Improve test coverage a bit

[etc/vim.git] / black.py

diff --git a/black.py b/black.py
index de861564a44bc7d05f89007e58ce687653c69dd8..c2c118a4883c979d68439ef2636534d1c952a5a1 100644 (file)
--- a/black.py
+++ b/black.py
@@ -3,14 +3,30 @@
 import asyncio
 from asyncio.base_events import BaseEventLoop
 from concurrent.futures import Executor, ProcessPoolExecutor
 import asyncio
 from asyncio.base_events import BaseEventLoop
 from concurrent.futures import Executor, ProcessPoolExecutor

-from functools import partial
+from enum import Enum
+from functools import partial, wraps

 import keyword
 import keyword

+import logging
+from multiprocessing import Manager

 import os
 from pathlib import Path
 import tokenize
 import os
 from pathlib import Path
 import tokenize

+import signal

 import sys
 from typing import (
 import sys
 from typing import (

-    Dict, Generic, Iterable, Iterator, List, Optional, Set, Tuple, Type, TypeVar, Union
+    Any,
+    Callable,
+    Dict,
+    Generic,
+    Iterable,
+    Iterator,
+    List,
+    Optional,
+    Set,
+    Tuple,
+    Type,
+    TypeVar,
+    Union,

 )
 
 from attr import dataclass, Factory
 )
 
 from attr import dataclass, Factory


@@ -32,19 +48,22 @@ Depth = int
 NodeType = int
 LeafID = int
 Priority = int
 NodeType = int
 LeafID = int
 Priority = int

+Index = int

 LN = Union[Leaf, Node]
 LN = Union[Leaf, Node]

+SplitFunc = Callable[["Line", bool], Iterator["Line"]]

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

-err = partial(click.secho, fg='red', err=True)
+err = partial(click.secho, fg="red", err=True)

 
 
 class NothingChanged(UserWarning):
 
 
 class NothingChanged(UserWarning):

-    """Raised by `format_file()` when the reformatted code is the same as source."""
+    """Raised by :func:`format_file` when reformatted code is the same as source."""

 
 
 class CannotSplit(Exception):
     """A readable split that fits the allotted line length is impossible.
 
 
 
 class CannotSplit(Exception):
     """A readable split that fits the allotted line length is impossible.
 

-    Raised by `left_hand_split()`, `right_hand_split()`, and `delimiter_split()`.
+    Raised by :func:`left_hand_split`, :func:`right_hand_split`, and
+    :func:`delimiter_split`.

     """
 
 
     """
 
 


@@ -76,32 +95,43 @@ class FormatOff(FormatError):
     """Found a comment like `# fmt: off` in the file."""
 
 
     """Found a comment like `# fmt: off` in the file."""
 
 

+class WriteBack(Enum):
+    NO = 0
+    YES = 1
+    DIFF = 2
+
+

 @click.command()
 @click.option(
 @click.command()
 @click.option(

-    '-l',
-    '--line-length',
+    "-l",
+    "--line-length",

     type=int,
     default=DEFAULT_LINE_LENGTH,
     type=int,
     default=DEFAULT_LINE_LENGTH,

-    help='How many character per line to allow.',
+    help="How many character per line to allow.",

     show_default=True,
 )
 @click.option(
     show_default=True,
 )
 @click.option(

-    '--check',
+    "--check",

     is_flag=True,
     help=(
     is_flag=True,
     help=(

-        "Don't write back the files, just return the status.  Return code 0 "
+        "Don't write the files back, just return the status.  Return code 0 "

         "means nothing would change.  Return code 1 means some files would be "
         "reformatted.  Return code 123 means there was an internal error."
     ),
 )
 @click.option(
         "means nothing would change.  Return code 1 means some files would be "
         "reformatted.  Return code 123 means there was an internal error."
     ),
 )
 @click.option(

-    '--fast/--safe',
+    "--diff",
+    is_flag=True,
+    help="Don't write the files back, just output a diff for each file on stdout.",
+)
+@click.option(
+    "--fast/--safe",

     is_flag=True,
     is_flag=True,

-    help='If --fast given, skip temporary sanity checks. [default: --safe]',
+    help="If --fast given, skip temporary sanity checks. [default: --safe]",

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

-    'src',
+    "src",

     nargs=-1,
     type=click.Path(
         exists=True, file_okay=True, dir_okay=True, readable=True, allow_dash=True
     nargs=-1,
     type=click.Path(
         exists=True, file_okay=True, dir_okay=True, readable=True, allow_dash=True


@@ -109,7 +139,12 @@ class FormatOff(FormatError):
 )
 @click.pass_context
 def main(
 )
 @click.pass_context
 def main(

-    ctx: click.Context, line_length: int, check: bool, fast: bool, src: List[str]
+    ctx: click.Context,
+    line_length: int,
+    check: bool,
+    diff: bool,
+    fast: bool,
+    src: List[str],

 ) -> None:
     """The uncompromising code formatter."""
     sources: List[Path] = []
 ) -> None:
     """The uncompromising code formatter."""
     sources: List[Path] = []


@@ -120,23 +155,34 @@ def main(
         elif p.is_file():
             # if a file was explicitly given, we don't care about its extension
             sources.append(p)
         elif p.is_file():
             # if a file was explicitly given, we don't care about its extension
             sources.append(p)

-        elif s == '-':
-            sources.append(Path('-'))
+        elif s == "-":
+            sources.append(Path("-"))

         else:
         else:

-            err(f'invalid path: {s}')
+            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:
+        write_back = WriteBack.NO
+    elif diff:
+        write_back = WriteBack.DIFF
+    else:
+        write_back = WriteBack.YES

     if len(sources) == 0:
         ctx.exit(0)
     elif len(sources) == 1:
         p = sources[0]
         report = Report(check=check)
         try:
     if len(sources) == 0:
         ctx.exit(0)
     elif len(sources) == 1:
         p = sources[0]
         report = Report(check=check)
         try:

-            if not p.is_file() and str(p) == '-':
+            if not p.is_file() and str(p) == "-":

                 changed = format_stdin_to_stdout(
                 changed = format_stdin_to_stdout(

-                    line_length=line_length, fast=fast, write_back=not check
+                    line_length=line_length, fast=fast, write_back=write_back

                 )
             else:
                 changed = format_file_in_place(
                 )
             else:
                 changed = format_file_in_place(

-                    p, line_length=line_length, fast=fast, write_back=not check
+                    p, line_length=line_length, fast=fast, write_back=write_back

                 )
             report.done(p, changed)
         except Exception as exc:
                 )
             report.done(p, changed)
         except Exception as exc:


@@ -149,18 +195,18 @@ def main(
         try:
             return_code = loop.run_until_complete(
                 schedule_formatting(
         try:
             return_code = loop.run_until_complete(
                 schedule_formatting(

-                    sources, line_length, not check, fast, loop, executor
+                    sources, line_length, write_back, fast, loop, executor

                 )
             )
         finally:
                 )
             )
         finally:

-            loop.close()
+            shutdown(loop)

             ctx.exit(return_code)
 
 
 async def schedule_formatting(
     sources: List[Path],
     line_length: int,
             ctx.exit(return_code)
 
 
 async def schedule_formatting(
     sources: List[Path],
     line_length: int,

-    write_back: bool,
+    write_back: WriteBack,

     fast: bool,
     loop: BaseEventLoop,
     executor: Executor,
     fast: bool,
     loop: BaseEventLoop,
     executor: Executor,


@@ -172,33 +218,49 @@ 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(
     tasks = {
         src: loop.run_in_executor(

-            executor, format_file_in_place, src, line_length, fast, write_back
+            executor, format_file_in_place, src, line_length, fast, write_back, lock

         )
         for src in sources
     }
         )
         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())
     cancelled = []
     await asyncio.wait(tasks.values())
     cancelled = []

-    report = Report()
+    report = Report(check=not write_back)

     for src, task in tasks.items():
         if not task.done():
     for src, task in tasks.items():
         if not task.done():

-            report.failed(src, 'timed out, cancelling')
+            report.failed(src, "timed out, cancelling")

             task.cancel()
             cancelled.append(task)
             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())
     if cancelled:
         elif task.exception():
             report.failed(src, str(task.exception()))
         else:
             report.done(src, task.result())
     if cancelled:

-        await asyncio.wait(cancelled, timeout=2)
-    out('All done! ✨ 🍰 ✨')
+        await asyncio.gather(*cancelled, loop=loop, return_exceptions=True)
+    else:
+        out("All done! ✨ 🍰 ✨")

     click.echo(str(report))
     return report.return_code
 
 
 def format_file_in_place(
     click.echo(str(report))
     return report.return_code
 
 
 def format_file_in_place(

-    src: Path, line_length: int, fast: bool, write_back: bool = False
+    src: Path,
+    line_length: int,
+    fast: bool,
+    write_back: WriteBack = WriteBack.NO,
+    lock: Any = None,  # multiprocessing.Manager().Lock() is some crazy proxy

 ) -> bool:
     """Format file under `src` path. Return True if changed.
 
 ) -> bool:
     """Format file under `src` path. Return True if changed.
 


