More detailed isort configuration explanation

[etc/vim.git] / black.py

diff --git a/black.py b/black.py
index dccda9108df4741aa49570f96ffaca049cf5fa90..fdefb746de14fc28002c820e2a19454db5873298 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


@@ -32,6 +31,7 @@ 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 +41,9 @@ 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__ = "18.4a4"

 DEFAULT_LINE_LENGTH = 88
 DEFAULT_LINE_LENGTH = 88

+

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


@@ -54,6 +55,10 @@ 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 +87,11 @@ 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 +109,12 @@ 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 +183,82 @@ 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:
         ctx.exit(0)
     if len(sources) == 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 +266,52 @@ 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 +326,7 @@ 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 +340,8 @@ 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()


@@ -382,7 +427,6 @@ 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,
 ]
 


@@ -522,9 +566,40 @@ MATH_OPERATORS = {
     token.DOUBLESTAR,
     token.DOUBLESLASH,
 }
     token.DOUBLESTAR,
     token.DOUBLESLASH,
 }

-VARARGS = {token.STAR, token.DOUBLESTAR}
+STARS = {token.STAR, token.DOUBLESTAR}
+VARARGS_PARENTS = {
+    syms.arglist,
+    syms.argument,  # double star in arglist
+    syms.trailer,  # single argument to call
+    syms.typedargslist,
+    syms.varargslist,  # lambdas
+}
+UNPACKING_PARENTS = {
+    syms.atom,  # single element of a list or set literal
+    syms.dictsetmaker,
+    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,
+}

 COMPREHENSION_PRIORITY = 20
 COMMA_PRIORITY = 10
 COMPREHENSION_PRIORITY = 20
 COMMA_PRIORITY = 10

+TERNARY_PRIORITY = 7

 LOGIC_PRIORITY = 5
 STRING_PRIORITY = 4
 COMPARATOR_PRIORITY = 3
 LOGIC_PRIORITY = 5
 STRING_PRIORITY = 4
 COMPARATOR_PRIORITY = 3


@@ -539,6 +614,8 @@ 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: bool = False
+    _lambda_arguments: bool = False

 
     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.


@@ -558,6 +635,8 @@ 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))


@@ -575,6 +654,8 @@ 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."""


@@ -583,11 +664,59 @@ 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 = 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.depth -= 1
+            self._for_loop_variable = False
+            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 = True
+            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 = False
+            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:


@@ -598,8 +727,6 @@ 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.


@@ -618,12 +745,12 @@ class Line:
         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)


@@ -753,7 +880,7 @@ 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
 


@@ -770,29 +897,6 @@ 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 (


@@ -837,6 +941,24 @@ 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:


@@ -959,8 +1081,14 @@ 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


@@ -968,9 +1096,6 @@ 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


@@ -979,13 +1104,6 @@ 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
 
 


@@ -1077,7 +1195,16 @@ 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(


@@ -1215,8 +1342,12 @@ 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 = "  "


@@ -1230,7 +1361,10 @@ 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


@@ -1240,7 +1374,13 @@ 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:


@@ -1255,27 +1395,17 @@ def whitespace(leaf: Leaf) -> str:  # noqa C901
                     # that, too.
                     return prevp.prefix
 
                     # that, too.
                     return prevp.prefix
 

-        elif prevp.type == token.DOUBLESTAR:
-            if (
-                prevp.parent
-                and prevp.parent.type in {
-                    syms.arglist,
-                    syms.argument,
-                    syms.dictsetmaker,
-                    syms.parameters,
-                    syms.typedargslist,
-                    syms.varargslist,
-                }
-            ):
+        elif prevp.type in STARS:
+            if is_vararg(prevp, within=VARARGS_PARENTS | UNPACKING_PARENTS):

                 return NO
 
         elif prevp.type == token.COLON:
             if prevp.parent and prevp.parent.type in {syms.subscript, syms.sliceop}:
                 return NO
 
