) and child.value in parens_after
 
 

+def maybe_make_parens_invisible_in_atom(node: LN) -> bool:
+    """If it's safe, make the parens in the atom `node` invisible, recusively."""
+    if (
+        node.type != syms.atom
+        or is_empty_tuple(node)
+        or is_one_tuple(node)
+        or is_yield(node)
+        or max_delimiter_priority_in_atom(node) >= COMMA_PRIORITY
+    ):
+        return False
+
+    first = node.children[0]
+    last = node.children[-1]
+    if first.type == token.LPAR and last.type == token.RPAR:
+        # make parentheses invisible
+        first.value = ""  # type: ignore
+        last.value = ""  # type: ignore
+        if len(node.children) > 1:
+            maybe_make_parens_invisible_in_atom(node.children[1])
+        return True
+
+    return False
+
+

 def is_empty_tuple(node: LN) -> bool:
     """Return True if `node` holds an empty tuple."""
     return (
 def is_empty_tuple(node: LN) -> bool:
     """Return True if `node` holds an empty tuple."""
     return (


@@ -2088,12 +2344,33 @@ def is_one_tuple(node: LN) -> bool:
     )
 
 
     )
 
 

+def is_yield(node: LN) -> bool:
+    """Return True if `node` holds a `yield` or `yield from` expression."""
+    if node.type == syms.yield_expr:
+        return True
+
+    if node.type == token.NAME and node.value == "yield":  # type: ignore
+        return True
+
+    if node.type != syms.atom:
+        return False
+
+    if len(node.children) != 3:
+        return False
+
+    lpar, expr, rpar = node.children
+    if lpar.type == token.LPAR and rpar.type == token.RPAR:
+        return is_yield(expr)
+
+    return False
+
+

 def is_vararg(leaf: Leaf, within: Set[NodeType]) -> bool:
     """Return True if `leaf` is a star or double star in a vararg or kwarg.
 
     If `within` includes VARARGS_PARENTS, this applies to function signatures.
 def is_vararg(leaf: Leaf, within: Set[NodeType]) -> bool:
     """Return True if `leaf` is a star or double star in a vararg or kwarg.
 
     If `within` includes VARARGS_PARENTS, this applies to function signatures.

-    If `within` includes COLLECTION_LIBERALS_PARENTS, it applies to right
-    hand-side extended iterable unpacking (PEP 3132) and additional unpacking
+    If `within` includes UNPACKING_PARENTS, it applies to right hand-side
+    extended iterable unpacking (PEP 3132) and additional unpacking

     generalizations (PEP 448).
     """
     if leaf.type not in STARS or not leaf.parent:
     generalizations (PEP 448).
     """
     if leaf.type not in STARS or not leaf.parent:


@@ -2112,6 +2389,11 @@ def is_vararg(leaf: Leaf, within: Set[NodeType]) -> bool:
 
 
 def max_delimiter_priority_in_atom(node: LN) -> int:
 
 
 def max_delimiter_priority_in_atom(node: LN) -> int:

+    """Return maximum delimiter priority inside `node`.
+
+    This is specific to atoms with contents contained in a pair of parentheses.
+    If `node` isn't an atom or there are no enclosing parentheses, returns 0.
+    """

     if node.type != syms.atom:
         return 0
 
     if node.type != syms.atom:
         return 0
 


@@ -2151,7 +2433,7 @@ def is_python36(node: Node) -> bool:
 
     Currently looking for:
     - f-strings; and
 
     Currently looking for:
     - f-strings; and

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

     """
     for n in node.pre_order():
         if n.type == token.STRING:
     """
     for n in node.pre_order():
         if n.type == token.STRING:


@@ -2160,17 +2442,118 @@ def is_python36(node: Node) -> bool:
                 return True
 
         elif (
                 return True
 
         elif (

-            n.type == syms.typedargslist
+            n.type in {syms.typedargslist, syms.arglist}

             and n.children
             and n.children[-1].type == token.COMMA
         ):
             for ch in n.children:
             and n.children
             and n.children[-1].type == token.COMMA
         ):
             for ch in n.children:

-                if ch.type == token.STAR or ch.type == token.DOUBLESTAR:
+                if ch.type in STARS:

                     return True
 
                     return True
 

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

     return False
 
 
     return False
 
 

