Consistent empty lines in Change Log

[etc/vim.git] / black.py

diff --git a/black.py b/black.py
index e2c427cb536c752b6e5d0e00c07ca2b2a2a6a23e..537ba599807ffea932a223884a40488d22c8ab45 100644 (file)
--- a/black.py
+++ b/black.py
@@ -1,17 +1,37 @@
 #!/usr/bin/env python3
 #!/usr/bin/env python3

+

 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 os
 from pathlib import Path

+import re

 import tokenize
 import tokenize

+import signal
+import sys

 from typing import (
 from typing import (

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

 )
 
 )
 

-from attr import attrib, dataclass, Factory
+from attr import dataclass, Factory

 import click
 
 # lib2to3 fork
 import click
 
 # lib2to3 fork


@@ -20,7 +40,7 @@ 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__ = "18.3a1"
+__version__ = "18.4a1"

 DEFAULT_LINE_LENGTH = 88
 # types
 syms = pygram.python_symbols
 DEFAULT_LINE_LENGTH = 88
 # types
 syms = pygram.python_symbols


@@ -30,54 +50,113 @@ 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()` and `right_hand_split()`.
+    Raised by :func:`left_hand_split`, :func:`right_hand_split`, and
+    :func:`delimiter_split`.

     """
 
 
     """
 
 

+class FormatError(Exception):
+    """Base exception for `# fmt: on` and `# fmt: off` handling.
+
+    It holds the number of bytes of the prefix consumed before the format
+    control comment appeared.
+    """
+
+    def __init__(self, consumed: int) -> None:
+        super().__init__(consumed)
+        self.consumed = consumed
+
+    def trim_prefix(self, leaf: Leaf) -> None:
+        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."""
+        unformatted_prefix = leaf.prefix[:self.consumed]
+        return Leaf(token.NEWLINE, unformatted_prefix)
+
+
+class FormatOn(FormatError):
+    """Found a comment like `# fmt: on` in the file."""
+
+
+class FormatOff(FormatError):
+    """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 "
-        "means nothing changed.  Return code 1 means some files were "
+        "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(
         "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,
+    help="If --fast given, skip temporary sanity checks. [default: --safe]",
+)
+@click.option(
+    "-q",
+    "--quiet",

     is_flag=True,
     is_flag=True,

-    help='If --fast given, skip temporary sanity checks. [default: --safe]',
+    help=(
+        "Don't emit non-error messages to stderr. Errors are still emitted, "
+        "silence those with 2>/dev/null."
+    ),

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

-    'src',
+    "src",

     nargs=-1,
     nargs=-1,

-    type=click.Path(exists=True, file_okay=True, dir_okay=True, readable=True),
+    type=click.Path(
+        exists=True, file_okay=True, dir_okay=True, readable=True, allow_dash=True
+    ),

 )
 @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,
+    quiet: bool,
+    src: List[str],

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


@@ -88,17 +167,35 @@ 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("-"))

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

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

         try:
         try:

-            changed = format_file_in_place(
-                p, line_length=line_length, fast=fast, write_back=not check
-            )
+            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))
             report.done(p, changed)
         except Exception as exc:
             report.failed(p, str(exc))


@@ -110,88 +207,164 @@ 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, quiet, 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,
     fast: bool,

+    quiet: bool,

     loop: BaseEventLoop,
     executor: Executor,
 ) -> int:
     loop: BaseEventLoop,
     executor: Executor,
 ) -> int:

+    """Run formatting of `sources` in parallel using the provided `executor`.
+
+    (Use ProcessPoolExecutors for actual parallelism.)
+
+    `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=write_back is WriteBack.NO, quiet=quiet)

     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! ✨ 🍰 ✨')
-    click.echo(str(report))
+        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
 
 
 def format_file_in_place(
     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:
 ) -> bool:

-    """Format the file and rewrite if changed. Return True if changed."""
+    """Format file under `src` path. Return True if changed.
+
+    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:
     try:

-        contents, encoding = format_file(src, line_length=line_length, fast=fast)
+        dst_contents = format_file_contents(
+            src_contents, line_length=line_length, fast=fast
+        )

     except NothingChanged:
         return False
 
     except NothingChanged:
         return False
 

-    if write_back:
-        with open(src, "w", encoding=encoding) as f:
-            f.write(contents)
+    if write_back == write_back.YES:
+        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)"
+        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
 
 
     return True
 
 

-def format_file(
-    src: Path, line_length: int, fast: bool
-) -> Tuple[FileContent, Encoding]:
-    """Reformats a file and returns its contents and encoding."""
-    with tokenize.open(src) as src_buffer:
-        src_contents = src_buffer.read()
-    if src_contents.strip() == '':
-        raise NothingChanged(src)
+def format_stdin_to_stdout(
+    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`.
+    """
+    src = sys.stdin.read()
+    dst = src
+    try:
+        dst = format_file_contents(src, line_length=line_length, fast=fast)
+        return True
+
+    except NothingChanged:
+        return False
+
+    finally:
+        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:
+    """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 src_contents.strip() == "":
+        raise NothingChanged

 
     dst_contents = format_str(src_contents, line_length=line_length)
     if src_contents == dst_contents:
 
     dst_contents = format_str(src_contents, line_length=line_length)
     if src_contents == dst_contents:

-        raise NothingChanged(src)
+        raise NothingChanged

 
     if not fast:
         assert_equivalent(src_contents, dst_contents)
         assert_stable(src_contents, dst_contents, line_length=line_length)
 
     if not fast:
         assert_equivalent(src_contents, dst_contents)
         assert_stable(src_contents, dst_contents, line_length=line_length)

-    return dst_contents, src_buffer.encoding
+    return dst_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.
+    """

     src_node = lib2to3_parse(src_contents)
     dst_contents = ""
     src_node = lib2to3_parse(src_contents)
     dst_contents = ""

-    comments: List[Line] = []

     lines = LineGenerator()
     elt = EmptyLineTracker()
     lines = LineGenerator()
     elt = EmptyLineTracker()

+    py36 = is_python36(src_node)

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


@@ -200,36 +373,41 @@ def format_str(src_contents: str, line_length: int) -> FileContent:
         before, after = elt.maybe_empty_lines(current_line)
         for _ in range(before):
             dst_contents += str(empty_line)
         before, after = elt.maybe_empty_lines(current_line)
         for _ in range(before):
             dst_contents += str(empty_line)

-        if not current_line.is_comment:
-            for comment in comments:
-                dst_contents += str(comment)
-            comments = []
-            for line in split_line(current_line, line_length=line_length):
-                dst_contents += str(line)
-        else:
-            comments.append(current_line)
-    for comment in comments:
-        dst_contents += str(comment)
+        for line in split_line(current_line, line_length=line_length, py36=py36):
+            dst_contents += str(line)

     return dst_contents
 
 
     return dst_contents
 
 

+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,
+]
+
+

 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

-    drv = driver.Driver(grammar, pytree.convert)
-    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
         src_txt += nl

-    try:
-        result = drv.parse_string(src_txt, True)
-    except ParseError as pe:
-        lineno, column = pe.context[1]
-        lines = src_txt.splitlines()
+    for grammar in GRAMMARS:
+        drv = driver.Driver(grammar, pytree.convert)

         try:
         try:

-            faulty_line = lines[lineno - 1]
-        except IndexError:
-            faulty_line = "<line number missing in source>"
-        raise ValueError(f"Cannot parse: {lineno}:{column}: {faulty_line}") from None
+            result = drv.parse_string(src_txt, True)
+            break
+
+        except ParseError as pe:
+            lineno, column = pe.context[1]
+            lines = src_txt.splitlines()
+            try:
+                faulty_line = lines[lineno - 1]
+            except IndexError:
+                faulty_line = "<line number missing in source>"
+            exc = ValueError(f"Cannot parse: {lineno}:{column}: {faulty_line}")
+    else:
+        raise exc from None

 
     if isinstance(result, Leaf):
         result = Node(syms.file_input, [result])
 
     if isinstance(result, Leaf):
         result = Node(syms.file_input, [result])


@@ -242,20 +420,30 @@ def lib2to3_unparse(node: Node) -> str:
     return code
 
 
     return code
 
 

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

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

-    """Basic lib2to3 visitor that yields things on visiting."""
+    """Basic lib2to3 visitor that yields things of type `T` on `visit()`."""

 
     def visit(self, node: LN) -> Iterator[T]:
 
     def visit(self, node: LN) -> Iterator[T]:

+        """Main method to visit `node` and its children.
+
+        It tries to find a `visit_*()` method for the given `node.type`, like
+        `visit_simple_stmt` for Node objects or `visit_INDENT` for Leaf objects.
+        If no dedicated `visit_*()` method is found, chooses `visit_default()`
+        instead.
+
+        Then yields objects of type `T` from the selected visitor.
+        """

         if node.type < 256:
             name = token.tok_name[node.type]
         else:
             name = type_repr(node.type)
         if node.type < 256:
             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]:
 
     def visit_default(self, node: LN) -> Iterator[T]:

+        """Default `visit_*()` implementation. Recurses to children of `node`."""

         if isinstance(node, Node):
             for child in node.children:
                 yield from self.visit(child)
         if isinstance(node, Node):
             for child in node.children:
                 yield from self.visit(child)


@@ -263,32 +451,41 @@ class Visitor(Generic[T]):
 
 @dataclass
 class DebugVisitor(Visitor[T]):
 
 @dataclass
 class DebugVisitor(Visitor[T]):

-    tree_depth: int = attrib(default=0)
+    tree_depth: int = 0

 
     def visit_default(self, node: LN) -> Iterator[T]:
 
     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:
+        """Pretty-print the lib2to3 AST of a given string of `code`.
+
+        Convenience method for debugging.
+        """
+        v: DebugVisitor[None] = DebugVisitor()
+        list(v.visit(lib2to3_parse(code)))