         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

-            and prevp.parent.type in {syms.factor, syms.star_expr}
+            and prevp.parent.type == syms.factor

             and prevp.type in MATH_OPERATORS
         ):
             return NO
             and prevp.type in MATH_OPERATORS
         ):
             return NO


@@ -1296,17 +1426,11 @@ 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
 


@@ -1361,7 +1485,7 @@ 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 == token.EQUAL or prev.type == token.DOUBLESTAR:
+        elif prev.type in {token.EQUAL} | STARS:

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


@@ -1383,7 +1507,7 @@ 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"


@@ -1392,7 +1516,7 @@ 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:


@@ -1400,21 +1524,9 @@ 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}:


@@ -1474,6 +1586,14 @@ 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.
 


@@ -1496,11 +1616,7 @@ def is_split_before_delimiter(leaf: Leaf, previous: Leaf = None) -> int:
 
     Higher numbers are higher priority.
     """
 
     Higher numbers are higher priority.
     """

-    if (
-        leaf.type in VARARGS
-        and leaf.parent
-        and leaf.parent.type in {syms.argument, syms.typedargslist, syms.dictsetmaker}
-    ):
+    if is_vararg(leaf, within=VARARGS_PARENTS | UNPACKING_PARENTS):

         # * and ** might also be MATH_OPERATORS but in this case they are not.
         # Don't treat them as a delimiter.
         return 0
         # * and ** might also be MATH_OPERATORS but in this case they are not.
         # Don't treat them as a delimiter.
         return 0


@@ -1538,23 +1654,20 @@ 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.
 


@@ -1660,6 +1773,8 @@ def split_line(
     split_funcs: List[SplitFunc]
     if line.is_def:
         split_funcs = [left_hand_split]
     split_funcs: List[SplitFunc]
     if line.is_def:
         split_funcs = [left_hand_split]

+    elif line.is_import:
+        split_funcs = [explode_split]

     elif line.inside_brackets:
         split_funcs = [delimiter_split, standalone_comment_split, right_hand_split]
     else:
     elif line.inside_brackets:
         split_funcs = [delimiter_split, standalone_comment_split, right_hand_split]
     else:


@@ -1878,8 +1993,7 @@ def delimiter_split(line: Line, py36: bool = False) -> Iterator[Line]:
         lowest_depth = min(lowest_depth, leaf.bracket_depth)
         if (
             leaf.bracket_depth == lowest_depth
         lowest_depth = min(lowest_depth, leaf.bracket_depth)
         if (
             leaf.bracket_depth == lowest_depth

-            and leaf.type == token.STAR
-            or leaf.type == token.DOUBLESTAR
+            and is_vararg(leaf, within=VARARGS_PARENTS)

         ):
             trailing_comma_safe = trailing_comma_safe and py36
         leaf_priority = delimiters.get(id(leaf))
         ):
             trailing_comma_safe = trailing_comma_safe and py36
         leaf_priority = delimiters.get(id(leaf))


@@ -1927,6 +2041,26 @@ def standalone_comment_split(line: Line, py36: bool = False) -> Iterator[Line]:
         yield 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


@@ -1988,7 +2122,7 @@ 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


@@ -2090,7 +2224,35 @@ def is_one_tuple(node: LN) -> bool:
     )
 
 
     )
 
 

+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
+    generalizations (PEP 448).
+    """
+    if leaf.type not in STARS or not leaf.parent:
+        return False
+
+    p = leaf.parent
+    if p.type == syms.star_expr:
+        # Star expressions are also used as assignment targets in extended
+        # iterable unpacking (PEP 3132).  See what its parent is instead.
+        if not p.parent:
+            return False
+
+        p = p.parent
+
+    return p.type in within
+
+

 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
 


@@ -2144,7 +2306,7 @@ def is_python36(node: Node) -> bool:
             and n.children[-1].type == token.COMMA
         ):
             for ch in 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 False
                     return True
 
     return False


@@ -2180,16 +2342,20 @@ 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:


@@ -2388,5 +2554,67 @@ 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))
 
 

+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()