+def generate_trailers_to_omit(line: Line, line_length: int) -> Iterator[Set[LeafID]]:
+    """Generate sets of closing bracket IDs that should be omitted in a RHS.
+
+    Brackets can be omitted if the entire trailer up to and including
+    a preceding closing bracket fits in one line.
+
+    Yielded sets are cumulative (contain results of previous yields, too).  First
+    set is empty.
+    """
+
+    omit: Set[LeafID] = set()
+    yield omit
+
+    length = 4 * line.depth
+    opening_bracket = None
+    closing_bracket = None
+    optional_brackets: Set[LeafID] = set()
+    inner_brackets: Set[LeafID] = set()
+    for index, leaf in enumerate_reversed(line.leaves):
+        length += len(leaf.prefix) + len(leaf.value)
+        if length > line_length:
+            break
+
+        comment: Optional[Leaf]
+        for comment in line.comments_after(leaf, index):
+            if "\n" in comment.prefix:
+                break  # Oops, standalone comment!
+
+            length += len(comment.value)
+        else:
+            comment = None
+        if comment is not None:
+            break  # There was a standalone comment, we can't continue.
+
+        optional_brackets.discard(id(leaf))
+        if opening_bracket:
+            if leaf is opening_bracket:
+                opening_bracket = None
+            elif leaf.type in CLOSING_BRACKETS:
+                inner_brackets.add(id(leaf))
+        elif leaf.type in CLOSING_BRACKETS:
+            if not leaf.value:
+                optional_brackets.add(id(opening_bracket))
+                continue
+
+            if index > 0 and line.leaves[index - 1].type in OPENING_BRACKETS:
+                # Empty brackets would fail a split so treat them as "inner"
+                # brackets (e.g. only add them to the `omit` set if another
+                # pair of brackets was good enough.
+                inner_brackets.add(id(leaf))
+                continue
+
+            opening_bracket = leaf.opening_bracket
+            if closing_bracket:
+                omit.add(id(closing_bracket))
+                omit.update(inner_brackets)
+                inner_brackets.clear()
+                yield omit
+            closing_bracket = leaf
+
+
+def get_future_imports(node: Node) -> Set[str]:
+    """Return a set of __future__ imports in the file."""
+    imports = set()
+    for child in node.children:
+        if child.type != syms.simple_stmt:
+            break
+        first_child = child.children[0]
+        if isinstance(first_child, Leaf):
+            # Continue looking if we see a docstring; otherwise stop.
+            if (
+                len(child.children) == 2
+                and first_child.type == token.STRING
+                and child.children[1].type == token.NEWLINE
+            ):
+                continue
+            else:
+                break
+        elif first_child.type == syms.import_from:
+            module_name = first_child.children[1]
+            if not isinstance(module_name, Leaf) or module_name.value != "__future__":
+                break
+            for import_from_child in first_child.children[3:]:
+                if isinstance(import_from_child, Leaf):
+                    if import_from_child.type == token.NAME:
+                        imports.add(import_from_child.value)
+                else:
+                    assert import_from_child.type == syms.import_as_names
+                    for leaf in import_from_child.children:
+                        if isinstance(leaf, Leaf) and leaf.type == token.NAME:
+                            imports.add(leaf.value)
+        else:
+            break
+    return imports
+
+

 PYTHON_EXTENSIONS = {".py"}
 BLACKLISTED_DIRECTORIES = {
     "build", "buck-out", "dist", "_build", ".git", ".hg", ".mypy_cache", ".tox", ".venv"
 PYTHON_EXTENSIONS = {".py"}
 BLACKLISTED_DIRECTORIES = {
     "build", "buck-out", "dist", "_build", ".git", ".hg", ".mypy_cache", ".tox", ".venv"


@@ -2188,7 +2571,7 @@ def gen_python_files_in_dir(path: Path) -> Iterator[Path]:
 
             yield from gen_python_files_in_dir(child)
 
 
             yield from gen_python_files_in_dir(child)
 

-        elif child.suffix in PYTHON_EXTENSIONS:
+        elif child.is_file() and child.suffix in PYTHON_EXTENSIONS:

             yield child
 
 
             yield child
 
 


@@ -2201,16 +2584,20 @@ class Report:
     same_count: int = 0
     failure_count: int = 0
 
     same_count: int = 0
     failure_count: int = 0
 

-    def done(self, src: Path, changed: bool) -> None:
+    def done(self, src: Path, changed: Changed) -> None:

         """Increment the counter for successful reformatting. Write out a message."""
         """Increment the counter for successful reformatting. Write out a message."""