@@ -208,49 +270,65 @@ def format_file_in_place(
     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:

-        contents = format_file_contents(
+        dst_contents = format_file_contents(

             src_contents, line_length=line_length, fast=fast
         )
     except NothingChanged:
         return False
 
             src_contents, line_length=line_length, fast=fast
         )
     except NothingChanged:
         return False
 

-    if write_back:
+    if write_back == write_back.YES:

         with open(src, "w", encoding=src_buffer.encoding) as f:
         with open(src, "w", encoding=src_buffer.encoding) as f:

-            f.write(contents)
+            f.write(dst_contents)
+    elif write_back == write_back.DIFF:
+        src_name = f"{src.name}  (original)"
+        dst_name = f"{src.name}  (formatted)"
+        diff_contents = diff(src_contents, dst_contents, src_name, dst_name)
+        if lock:
+            lock.acquire()
+        try:
+            sys.stdout.write(diff_contents)
+        finally:
+            if lock:
+                lock.release()

     return True
 
 
 def format_stdin_to_stdout(
     return True
 
 
 def format_stdin_to_stdout(

-    line_length: int, fast: bool, write_back: bool = False
+    line_length: int, fast: bool, write_back: WriteBack = WriteBack.NO

 ) -> bool:
     """Format file on stdin. Return True if changed.
 
     If `write_back` is True, write reformatted code back to stdout.
     `line_length` and `fast` arguments are passed to :func:`format_file_contents`.
     """
 ) -> bool:
     """Format file on stdin. Return True if changed.
 
     If `write_back` is True, write reformatted code back to stdout.
     `line_length` and `fast` arguments are passed to :func:`format_file_contents`.
     """

-    contents = sys.stdin.read()
+    src = sys.stdin.read()

     try:
     try:

-        contents = format_file_contents(contents, line_length=line_length, fast=fast)
+        dst = format_file_contents(src, line_length=line_length, fast=fast)

         return True
 
     except NothingChanged:
         return True
 
     except NothingChanged:

+        dst = src

         return False
 
     finally:
         return False
 
     finally:

-        if write_back:
-            sys.stdout.write(contents)
+        if write_back == WriteBack.YES:
+            sys.stdout.write(dst)
+        elif write_back == WriteBack.DIFF:
+            src_name = "<stdin>  (original)"
+            dst_name = "<stdin>  (formatted)"
+            sys.stdout.write(diff(src, dst, src_name, dst_name))

 
 
 def format_file_contents(
     src_contents: str, line_length: int, fast: bool
 ) -> FileContent:
 
 
 def format_file_contents(
     src_contents: str, line_length: int, fast: bool
 ) -> FileContent:

-    """Reformats a file and returns its contents and encoding.
+    """Reformat contents a file and return new contents.

 
     If `fast` is False, additionally confirm that the reformatted code is
     valid by calling :func:`assert_equivalent` and :func:`assert_stable` on it.
     `line_length` is passed to :func:`format_str`.
     """
 
     If `fast` is False, additionally confirm that the reformatted code is
     valid by calling :func:`assert_equivalent` and :func:`assert_stable` on it.
     `line_length` is passed to :func:`format_str`.
     """

-    if src_contents.strip() == '':
+    if src_contents.strip() == "":

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


@@ -264,7 +342,7 @@ def format_file_contents(
 
 
 def format_str(src_contents: str, line_length: int) -> FileContent:
 
 
 def format_str(src_contents: str, line_length: int) -> FileContent:

-    """Reformats a string and returns new contents.
+    """Reformat a string and return new contents.

 
     `line_length` determines how many characters per line are allowed.
     """
 
     `line_length` determines how many characters per line are allowed.
     """


@@ -297,8 +375,8 @@ GRAMMARS = [
 def lib2to3_parse(src_txt: str) -> Node:
     """Given a string with source, return the lib2to3 Node."""
     grammar = pygram.python_grammar_no_print_statement
 def lib2to3_parse(src_txt: str) -> Node:
     """Given a string with source, return the lib2to3 Node."""
     grammar = pygram.python_grammar_no_print_statement

-    if src_txt[-1] != '\n':
-        nl = '\r\n' if '\r\n' in src_txt[:1024] else '\n'
+    if src_txt[-1] != "\n":
+        nl = "\r\n" if "\r\n" in src_txt[:1024] else "\n"

         src_txt += nl
     for grammar in GRAMMARS:
         drv = driver.Driver(grammar, pytree.convert)
         src_txt += nl
     for grammar in GRAMMARS:
         drv = driver.Driver(grammar, pytree.convert)


@@ -328,7 +406,7 @@ def lib2to3_unparse(node: Node) -> str:
     return code
 
 
     return code
 
 

-T = TypeVar('T')
+T = TypeVar("T")

 
 
 class Visitor(Generic[T]):
 
 
 class Visitor(Generic[T]):


@@ -348,7 +426,7 @@ class Visitor(Generic[T]):
             name = token.tok_name[node.type]
         else:
             name = type_repr(node.type)
             name = token.tok_name[node.type]
         else:
             name = type_repr(node.type)

-        yield from getattr(self, f'visit_{name}', self.visit_default)(node)
+        yield from getattr(self, f"visit_{name}", self.visit_default)(node)

 
     def visit_default(self, node: LN) -> Iterator[T]:
         """Default `visit_*()` implementation. Recurses to children of `node`."""
 
     def visit_default(self, node: LN) -> Iterator[T]:
         """Default `visit_*()` implementation. Recurses to children of `node`."""


@@ -362,28 +440,28 @@ class DebugVisitor(Visitor[T]):
     tree_depth: int = 0
 
     def visit_default(self, node: LN) -> Iterator[T]:
     tree_depth: int = 0
 
     def visit_default(self, node: LN) -> Iterator[T]:

-        indent = ' ' * (2 * self.tree_depth)
+        indent = " " * (2 * self.tree_depth)

         if isinstance(node, Node):
             _type = type_repr(node.type)
         if isinstance(node, Node):
             _type = type_repr(node.type)

-            out(f'{indent}{_type}', fg='yellow')
+            out(f"{indent}{_type}", fg="yellow")

             self.tree_depth += 1
             for child in node.children:
                 yield from self.visit(child)
 
             self.tree_depth -= 1
             self.tree_depth += 1
             for child in node.children:
                 yield from self.visit(child)
 
             self.tree_depth -= 1

-            out(f'{indent}/{_type}', fg='yellow', bold=False)
+            out(f"{indent}/{_type}", fg="yellow", bold=False)

         else:
             _type = token.tok_name.get(node.type, str(node.type))
         else:
             _type = token.tok_name.get(node.type, str(node.type))

-            out(f'{indent}{_type}', fg='blue', nl=False)
+            out(f"{indent}{_type}", fg="blue", nl=False)

             if node.prefix:
                 # We don't have to handle prefixes for `Node` objects since
                 # that delegates to the first child anyway.
             if node.prefix:
                 # We don't have to handle prefixes for `Node` objects since
                 # that delegates to the first child anyway.

-                out(f' {node.prefix!r}', fg='green', bold=False, nl=False)
-            out(f' {node.value!r}', fg='blue', bold=False)
+                out(f" {node.prefix!r}", fg="green", bold=False, nl=False)
+            out(f" {node.value!r}", fg="blue", bold=False)

 
     @classmethod
     def show(cls, code: str) -> None:
 
     @classmethod
     def show(cls, code: str) -> None:

-        """Pretty-prints a given string of `code`.
+        """Pretty-print the lib2to3 AST of a given string of `code`.

 
         Convenience method for debugging.
         """
 
         Convenience method for debugging.
         """


@@ -393,7 +471,7 @@ class DebugVisitor(Visitor[T]):
 
 KEYWORDS = set(keyword.kwlist)
 WHITESPACE = {token.DEDENT, token.INDENT, token.NEWLINE}
 
 KEYWORDS = set(keyword.kwlist)
 WHITESPACE = {token.DEDENT, token.INDENT, token.NEWLINE}

-FLOW_CONTROL = {'return', 'raise', 'break', 'continue'}
+FLOW_CONTROL = {"return", "raise", "break", "continue"}

 STATEMENT = {
     syms.if_stmt,
     syms.while_stmt,
 STATEMENT = {
     syms.if_stmt,
     syms.while_stmt,


@@ -405,7 +483,7 @@ STATEMENT = {
     syms.classdef,
 }
 STANDALONE_COMMENT = 153
     syms.classdef,
 }
 STANDALONE_COMMENT = 153

-LOGIC_OPERATORS = {'and', 'or'}
+LOGIC_OPERATORS = {"and", "or"}

 COMPARATORS = {
     token.LESS,
     token.GREATER,
 COMPARATORS = {
     token.LESS,
     token.GREATER,


@@ -429,6 +507,7 @@ MATH_OPERATORS = {
     token.DOUBLESTAR,
     token.DOUBLESLASH,
 }
     token.DOUBLESTAR,
     token.DOUBLESLASH,
 }

+VARARGS = {token.STAR, token.DOUBLESTAR}

 COMPREHENSION_PRIORITY = 20
 COMMA_PRIORITY = 10
 LOGIC_PRIORITY = 5
 COMPREHENSION_PRIORITY = 20
 COMMA_PRIORITY = 10
 LOGIC_PRIORITY = 5


@@ -447,7 +526,7 @@ class BracketTracker:
     previous: Optional[Leaf] = None
 
     def mark(self, leaf: Leaf) -> None:
     previous: Optional[Leaf] = None
 
     def mark(self, leaf: Leaf) -> None:

-        """Marks `leaf` with bracket-related metadata. Keeps track of delimiters.
+        """Mark `leaf` with bracket-related metadata. Keep track of delimiters.

 
         All leaves receive an int `bracket_depth` field that stores how deep
         within brackets a given leaf is. 0 means there are no enclosing brackets
 
         All leaves receive an int `bracket_depth` field that stores how deep
         within brackets a given leaf is. 0 means there are no enclosing brackets


@@ -470,43 +549,23 @@ class BracketTracker:
             leaf.opening_bracket = opening_bracket
         leaf.bracket_depth = self.depth
         if self.depth == 0:
             leaf.opening_bracket = opening_bracket
         leaf.bracket_depth = self.depth
         if self.depth == 0:

-            delim = is_delimiter(leaf)
-            if delim:
-                self.delimiters[id(leaf)] = delim
-            elif self.previous is not None:
-                if leaf.type == token.STRING and self.previous.type == token.STRING:
-                    self.delimiters[id(self.previous)] = STRING_PRIORITY
-                elif (
-                    leaf.type == token.NAME
-                    and leaf.value == 'for'
-                    and leaf.parent
-                    and leaf.parent.type in {syms.comp_for, syms.old_comp_for}
-                ):
-                    self.delimiters[id(self.previous)] = COMPREHENSION_PRIORITY
-                elif (
-                    leaf.type == token.NAME
-                    and leaf.value == 'if'
-                    and leaf.parent
-                    and leaf.parent.type in {syms.comp_if, syms.old_comp_if}
-                ):
-                    self.delimiters[id(self.previous)] = COMPREHENSION_PRIORITY
-                elif (
-                    leaf.type == token.NAME
-                    and leaf.value in LOGIC_OPERATORS
-                    and leaf.parent
-                ):
-                    self.delimiters[id(self.previous)] = LOGIC_PRIORITY
+            after_delim = is_split_after_delimiter(leaf, self.previous)
+            before_delim = is_split_before_delimiter(leaf, self.previous)
+            if after_delim > before_delim:
+                self.delimiters[id(leaf)] = after_delim
+            elif before_delim > after_delim and self.previous is not None:
+                self.delimiters[id(self.previous)] = before_delim

         if leaf.type in OPENING_BRACKETS:
             self.bracket_match[self.depth, BRACKET[leaf.type]] = leaf
             self.depth += 1
         self.previous = leaf
 
     def any_open_brackets(self) -> bool:
         if leaf.type in OPENING_BRACKETS:
             self.bracket_match[self.depth, BRACKET[leaf.type]] = leaf
             self.depth += 1
         self.previous = leaf
 
     def any_open_brackets(self) -> bool:

-        """Returns True if there is an yet unmatched open bracket on the line."""
+        """Return True if there is an yet unmatched open bracket on the line."""

         return bool(self.bracket_match)
 
     def max_delimiter_priority(self, exclude: Iterable[LeafID] = ()) -> int:
         return bool(self.bracket_match)
 
     def max_delimiter_priority(self, exclude: Iterable[LeafID] = ()) -> int:

-        """Returns the highest priority of a delimiter found on the line.
+        """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_delimiter()` returns.
         """


@@ -519,7 +578,7 @@ class Line:
 
     depth: int = 0
     leaves: List[Leaf] = Factory(list)
 
     depth: int = 0
     leaves: List[Leaf] = Factory(list)

-    comments: Dict[LeafID, Leaf] = Factory(dict)
+    comments: List[Tuple[Index, Leaf]] = Factory(list)

     bracket_tracker: BracketTracker = Factory(BracketTracker)
     inside_brackets: bool = False
     has_for: bool = False
     bracket_tracker: BracketTracker = Factory(BracketTracker)
     inside_brackets: bool = False
     has_for: bool = False


@@ -548,16 +607,31 @@ class Line:
             self.bracket_tracker.mark(leaf)
             self.maybe_remove_trailing_comma(leaf)
             self.maybe_increment_for_loop_variable(leaf)
             self.bracket_tracker.mark(leaf)
             self.maybe_remove_trailing_comma(leaf)
             self.maybe_increment_for_loop_variable(leaf)

-            if self.maybe_adapt_standalone_comment(leaf):
-                return

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

+    def append_safe(self, leaf: Leaf, preformatted: bool = False) -> None:
+        """Like :func:`append()` but disallow invalid standalone comment structure.
+
+        Raises ValueError when any `leaf` is appended after a standalone comment
+        or when a standalone comment is not the first leaf on the line.
+        """
+        if self.bracket_tracker.depth == 0:
+            if self.is_comment:
+                raise ValueError("cannot append to standalone comments")
+
+            if self.leaves and leaf.type == STANDALONE_COMMENT:
+                raise ValueError(
+                    "cannot append standalone comments to a populated line"
+                )
+
+        self.append(leaf, preformatted=preformatted)
+

     @property
     def is_comment(self) -> bool:
         """Is this line a standalone comment?"""
     @property
     def is_comment(self) -> bool:
         """Is this line a standalone comment?"""

-        return bool(self) and self.leaves[0].type == STANDALONE_COMMENT
+        return len(self.leaves) == 1 and self.leaves[0].type == STANDALONE_COMMENT

 
     @property
     def is_decorator(self) -> bool:
 
     @property
     def is_decorator(self) -> bool:


@@ -571,11 +645,11 @@ class Line:
 
     @property
     def is_class(self) -> bool:
 
     @property
     def is_class(self) -> bool:

-        """Is this a class definition?"""
+        """Is this line a class definition?"""

         return (
             bool(self)
             and self.leaves[0].type == token.NAME
         return (
             bool(self)
             and self.leaves[0].type == token.NAME

-            and self.leaves[0].value == 'class'
+            and self.leaves[0].value == "class"

         )
 
     @property
         )
 
     @property


@@ -591,18 +665,18 @@ class Line:
         except IndexError:
             second_leaf = None
         return (
         except IndexError:
             second_leaf = None
         return (

-            (first_leaf.type == token.NAME and first_leaf.value == 'def')
+            (first_leaf.type == token.NAME and first_leaf.value == "def")

             or (
                 first_leaf.type == token.ASYNC
                 and second_leaf is not None
                 and second_leaf.type == token.NAME
             or (
                 first_leaf.type == token.ASYNC
                 and second_leaf is not None
                 and second_leaf.type == token.NAME

-                and second_leaf.value == 'def'
+                and second_leaf.value == "def"

             )
         )
 
     @property
     def is_flow_control(self) -> bool:
             )
         )
 
     @property
     def is_flow_control(self) -> bool:

-        """Is this a flow control statement?
+        """Is this line a flow control statement?

 
         Those are `return`, `raise`, `break`, and `continue`.
         """
 
         Those are `return`, `raise`, `break`, and `continue`.
         """


@@ -614,13 +688,22 @@ class Line:
 
     @property
     def is_yield(self) -> bool:
 
     @property
     def is_yield(self) -> bool:

-        """Is this a yield statement?"""
+        """Is this line a yield statement?"""

         return (
             bool(self)
             and self.leaves[0].type == token.NAME
         return (
             bool(self)
             and self.leaves[0].type == token.NAME

-            and self.leaves[0].value == 'yield'
+            and self.leaves[0].value == "yield"

         )
 
         )
 

+    @property
+    def contains_standalone_comments(self) -> bool:
+        """If so, needs to be split before emitting."""
+        for leaf in self.leaves:
+            if leaf.type == STANDALONE_COMMENT:
+                return True
+
+        return False
+

     def maybe_remove_trailing_comma(self, closing: Leaf) -> bool:
         """Remove trailing comma if there is one and it's safe."""
         if not (
     def maybe_remove_trailing_comma(self, closing: Leaf) -> bool:
         """Remove trailing comma if there is one and it's safe."""
         if not (


@@ -631,13 +714,13 @@ class Line:
             return False
 
         if closing.type == token.RBRACE:
             return False
 
         if closing.type == token.RBRACE:

-            self.leaves.pop()
+            self.remove_trailing_comma()

             return True
 
         if closing.type == token.RSQB:
             comma = self.leaves[-1]
             if comma.parent and comma.parent.type == syms.listmaker:
             return True
 
         if closing.type == token.RSQB:
             comma = self.leaves[-1]
             if comma.parent and comma.parent.type == syms.listmaker:

-                self.leaves.pop()
+                self.remove_trailing_comma()

                 return True
 
         # For parens let's check if it's safe to remove the comma.  If the
                 return True
 
         # For parens let's check if it's safe to remove the comma.  If the


@@ -665,7 +748,7 @@ class Line:
                     break
 
         if commas > 1:
                     break
 
         if commas > 1:

-            self.leaves.pop()
+            self.remove_trailing_comma()

             return True
 
         return False
             return True
 
         return False


@@ -673,10 +756,10 @@ class Line:
     def maybe_increment_for_loop_variable(self, leaf: Leaf) -> bool:
         """In a for loop, or comprehension, the variables are often unpacks.
 