-        if changed:
+        if changed is Changed.YES:

             reformatted = "would reformat" if self.check else "reformatted"
             if not self.quiet:
                 out(f"{reformatted} {src}")
             self.change_count += 1
         else:
             if not self.quiet:
             reformatted = "would reformat" if self.check else "reformatted"
             if not self.quiet:
                 out(f"{reformatted} {src}")
             self.change_count += 1
         else:
             if not self.quiet:

-                out(f"{src} already well formatted, good job.", bold=False)
+                if changed is Changed.NO:
+                    msg = f"{src} already well formatted, good job."
+                else:
+                    msg = f"{src} wasn't modified on disk since last run."
+                out(msg, bold=False)

             self.same_count += 1
 
     def failed(self, src: Path, message: str) -> None:
             self.same_count += 1
 
     def failed(self, src: Path, message: str) -> None:


@@ -2409,5 +2796,89 @@ def sub_twice(regex: Pattern[str], replacement: str, original: str) -> str:
     return regex.sub(replacement, regex.sub(replacement, original))
 
 
     return regex.sub(replacement, regex.sub(replacement, original))
 
 

+def enumerate_reversed(sequence: Sequence[T]) -> Iterator[Tuple[Index, T]]:
+    """Like `reversed(enumerate(sequence))` if that were possible."""
+    index = len(sequence) - 1
+    for element in reversed(sequence):
+        yield (index, element)
+        index -= 1
+
+
+def is_line_short_enough(line: Line, *, line_length: int, line_str: str = "") -> bool:
+    """Return True if `line` is no longer than `line_length`.
+
+    Uses the provided `line_str` rendering, if any, otherwise computes a new one.
+    """
+    if not line_str:
+        line_str = str(line).strip("\n")
+    return (
+        len(line_str) <= line_length
+        and "\n" not in line_str  # multiline strings
+        and not line.contains_standalone_comments()
+    )
+
+
+CACHE_DIR = Path(user_cache_dir("black", version=__version__))
+
+
+def get_cache_file(line_length: int) -> Path:
+    return CACHE_DIR / f"cache.{line_length}.pickle"
+
+
+def read_cache(line_length: int) -> Cache:
+    """Read the cache if it exists and is well formed.
+
+    If it is not well formed, the call to write_cache later should resolve the issue.
+    """
+    cache_file = get_cache_file(line_length)
+    if not cache_file.exists():
+        return {}
+
+    with cache_file.open("rb") as fobj:
+        try:
+            cache: Cache = pickle.load(fobj)
+        except pickle.UnpicklingError:
+            return {}
+
+    return cache
+
+
+def get_cache_info(path: Path) -> CacheInfo:
+    """Return the information used to check if a file is already formatted or not."""
+    stat = path.stat()
+    return stat.st_mtime, stat.st_size
+
+
+def filter_cached(
+    cache: Cache, sources: Iterable[Path]
+) -> Tuple[List[Path], List[Path]]:
+    """Split a list of paths into two.
+
+    The first list contains paths of files that modified on disk or are not in the
+    cache. The other list contains paths to non-modified files.
+    """
+    todo, done = [], []
+    for src in sources:
+        src = src.resolve()
+        if cache.get(src) != get_cache_info(src):
+            todo.append(src)
+        else:
+            done.append(src)
+    return todo, done
+
+
+def write_cache(cache: Cache, sources: List[Path], line_length: int) -> None:
+    """Update the cache file."""
+    cache_file = get_cache_file(line_length)
+    try:
+        if not CACHE_DIR.exists():
+            CACHE_DIR.mkdir(parents=True)
+        new_cache = {**cache, **{src.resolve(): get_cache_info(src) for src in sources}}
+        with cache_file.open("wb") as fobj:
+            pickle.dump(new_cache, fobj, protocol=pickle.HIGHEST_PROTOCOL)
+    except OSError:
+        pass
+
+

 if __name__ == "__main__":
     main()
 if __name__ == "__main__":
     main()