     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, we will increase
-        the depth of tokens between `for` and `in`.
+        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':
+        if leaf.type == token.NAME and leaf.value == "for":

             self.has_for = True
             self.bracket_tracker.depth += 1
             self._for_loop_variable = True
             self.has_for = True
             self.bracket_tracker.depth += 1
             self._for_loop_variable = True


@@ -686,77 +769,74 @@ class Line:
 
     def maybe_decrement_after_for_loop_variable(self, leaf: Leaf) -> bool:
         """See `maybe_increment_for_loop_variable` above for explanation."""
 
     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':
+        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
 
             self.bracket_tracker.depth -= 1
             self._for_loop_variable = False
             return True
 
         return False
 

-    def maybe_adapt_standalone_comment(self, comment: Leaf) -> bool:
-        """Hack a standalone comment to act as a trailing comment for line splitting.
-
-        If this line has brackets and a standalone `comment`, we need to adapt
-        it to be able to still reformat the line.
-
-        This is not perfect, the line to which the standalone comment gets
-        appended will appear "too long" when splitting.
-        """
-        if not (
+    def append_comment(self, comment: Leaf) -> bool:
+        """Add an inline or standalone comment to the line."""
+        if (

             comment.type == STANDALONE_COMMENT
             and self.bracket_tracker.any_open_brackets()
         ):
             comment.type == STANDALONE_COMMENT
             and self.bracket_tracker.any_open_brackets()
         ):

+            comment.prefix = ""

             return False
 
             return False
 

-        comment.type = token.COMMENT
-        comment.prefix = '\n' + '    ' * (self.depth + 1)
-        return self.append_comment(comment)
-
-    def append_comment(self, comment: Leaf) -> bool:
-        """Add an inline comment to the line."""

         if comment.type != token.COMMENT:
             return False
 
         if comment.type != token.COMMENT:
             return False
 

-        try:
-            after = id(self.last_non_delimiter())
-        except LookupError:
+        after = len(self.leaves) - 1
+        if after == -1:

             comment.type = STANDALONE_COMMENT
             comment.type = STANDALONE_COMMENT

-            comment.prefix = ''
+            comment.prefix = ""

             return False
 
         else:
             return False
 
         else:

-            if after in self.comments:
-                self.comments[after].value += str(comment)
-            else:
-                self.comments[after] = comment
+            self.comments.append((after, comment))

             return True
 
             return True
 

-    def last_non_delimiter(self) -> Leaf:
-        """Returns the last non-delimiter on the line. Raises LookupError otherwise."""
-        for i in range(len(self.leaves)):
-            last = self.leaves[-i - 1]
-            if not is_delimiter(last):
-                return last
+    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
+
+        else:
+            return

 
 

-        raise LookupError("No non-delimiters found")
+        for index, comment_after in self.comments:
+            if _leaf_index == index:
+                yield comment_after
+
+    def remove_trailing_comma(self) -> None:
+        """Remove the trailing comma and moves the comments attached to it."""
+        comma_index = len(self.leaves) - 1
+        for i in range(len(self.comments)):
+            comment_index, comment = self.comments[i]
+            if comment_index == comma_index:
+                self.comments[i] = (comma_index - 1, comment)
+        self.leaves.pop()

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

-            return '\n'
+            return "\n"

 
 

-        indent = '    ' * self.depth
+        indent = "    " * self.depth

         leaves = iter(self.leaves)
         first = next(leaves)
         leaves = iter(self.leaves)
         first = next(leaves)

-        res = f'{first.prefix}{indent}{first.value}'
+        res = f"{first.prefix}{indent}{first.value}"

         for leaf in leaves:
             res += str(leaf)
         for leaf in leaves:
             res += str(leaf)

-        for comment in self.comments.values():
+        for _, comment in self.comments:

             res += str(comment)
             res += str(comment)

-        return res + '\n'
+        return res + "\n"

 
     def __bool__(self) -> bool:
 
     def __bool__(self) -> bool:

-        """Returns True if the line has leaves or comments."""
+        """Return True if the line has leaves or comments."""

         return bool(self.leaves or self.comments)
 
 
         return bool(self.leaves or self.comments)
 
 


@@ -783,8 +863,21 @@ class UnformattedLines(Line):
         elif leaf.type == token.DEDENT:
             self.depth -= 1
 
         elif leaf.type == token.DEDENT:
             self.depth -= 1
 

+    def __str__(self) -> str:
+        """Render unformatted lines from leaves which were added with `append()`.
+
+        `depth` is not used for indentation in this case.
+        """
+        if not self:
+            return "\n"
+
+        res = ""
+        for leaf in self.leaves:
+            res += str(leaf)
+        return res
+

     def append_comment(self, comment: Leaf) -> bool:
     def append_comment(self, comment: Leaf) -> bool:

-        """Not implemented in this class."""
+        """Not implemented in this class. Raises `NotImplementedError`."""

         raise NotImplementedError("Unformatted lines don't store comments separately.")
 
     def maybe_remove_trailing_comma(self, closing: Leaf) -> bool:
         raise NotImplementedError("Unformatted lines don't store comments separately.")
 
     def maybe_remove_trailing_comma(self, closing: Leaf) -> bool:


@@ -795,23 +888,6 @@ class UnformattedLines(Line):
         """Does nothing and returns False."""
         return False
 
         """Does nothing and returns False."""
         return False
 

-    def maybe_adapt_standalone_comment(self, comment: Leaf) -> bool:
-        """Does nothing and returns False."""
-        return False
-
-    def __str__(self) -> str:
-        """Renders unformatted lines from leaves which were added with `append()`.
-
-        `depth` is not used for indentation in this case.
-        """
-        if not self:
-            return '\n'
-
-        res = ''
-        for leaf in self.leaves:
-            res += str(leaf)
-        return res
-

 
 @dataclass
 class EmptyLineTracker:
 
 @dataclass
 class EmptyLineTracker:


@@ -827,11 +903,11 @@ class EmptyLineTracker:
     previous_defs: List[int] = Factory(list)
 
     def maybe_empty_lines(self, current_line: Line) -> Tuple[int, int]:
     previous_defs: List[int] = Factory(list)
 
     def maybe_empty_lines(self, current_line: Line) -> Tuple[int, int]:

-        """Returns the number of extra empty lines before and after the `current_line`.
+        """Return the number of extra empty lines before and after the `current_line`.

 
 

-        This is for separating `def`, `async def` and `class` with extra empty lines
-        (two on module-level), as well as providing an extra empty line after flow
-        control keywords to make them more prominent.
+        This is for separating `def`, `async def` and `class` with extra empty
+        lines (two on module-level), as well as providing an extra empty line
+        after flow control keywords to make them more prominent.

         """
         if isinstance(current_line, UnformattedLines):
             return 0, 0
         """
         if isinstance(current_line, UnformattedLines):
             return 0, 0


@@ -849,9 +925,9 @@ class EmptyLineTracker:
         if current_line.leaves:
             # Consume the first leaf's extra newlines.
             first_leaf = current_line.leaves[0]
         if current_line.leaves:
             # Consume the first leaf's extra newlines.
             first_leaf = current_line.leaves[0]

-            before = first_leaf.prefix.count('\n')
+            before = first_leaf.prefix.count("\n")

             before = min(before, max_allowed)
             before = min(before, max_allowed)

-            first_leaf.prefix = ''
+            first_leaf.prefix = ""

         else:
             before = 0
         depth = current_line.depth
         else:
             before = 0
         depth = current_line.depth


@@ -925,7 +1001,10 @@ class LineGenerator(Visitor[Line]):
         yield complete_line
 
     def visit(self, node: LN) -> Iterator[Line]:
         yield complete_line
 
     def visit(self, node: LN) -> Iterator[Line]:

-        """Main method to start the visit process. Yields :class:`Line` objects."""
+        """Main method to visit `node` and its children.
+
+        Yields :class:`Line` objects.
+        """

         if isinstance(self.current_line, UnformattedLines):
             # File contained `# fmt: off`
             yield from self.visit_unformatted(node)
         if isinstance(self.current_line, UnformattedLines):
             # File contained `# fmt: off`
             yield from self.visit_unformatted(node)


@@ -967,23 +1046,25 @@ class LineGenerator(Visitor[Line]):
 
             else:
                 normalize_prefix(node, inside_brackets=any_open_brackets)
 
             else:
                 normalize_prefix(node, inside_brackets=any_open_brackets)

+                if node.type == token.STRING:
+                    normalize_string_quotes(node)

                 if node.type not in WHITESPACE:
                     self.current_line.append(node)
         yield from super().visit_default(node)
 
     def visit_INDENT(self, node: Node) -> Iterator[Line]:
                 if node.type not in WHITESPACE:
                     self.current_line.append(node)
         yield from super().visit_default(node)
 
     def visit_INDENT(self, node: Node) -> Iterator[Line]:

-        """Increases indentation level, maybe yields a line."""
+        """Increase indentation level, maybe yield a line."""

         # In blib2to3 INDENT never holds comments.
         yield from self.line(+1)
         yield from self.visit_default(node)
 
     def visit_DEDENT(self, node: Node) -> Iterator[Line]:
         # In blib2to3 INDENT never holds comments.
         yield from self.line(+1)
         yield from self.visit_default(node)
 
     def visit_DEDENT(self, node: Node) -> Iterator[Line]:

-        """Decreases indentation level, maybe yields a line."""
+        """Decrease indentation level, maybe yield a line."""

         # DEDENT has no value. Additionally, in blib2to3 it never holds comments.
         yield from self.line(-1)
 
     def visit_stmt(self, node: Node, keywords: Set[str]) -> Iterator[Line]:
         # DEDENT has no value. Additionally, in blib2to3 it never holds comments.
         yield from self.line(-1)
 
     def visit_stmt(self, node: Node, keywords: Set[str]) -> Iterator[Line]:

-        """Visits a statement.
+        """Visit a statement.

 
         This implementation is shared for `if`, `while`, `for`, `try`, `except`,
         `def`, `with`, and `class`.
 
         This implementation is shared for `if`, `while`, `for`, `try`, `except`,
         `def`, `with`, and `class`.


@@ -998,7 +1079,7 @@ class LineGenerator(Visitor[Line]):
             yield from self.visit(child)
 
     def visit_simple_stmt(self, node: Node) -> Iterator[Line]:
             yield from self.visit(child)
 
     def visit_simple_stmt(self, node: Node) -> Iterator[Line]:

-        """Visits a statement without nested statements."""
+        """Visit a statement without nested statements."""

         is_suite_like = node.parent and node.parent.type in STATEMENT
         if is_suite_like:
             yield from self.line(+1)
         is_suite_like = node.parent and node.parent.type in STATEMENT
         if is_suite_like:
             yield from self.line(+1)


@@ -1010,7 +1091,7 @@ class LineGenerator(Visitor[Line]):
             yield from self.visit_default(node)
 
     def visit_async_stmt(self, node: Node) -> Iterator[Line]:
             yield from self.visit_default(node)
 
     def visit_async_stmt(self, node: Node) -> Iterator[Line]:

-        """Visits `async def`, `async for`, `async with`."""
+        """Visit `async def`, `async for`, `async with`."""

         yield from self.line()
 
         children = iter(node.children)
         yield from self.line()
 
         children = iter(node.children)


@@ -1025,23 +1106,17 @@ class LineGenerator(Visitor[Line]):
             yield from self.visit(child)
 
     def visit_decorators(self, node: Node) -> Iterator[Line]:
             yield from self.visit(child)
 
     def visit_decorators(self, node: Node) -> Iterator[Line]:

-        """Visits decorators."""
+        """Visit decorators."""

         for child in node.children:
             yield from self.line()
             yield from self.visit(child)
 
     def visit_SEMI(self, leaf: Leaf) -> Iterator[Line]:
         for child in node.children:
             yield from self.line()
             yield from self.visit(child)
 
     def visit_SEMI(self, leaf: Leaf) -> Iterator[Line]:

-        """Semicolons are always removed.
-
-        Statements between them are put on separate lines.
-        """
+        """Remove a semicolon and put the other statement on a separate line."""

         yield from self.line()
 
     def visit_ENDMARKER(self, leaf: Leaf) -> Iterator[Line]:
         yield from self.line()
 
     def visit_ENDMARKER(self, leaf: Leaf) -> Iterator[Line]:

-        """End of file.
-
-        Process outstanding comments and end with a newline.
-        """
+        """End of file. Process outstanding comments and end with a newline."""

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


@@ -1062,14 +1137,14 @@ class LineGenerator(Visitor[Line]):
     def __attrs_post_init__(self) -> None:
         """You are in a twisty little maze of passages."""
         v = self.visit_stmt
     def __attrs_post_init__(self) -> None:
         """You are in a twisty little maze of passages."""
         v = self.visit_stmt

-        self.visit_if_stmt = partial(v, keywords={'if', 'else', 'elif'})
-        self.visit_while_stmt = partial(v, keywords={'while', 'else'})
-        self.visit_for_stmt = partial(v, keywords={'for', 'else'})
-        self.visit_try_stmt = partial(v, keywords={'try', 'except', 'else', 'finally'})
-        self.visit_except_clause = partial(v, keywords={'except'})
-        self.visit_funcdef = partial(v, keywords={'def'})
-        self.visit_with_stmt = partial(v, keywords={'with'})
-        self.visit_classdef = partial(v, keywords={'class'})
+        self.visit_if_stmt = partial(v, keywords={"if", "else", "elif"})
+        self.visit_while_stmt = partial(v, keywords={"while", "else"})
+        self.visit_for_stmt = partial(v, keywords={"for", "else"})
+        self.visit_try_stmt = partial(v, keywords={"try", "except", "else", "finally"})
+        self.visit_except_clause = partial(v, keywords={"except"})
+        self.visit_funcdef = partial(v, keywords={"def"})
+        self.visit_with_stmt = partial(v, keywords={"with"})
+        self.visit_classdef = partial(v, keywords={"class"})

         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
 


@@ -1083,9 +1158,9 @@ 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) -> str:  # noqa C901
     """Return whitespace prefix if needed for the given `leaf`."""

-    NO = ''
-    SPACE = ' '
-    DOUBLESPACE = '  '
+    NO = ""
+    SPACE = " "
+    DOUBLESPACE = "  "

     t = leaf.type
     p = leaf.parent
     v = leaf.value
     t = leaf.type
     p = leaf.parent
     v = leaf.value


@@ -1149,7 +1224,7 @@ def whitespace(leaf: Leaf) -> str:  # noqa C901
             and prevp.parent.type == syms.shift_expr
             and prevp.prev_sibling
             and prevp.prev_sibling.type == token.NAME
             and prevp.parent.type == syms.shift_expr
             and prevp.prev_sibling
             and prevp.prev_sibling.type == token.NAME

-            and prevp.prev_sibling.value == 'print'  # type: ignore
+            and prevp.prev_sibling.value == "print"  # type: ignore

         ):
             # Python 2 print chevron
             return NO
         ):
             # Python 2 print chevron
             return NO


@@ -1306,7 +1381,7 @@ def whitespace(leaf: Leaf) -> str:  # noqa C901
                 return NO
 
         elif t == token.NAME:
                 return NO
 
         elif t == token.NAME:

-            if v == 'import':
+            if v == "import":

                 return SPACE
 
             if prev and prev.type == token.DOT:
                 return SPACE
 
             if prev and prev.type == token.DOT:


@@ -1319,7 +1394,7 @@ def whitespace(leaf: Leaf) -> str:  # noqa C901
 
 
 def preceding_leaf(node: Optional[LN]) -> Optional[Leaf]:
 
 
 def preceding_leaf(node: Optional[LN]) -> Optional[Leaf]:

-    """Returns the first leaf that precedes `node`, if any."""
+    """Return the first leaf that precedes `node`, if any."""

     while node:
         res = node.prev_sibling
         if res:
     while node:
         res = node.prev_sibling
         if res:


@@ -1336,17 +1411,35 @@ def preceding_leaf(node: Optional[LN]) -> Optional[Leaf]:
     return None
 
 
     return None
 
 

-def is_delimiter(leaf: Leaf) -> int:
-    """Returns the priority of the `leaf` delimiter. Returns 0 if not delimiter.
+def is_split_after_delimiter(leaf: Leaf, previous: Leaf = None) -> int:
+    """Return the priority of the `leaf` delimiter, given a line break after it.
+
+    The delimiter priorities returned here are from those delimiters that would
+    cause a line break after themselves.

 
     Higher numbers are higher priority.
     """
     if leaf.type == token.COMMA:
         return COMMA_PRIORITY
 
 
     Higher numbers are higher priority.
     """
     if leaf.type == token.COMMA:
         return COMMA_PRIORITY
 

-    if leaf.type in COMPARATORS:
-        return COMPARATOR_PRIORITY
+    if (
+        leaf.type in VARARGS
+        and leaf.parent
+        and leaf.parent.type in {syms.argument, syms.typedargslist}
+    ):
+        return MATH_PRIORITY
+
+    return 0
+
+
+def is_split_before_delimiter(leaf: Leaf, previous: Leaf = None) -> int:
+    """Return the priority of the `leaf` delimiter, given a line before after it.

 
 

+    The delimiter priorities returned here are from those delimiters that would
+    cause a line break before themselves.
+
+    Higher numbers are higher priority.
+    """

     if (
         leaf.type in MATH_OPERATORS
         and leaf.parent
     if (
         leaf.type in MATH_OPERATORS
         and leaf.parent


@@ -1354,11 +1447,51 @@ def is_delimiter(leaf: Leaf) -> int:
     ):
         return MATH_PRIORITY
 
     ):
         return MATH_PRIORITY
 

+    if leaf.type in COMPARATORS:
+        return COMPARATOR_PRIORITY
+
+    if (
+        leaf.type == token.STRING
+        and previous is not None
+        and previous.type == token.STRING
+    ):
+        return STRING_PRIORITY
+
+    if (
+        leaf.type == token.NAME
+        and leaf.value == "for"
+        and leaf.parent
+        and leaf.parent.type in {syms.comp_for, syms.old_comp_for}
+    ):
+        return COMPREHENSION_PRIORITY
+
+    if (
+        leaf.type == token.NAME
+        and leaf.value == "if"
+        and leaf.parent
+        and leaf.parent.type in {syms.comp_if, syms.old_comp_if}
+    ):
+        return COMPREHENSION_PRIORITY
+
+    if leaf.type == token.NAME and leaf.value in LOGIC_OPERATORS and leaf.parent:
+        return LOGIC_PRIORITY
+

     return 0
 
 
     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]:
 def generate_comments(leaf: Leaf) -> Iterator[Leaf]:

-    """Cleans the prefix of the `leaf` and generates comments from it, if any.
+    """Clean the prefix of the `leaf` and generate comments from it, if any.

 
     Comments in lib2to3 are shoved into the whitespace prefix.  This happens
     in `pgen2/driver.py:Driver.parse_tokens()`.  This was a brilliant implementation
 
     Comments in lib2to3 are shoved into the whitespace prefix.  This happens
     in `pgen2/driver.py:Driver.parse_tokens()`.  This was a brilliant implementation


@@ -1380,17 +1513,17 @@ def generate_comments(leaf: Leaf) -> Iterator[Leaf]:
     if not p:
         return
 
     if not p:
         return
 

-    if '#' not in p:
+    if "#" not in p:

         return
 
     consumed = 0
     nlines = 0
         return
 
     consumed = 0
     nlines = 0

-    for index, line in enumerate(p.split('\n')):
+    for index, line in enumerate(p.split("\n")):

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

-        if not line.startswith('#'):
+        if not line.startswith("#"):

             continue
 
         if index == 0 and leaf.type != token.ENDMARKER:
             continue
 
         if index == 0 and leaf.type != token.ENDMARKER:


@@ -1398,19 +1531,19 @@ def generate_comments(leaf: Leaf) -> Iterator[Leaf]:
         else:
             comment_type = STANDALONE_COMMENT
         comment = make_comment(line)
         else:
             comment_type = STANDALONE_COMMENT
         comment = make_comment(line)

-        yield Leaf(comment_type, comment, prefix='\n' * nlines)
+        yield Leaf(comment_type, comment, prefix="\n" * nlines)

 
 

-        if comment in {'# fmt: on', '# yapf: enable'}:
+        if comment in {"# fmt: on", "# yapf: enable"}:

             raise FormatOn(consumed)
 
             raise FormatOn(consumed)
 

-        if comment in {'# fmt: off', '# yapf: disable'}:
+        if comment in {"# fmt: off", "# yapf: disable"}:

             raise FormatOff(consumed)
 
         nlines = 0
 
 
 def make_comment(content: str) -> str:
             raise FormatOff(consumed)
 
         nlines = 0
 
 
 def make_comment(content: str) -> str:

-    """Returns a consistently formatted comment from the given `content` string.
+    """Return a consistently formatted comment from the given `content` string.

 
     All comments (except for "##", "#!", "#:") should have a single space between
     the hash sign and the content.
 
     All comments (except for "##", "#!", "#:") should have a single space between
     the hash sign and the content.


@@ -1419,19 +1552,19 @@ def make_comment(content: str) -> str:
     """
     content = content.rstrip()
     if not content:
     """
     content = content.rstrip()
     if not content:

-        return '#'
+        return "#"

 
 

-    if content[0] == '#':
+    if content[0] == "#":

         content = content[1:]
         content = content[1:]

-    if content and content[0] not in ' !:#':
-        content = ' ' + content
-    return '#' + content
+    if content and content[0] not in " !:#":
+        content = " " + content
+    return "#" + content

 
 
 def split_line(
     line: Line, line_length: int, inner: bool = False, py36: bool = False
 ) -> Iterator[Line]:
 
 
 def split_line(
     line: Line, line_length: int, inner: bool = False, py36: bool = False
 ) -> Iterator[Line]:

-    """Splits a `line` into potentially many lines.
+    """Split a `line` into potentially many lines.

 
     They should fit in the allotted `line_length` but might not be able to.
     `inner` signifies that there were a pair of brackets somewhere around the
 
     They should fit in the allotted `line_length` but might not be able to.
     `inner` signifies that there were a pair of brackets somewhere around the


@@ -1441,23 +1574,24 @@ def split_line(
     If `py36` is True, splitting may generate syntax that is only compatible
     with Python 3.6 and later.
     """
     If `py36` is True, splitting may generate syntax that is only compatible
     with Python 3.6 and later.
     """

-    if isinstance(line, UnformattedLines):
+    if isinstance(line, UnformattedLines) or line.is_comment:

         yield line
         return
 
         yield line
         return
 

-    line_str = str(line).strip('\n')
-    if len(line_str) <= line_length and '\n' not in line_str:
+    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
+    ):

         yield line
         return
 
         yield line
         return
 

+    split_funcs: List[SplitFunc]

     if line.is_def:
         split_funcs = [left_hand_split]
     elif line.inside_brackets:
     if line.is_def:
         split_funcs = [left_hand_split]
     elif line.inside_brackets:

-        split_funcs = [delimiter_split]
-        if '\n' not in line_str:
-            # Only attempt RHS if we don't have multiline strings or comments
-            # on this line.
-            split_funcs.append(right_hand_split)
+        split_funcs = [delimiter_split, standalone_comment_split, right_hand_split]

     else:
         split_funcs = [right_hand_split]
     for split_func in split_funcs:
     else:
         split_funcs = [right_hand_split]
     for split_func in split_funcs:


@@ -1466,8 +1600,8 @@ def split_line(
         # split altogether.
         result: List[Line] = []
         try:
         # split altogether.
         result: List[Line] = []
         try:

-            for l in split_func(line, py36=py36):
-                if str(l).strip('\n') == line_str:
+            for l in split_func(line, py36):
+                if str(l).strip("\n") == line_str:

                     raise CannotSplit("Split function returned an unchanged result")
 
                 result.extend(
                     raise CannotSplit("Split function returned an unchanged result")
 
                 result.extend(


@@ -1485,7 +1619,7 @@ def split_line(
 
 
 def left_hand_split(line: Line, py36: bool = False) -> Iterator[Line]:
 
 
 def left_hand_split(line: Line, py36: bool = False) -> Iterator[Line]:

-    """Splits line into many lines, starting with the first matching bracket pair.
+    """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.
 
     Note: this usually looks weird, only use this for function definitions.
     Prefer RHS otherwise.


@@ -1519,8 +1653,7 @@ def left_hand_split(line: Line, py36: bool = False) -> Iterator[Line]:
     ):
         for leaf in leaves:
             result.append(leaf, preformatted=True)
     ):
         for leaf in leaves:
             result.append(leaf, preformatted=True)

-            comment_after = line.comments.get(id(leaf))
-            if comment_after:
+            for comment_after in line.comments_after(leaf):

                 result.append(comment_after, preformatted=True)
     bracket_split_succeeded_or_raise(head, body, tail)
     for result in (head, body, tail):
                 result.append(comment_after, preformatted=True)
     bracket_split_succeeded_or_raise(head, body, tail)
     for result in (head, body, tail):


@@ -1529,7 +1662,7 @@ def left_hand_split(line: Line, py36: bool = False) -> Iterator[Line]:
 
 
 def right_hand_split(line: Line, py36: bool = False) -> Iterator[Line]:
 
 
 def right_hand_split(line: Line, py36: bool = False) -> Iterator[Line]:

-    """Splits line into many lines, starting with the last matching bracket pair."""
+    """Split line into many lines, starting with the last matching bracket pair."""

     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)


@@ -1559,8 +1692,7 @@ def right_hand_split(line: Line, py36: bool = False) -> Iterator[Line]:
     ):
         for leaf in leaves:
             result.append(leaf, preformatted=True)
     ):
         for leaf in leaves:
             result.append(leaf, preformatted=True)

-            comment_after = line.comments.get(id(leaf))
-            if comment_after:
+            for comment_after in line.comments_after(leaf):

                 result.append(comment_after, preformatted=True)
     bracket_split_succeeded_or_raise(head, body, tail)
     for result in (head, body, tail):
                 result.append(comment_after, preformatted=True)
     bracket_split_succeeded_or_raise(head, body, tail)
     for result in (head, body, tail):


@@ -1594,10 +1726,25 @@ def bracket_split_succeeded_or_raise(head: Line, body: Line, tail: Line) -> None
             )
 
 
             )
 
 

+def dont_increase_indentation(split_func: SplitFunc) -> SplitFunc:
+    """Normalize prefix of the first leaf in every line returned by `split_func`.
+
+    This is a decorator over relevant split functions.
+    """
+
+    @wraps(split_func)
+    def split_wrapper(line: Line, py36: bool = False) -> Iterator[Line]:
+        for l in split_func(line, py36):
+            normalize_prefix(l.leaves[0], inside_brackets=True)
+            yield l
+
+    return split_wrapper
+
+
+@dont_increase_indentation

 def delimiter_split(line: Line, py36: bool = False) -> Iterator[Line]:
 def delimiter_split(line: Line, py36: bool = False) -> Iterator[Line]:

-    """Splits according to delimiters of the highest priority.
+    """Split according to delimiters of the highest priority.

 
 

-    This kind of split doesn't increase indentation.

     If `py36` is True, the split will add trailing commas also in function
     signatures that contain `*` and `**`.
     """
     If `py36` is True, the split will add trailing commas also in function
     signatures that contain `*` and `**`.
     """


@@ -1617,11 +1764,24 @@ def delimiter_split(line: Line, py36: bool = False) -> Iterator[Line]:
     current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)
     lowest_depth = sys.maxsize
     trailing_comma_safe = True
     current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)
     lowest_depth = sys.maxsize
     trailing_comma_safe = True

+
+    def append_to_line(leaf: Leaf) -> Iterator[Line]:
+        """Append `leaf` to current line or to new line if appending impossible."""
+        nonlocal current_line
+        try:
+            current_line.append_safe(leaf, preformatted=True)
+        except ValueError as ve:
+            yield current_line
+
+            current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)
+            current_line.append(leaf)
+

     for leaf in line.leaves:
     for leaf in line.leaves:

-        current_line.append(leaf, preformatted=True)
-        comment_after = line.comments.get(id(leaf))
-        if comment_after:
-            current_line.append(comment_after, preformatted=True)
+        yield from append_to_line(leaf)
+
+        for comment_after in line.comments_after(leaf):
+            yield from append_to_line(comment_after)
+

         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


@@ -1631,7 +1791,6 @@ def delimiter_split(line: Line, py36: bool = False) -> Iterator[Line]:
             trailing_comma_safe = trailing_comma_safe and py36
         leaf_priority = delimiters.get(id(leaf))
         if leaf_priority == delimiter_priority:
             trailing_comma_safe = trailing_comma_safe and py36
         leaf_priority = delimiters.get(id(leaf))
         if leaf_priority == delimiter_priority:

-            normalize_prefix(current_line.leaves[0], inside_brackets=True)

             yield current_line
 
             current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)
             yield current_line
 
             current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)


@@ -1641,45 +1800,122 @@ def delimiter_split(line: Line, py36: bool = False) -> Iterator[Line]:
             and current_line.leaves[-1].type != token.COMMA
             and trailing_comma_safe
         ):
             and current_line.leaves[-1].type != token.COMMA
             and trailing_comma_safe
         ):

-            current_line.append(Leaf(token.COMMA, ','))
-        normalize_prefix(current_line.leaves[0], inside_brackets=True)
+            current_line.append(Leaf(token.COMMA, ","))
+        yield current_line
+
+
+@dont_increase_indentation
+def standalone_comment_split(line: Line, py36: bool = False) -> Iterator[Line]:
+    """Split standalone comments from the rest of the line."""
+    for leaf in line.leaves:
+        if leaf.type == STANDALONE_COMMENT:
+            if leaf.bracket_depth == 0:
+                break
+
+    else:
+        raise CannotSplit("Line does not have any standalone comments")
+
+    current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)
+
+    def append_to_line(leaf: Leaf) -> Iterator[Line]:
+        """Append `leaf` to current line or to new line if appending impossible."""
+        nonlocal current_line
+        try:
+            current_line.append_safe(leaf, preformatted=True)
+        except ValueError as ve:
+            yield current_line
+
+            current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)
+            current_line.append(leaf)
+
+    for leaf in line.leaves:
+        yield from append_to_line(leaf)
+
+        for comment_after in line.comments_after(leaf):
+            yield from append_to_line(comment_after)
+
+    if current_line:

         yield current_line
 
 
 def is_import(leaf: Leaf) -> bool:
         yield current_line
 
 
 def is_import(leaf: Leaf) -> bool:

-    """Returns True if the given leaf starts an import statement."""
+    """Return True if the given leaf starts an import statement."""

     p = leaf.parent
     t = leaf.type
     v = leaf.value
     return bool(
         t == token.NAME
         and (
     p = leaf.parent
     t = leaf.type
     v = leaf.value
     return bool(
         t == token.NAME
         and (

-            (v == 'import' and p and p.type == syms.import_name)
-            or (v == 'from' and p and p.type == syms.import_from)
+            (v == "import" and p and p.type == syms.import_name)
+            or (v == "from" and p and p.type == syms.import_from)

         )
     )
 
 
 def normalize_prefix(leaf: Leaf, *, inside_brackets: bool) -> None:
         )
     )
 
 
 def normalize_prefix(leaf: Leaf, *, inside_brackets: bool) -> None:

-    """Leaves existing extra newlines if not `inside_brackets`.
+    """Leave existing extra newlines if not `inside_brackets`. Remove everything
+    else.

 
 

-    Removes everything else.  Note: don't use backslashes for formatting or
-    you'll lose your voting rights.
+    Note: don't use backslashes for formatting or you'll lose your voting rights.

     """
     if not inside_brackets:
     """
     if not inside_brackets:

-        spl = leaf.prefix.split('#')
-        if '\\' not in spl[0]:
-            nl_count = spl[-1].count('\n')
+        spl = leaf.prefix.split("#")
+        if "\\" not in spl[0]:
+            nl_count = spl[-1].count("\n")

             if len(spl) > 1:
                 nl_count -= 1
             if len(spl) > 1:
                 nl_count -= 1

-            leaf.prefix = '\n' * nl_count
+            leaf.prefix = "\n" * nl_count

             return
 
             return
 

-    leaf.prefix = ''
+    leaf.prefix = ""
+
+
+def normalize_string_quotes(leaf: Leaf) -> None:
+    """Prefer double quotes but only if it doesn't cause more escaping.
+
+    Adds or removes backslashes as appropriate. Doesn't parse and fix
+    strings nested in f-strings (yet).
+
+    Note: Mutates its argument.
+    """
+    value = leaf.value.lstrip("furbFURB")
+    if value[:3] == '"""':
+        return
+
+    elif value[:3] == "'''":
+        orig_quote = "'''"
+        new_quote = '"""'
+    elif value[0] == '"':
+        orig_quote = '"'
+        new_quote = "'"
+    else:
+        orig_quote = "'"
+        new_quote = '"'
+    first_quote_pos = leaf.value.find(orig_quote)
+    if first_quote_pos == -1:
+        return  # There's an internal error
+
+    body = leaf.value[first_quote_pos + len(orig_quote):-len(orig_quote)]
+    new_body = body.replace(f"\\{orig_quote}", orig_quote).replace(
+        new_quote, f"\\{new_quote}"
+    )
+    if new_quote == '"""' and new_body[-1] == '"':
+        # edge case:
+        new_body = new_body[:-1] + '\\"'
+    orig_escape_count = body.count("\\")
+    new_escape_count = new_body.count("\\")
+    if new_escape_count > orig_escape_count:
+        return  # Do not introduce more escaping
+
+    if new_escape_count == orig_escape_count and orig_quote == '"':
+        return  # Prefer double quotes
+
+    prefix = leaf.value[:first_quote_pos]
+    leaf.value = f"{prefix}{new_quote}{new_body}{new_quote}"

 
 
 def is_python36(node: Node) -> bool:
 
 
 def is_python36(node: Node) -> bool:

-    """Returns True if the current file is using Python 3.6+ features.
+    """Return True if the current file is using Python 3.6+ features.

 
     Currently looking for:
     - f-strings; and
 
     Currently looking for:
     - f-strings; and


@@ -1688,7 +1924,7 @@ def is_python36(node: Node) -> bool:
     for n in node.pre_order():
         if n.type == token.STRING:
             value_head = n.value[:2]  # type: ignore
     for n in node.pre_order():
         if n.type == token.STRING:
             value_head = n.value[:2]  # type: ignore

-            if value_head in {'f"', 'F"', "f'", "F'", 'rf', 'fr', 'RF', 'FR'}:
+            if value_head in {'f"', 'F"', "f'", "F'", "rf", "fr", "RF", "FR"}:

                 return True
 
         elif (
                 return True
 
         elif (


@@ -1703,14 +1939,14 @@ def is_python36(node: Node) -> bool:
     return False
 
 
     return False
 
 

-PYTHON_EXTENSIONS = {'.py'}
+PYTHON_EXTENSIONS = {".py"}

 BLACKLISTED_DIRECTORIES = {
 BLACKLISTED_DIRECTORIES = {

-    'build', 'buck-out', 'dist', '_build', '.git', '.hg', '.mypy_cache', '.tox', '.venv'
+    "build", "buck-out", "dist", "_build", ".git", ".hg", ".mypy_cache", ".tox", ".venv"

 }
 
 
 def gen_python_files_in_dir(path: Path) -> Iterator[Path]:
 }
 
 
 def gen_python_files_in_dir(path: Path) -> Iterator[Path]:

-    """Generates all files under `path` which aren't under BLACKLISTED_DIRECTORIES
+    """Generate all files under `path` which aren't under BLACKLISTED_DIRECTORIES

     and have one of the PYTHON_EXTENSIONS.
     """
     for child in path.iterdir():
     and have one of the PYTHON_EXTENSIONS.
     """
     for child in path.iterdir():


@@ -1735,21 +1971,27 @@ class Report:
     def done(self, src: Path, changed: bool) -> None:
         """Increment the counter for successful reformatting. Write out a message."""
         if changed:
     def done(self, src: Path, changed: bool) -> None:
         """Increment the counter for successful reformatting. Write out a message."""
         if changed:

-            reformatted = 'would reformat' if self.check else 'reformatted'
-            out(f'{reformatted} {src}')
+            reformatted = "would reformat" if self.check else "reformatted"
+            out(f"{reformatted} {src}")

             self.change_count += 1
         else:
             self.change_count += 1
         else:

-            out(f'{src} already well formatted, good job.', bold=False)
+            out(f"{src} already well formatted, good job.", bold=False)

             self.same_count += 1
 
     def failed(self, src: Path, message: str) -> None:
         """Increment the counter for failed reformatting. Write out a message."""
             self.same_count += 1
 
     def failed(self, src: Path, message: str) -> None:
         """Increment the counter for failed reformatting. Write out a message."""

-        err(f'error: cannot format {src}: {message}')
+        err(f"error: cannot format {src}: {message}")

         self.failure_count += 1
 
     @property
     def return_code(self) -> int:
         self.failure_count += 1
 
     @property
     def return_code(self) -> int:

-        """Which return code should the app use considering the current state."""
+        """Return the exit code that the app should use.
+
+        This considers the current state of changed files and failures:
+        - if there were any failures, return 123;
+        - if any files were changed and --check is being used, return 1;
+        - otherwise return 0.
+        """

         # According to http://tldp.org/LDP/abs/html/exitcodes.html starting with
         # 126 we have special returncodes reserved by the shell.
         if self.failure_count:
         # According to http://tldp.org/LDP/abs/html/exitcodes.html starting with
         # 126 we have special returncodes reserved by the shell.
         if self.failure_count:


@@ -1761,7 +2003,7 @@ class Report:
         return 0
 
     def __str__(self) -> str:
         return 0
 
     def __str__(self) -> str:

-        """A color report of the current state.
+        """Render a color report of the current state.

 
         Use `click.unstyle` to remove colors.
         """
 
         Use `click.unstyle` to remove colors.
         """


@@ -1775,26 +2017,23 @@ class Report:
             failed = "failed to reformat"
         report = []
         if self.change_count:
             failed = "failed to reformat"
         report = []
         if self.change_count:

-            s = 's' if self.change_count > 1 else ''
+            s = "s" if self.change_count > 1 else ""

             report.append(
             report.append(

-                click.style(f'{self.change_count} file{s} {reformatted}', bold=True)
+                click.style(f"{self.change_count} file{s} {reformatted}", bold=True)

             )
         if self.same_count:
             )
         if self.same_count:

-            s = 's' if self.same_count > 1 else ''
-            report.append(f'{self.same_count} file{s} {unchanged}')
+            s = "s" if self.same_count > 1 else ""
+            report.append(f"{self.same_count} file{s} {unchanged}")

         if self.failure_count:
         if self.failure_count:

-            s = 's' if self.failure_count > 1 else ''
+            s = "s" if self.failure_count > 1 else ""

             report.append(
             report.append(

-                click.style(f'{self.failure_count} file{s} {failed}', fg='red')
+                click.style(f"{self.failure_count} file{s} {failed}", fg="red")

             )
             )

-        return ', '.join(report) + '.'
+        return ", ".join(report) + "."

 
 
 def assert_equivalent(src: str, dst: str) -> None:
 
 
 def assert_equivalent(src: str, dst: str) -> None:

-    """Raises AssertionError if `src` and `dst` aren't equivalent.
-
-    This is a temporary sanity check until Black becomes stable.
-    """
+    """Raise AssertionError if `src` and `dst` aren't equivalent."""

 
     import ast
     import traceback
 
     import ast
     import traceback


@@ -1837,17 +2076,17 @@ def assert_equivalent(src: str, dst: str) -> None:
     try:
         dst_ast = ast.parse(dst)
     except Exception as exc:
     try:
         dst_ast = ast.parse(dst)
     except Exception as exc:

-        log = dump_to_file(''.join(traceback.format_tb(exc.__traceback__)), dst)
+        log = dump_to_file("".join(traceback.format_tb(exc.__traceback__)), dst)

         raise AssertionError(
             f"INTERNAL ERROR: Black produced invalid code: {exc}. "
             f"Please report a bug on https://github.com/ambv/black/issues.  "
             f"This invalid output might be helpful: {log}"
         ) from None
 
         raise AssertionError(
             f"INTERNAL ERROR: Black produced invalid code: {exc}. "
             f"Please report a bug on https://github.com/ambv/black/issues.  "
             f"This invalid output might be helpful: {log}"
         ) from None
 

-    src_ast_str = '\n'.join(_v(src_ast))
-    dst_ast_str = '\n'.join(_v(dst_ast))
+    src_ast_str = "\n".join(_v(src_ast))
+    dst_ast_str = "\n".join(_v(dst_ast))

     if src_ast_str != dst_ast_str:
     if src_ast_str != dst_ast_str:

-        log = dump_to_file(diff(src_ast_str, dst_ast_str, 'src', 'dst'))
+        log = dump_to_file(diff(src_ast_str, dst_ast_str, "src", "dst"))

         raise AssertionError(
             f"INTERNAL ERROR: Black produced code that is not equivalent to "
             f"the source.  "
         raise AssertionError(
             f"INTERNAL ERROR: Black produced code that is not equivalent to "
             f"the source.  "


@@ -1857,15 +2096,12 @@ def assert_equivalent(src: str, dst: str) -> None:
 
 
 def assert_stable(src: str, dst: str, line_length: int) -> None:
 
 
 def assert_stable(src: str, dst: str, line_length: int) -> None:

-    """Raises AssertionError if `dst` reformats differently the second time.
-
-    This is a temporary sanity check until Black becomes stable.
-    """
+    """Raise AssertionError if `dst` reformats differently the second time."""

     newdst = format_str(dst, line_length=line_length)
     if dst != newdst:
         log = dump_to_file(
     newdst = format_str(dst, line_length=line_length)
     if dst != newdst:
         log = dump_to_file(

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

         )
         raise AssertionError(
             f"INTERNAL ERROR: Black produced different code on the second pass "
         )
         raise AssertionError(
             f"INTERNAL ERROR: Black produced different code on the second pass "


@@ -1876,28 +2112,58 @@ def assert_stable(src: str, dst: str, line_length: int) -> None:
 
 
 def dump_to_file(*output: str) -> str:
 
 
 def dump_to_file(*output: str) -> str:

-    """Dumps `output` to a temporary file. Returns path to the file."""
+    """Dump `output` to a temporary file. Return path to the file."""

     import tempfile
 
     with tempfile.NamedTemporaryFile(
     import tempfile
 
     with tempfile.NamedTemporaryFile(

-        mode='w', prefix='blk_', suffix='.log', delete=False
+        mode="w", prefix="blk_", suffix=".log", delete=False

     ) as f:
         for lines in output:
             f.write(lines)
     ) as f:
         for lines in output:
             f.write(lines)

-            f.write('\n')
+            if lines and lines[-1] != "\n":
+                f.write("\n")

     return f.name
 
 
 def diff(a: str, b: str, a_name: str, b_name: str) -> str:
     return f.name
 
 
 def diff(a: str, b: str, a_name: str, b_name: str) -> str:

-    """Returns a udiff string between strings `a` and `b`."""
+    """Return a unified diff string between strings `a` and `b`."""

     import difflib
 
     import difflib
 

-    a_lines = [line + '\n' for line in a.split('\n')]
-    b_lines = [line + '\n' for line in b.split('\n')]
-    return ''.join(
+    a_lines = [line + "\n" for line in a.split("\n")]
+    b_lines = [line + "\n" for line in b.split("\n")]
+    return "".join(

         difflib.unified_diff(a_lines, b_lines, fromfile=a_name, tofile=b_name, n=5)
     )
 
 
         difflib.unified_diff(a_lines, b_lines, fromfile=a_name, tofile=b_name, n=5)
     )
 
 

-if __name__ == '__main__':
+def cancel(tasks: List[asyncio.Task]) -> None:
+    """asyncio signal handler that cancels all `tasks` and reports to stderr."""
+    err("Aborted!")
+    for task in tasks:
+        task.cancel()
+
+
+def shutdown(loop: BaseEventLoop) -> None:
+    """Cancel all pending tasks on `loop`, wait for them, and close the loop."""
+    try:
+        # This part is borrowed from asyncio/runners.py in Python 3.7b2.
+        to_cancel = [task for task in asyncio.Task.all_tasks(loop) if not task.done()]
+        if not to_cancel:
+            return
+
+        for task in to_cancel:
+            task.cancel()
+        loop.run_until_complete(
+            asyncio.gather(*to_cancel, loop=loop, return_exceptions=True)
+        )
+    finally:
+        # `concurrent.futures.Future` objects cannot be cancelled once they
+        # are already running. There might be some when the `shutdown()` happened.
+        # Silence their logger's spew about the event loop being closed.
+        cf_logger = logging.getLogger("concurrent.futures")
+        cf_logger.setLevel(logging.CRITICAL)
+        loop.close()
+
+
+if __name__ == "__main__":

     main()
     main()