 
 
 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,


@@ -300,7 +497,7 @@ 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,


@@ -318,11 +515,13 @@ MATH_OPERATORS = {
     token.AMPER,
     token.PERCENT,
     token.CIRCUMFLEX,
     token.AMPER,
     token.PERCENT,
     token.CIRCUMFLEX,

+    token.TILDE,

     token.LEFTSHIFT,
     token.RIGHTSHIFT,
     token.DOUBLESTAR,
     token.DOUBLESLASH,
 }
     token.LEFTSHIFT,
     token.RIGHTSHIFT,
     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


@@ -333,68 +532,85 @@ MATH_PRIORITY = 1
 
 @dataclass
 class BracketTracker:
 
 @dataclass
 class BracketTracker:

-    depth: int = attrib(default=0)
-    bracket_match: Dict[Tuple[Depth, NodeType], Leaf] = attrib(default=Factory(dict))
-    delimiters: Dict[LeafID, Priority] = attrib(default=Factory(dict))
-    previous: Optional[Leaf] = attrib(default=None)
+    """Keeps track of brackets on a line."""
+
+    depth: int = 0
+    bracket_match: Dict[Tuple[Depth, NodeType], Leaf] = Factory(dict)
+    delimiters: Dict[LeafID, Priority] = Factory(dict)
+    previous: Optional[Leaf] = None

 
     def mark(self, leaf: Leaf) -> None:
 
     def mark(self, leaf: Leaf) -> None:

+        """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
+        that started on this line.
+
+        If a leaf is itself a closing bracket, it receives an `opening_bracket`
+        field that it forms a pair with. This is a one-directional link to
+        avoid reference cycles.
+
+        If a leaf is a delimiter (a token on which Black can split the line if
+        needed) and it's on depth 0, its `id()` is stored in the tracker's
+        `delimiters` field.
+        """

         if leaf.type == token.COMMENT:
             return
 
         if leaf.type in CLOSING_BRACKETS:
             self.depth -= 1
             opening_bracket = self.bracket_match.pop((self.depth, leaf.type))
         if leaf.type == token.COMMENT:
             return
 
         if leaf.type in CLOSING_BRACKETS:
             self.depth -= 1
             opening_bracket = self.bracket_match.pop((self.depth, leaf.type))

-            leaf.opening_bracket = opening_bracket  # type: ignore
-        leaf.bracket_depth = self.depth  # type: ignore
+            leaf.opening_bracket = opening_bracket
+        leaf.bracket_depth = self.depth

         if self.depth == 0:
         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
+            delim = is_split_before_delimiter(leaf, self.previous)
+            if delim and self.previous is not None:
+                self.delimiters[id(self.previous)] = delim
+            else:
+                delim = is_split_after_delimiter(leaf, self.previous)
+                if delim:
+                    self.delimiters[id(leaf)] = 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)
 
         return bool(self.bracket_match)
 

-    def max_priority(self, exclude: Iterable[LeafID] =()) -> int:
-        """Returns 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_delimiter()` returns.

+        Raises ValueError on no delimiters.

         """
         return max(v for k, v in self.delimiters.items() if k not in exclude)
 
 
 @dataclass
 class Line:
         """
         return max(v for k, v in self.delimiters.items() if k not in exclude)
 
 
 @dataclass
 class Line:

-    depth: int = attrib(default=0)
-    leaves: List[Leaf] = attrib(default=Factory(list))
-    comments: Dict[LeafID, Leaf] = attrib(default=Factory(dict))
-    bracket_tracker: BracketTracker = attrib(default=Factory(BracketTracker))
-    inside_brackets: bool = attrib(default=False)
+    """Holds leaves and comments. Can be printed with `str(line)`."""
+
+    depth: int = 0
+    leaves: List[Leaf] = Factory(list)
+    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:
 
     def append(self, leaf: Leaf, preformatted: bool = False) -> None:

-        has_value = leaf.value.strip()
+        """Add a new `leaf` to the end of the line.
+
+        Unless `preformatted` is True, the `leaf` will receive a new consistent
+        whitespace prefix and metadata applied by :class:`BracketTracker`.
+        Trailing commas are maybe removed, unpacked for loop variables are
+        demoted from being delimiters.
+
+        Inline comments are put aside.
+        """
+        has_value = leaf.type in BRACKETS or bool(leaf.value.strip())

         if not has_value:
             return
 
         if not has_value:
             return
 


@@ -403,37 +619,58 @@ class Line:
             # imports, for which we only preserve newlines.
             leaf.prefix += whitespace(leaf)
         if self.inside_brackets or not preformatted:
             # imports, for which we only preserve newlines.
             leaf.prefix += whitespace(leaf)
         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)

-            if self.maybe_adapt_standalone_comment(leaf):
-                return
+            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)
 

+    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:
     @property
     def is_comment(self) -> bool:

-        return bool(self) and self.leaves[0].type == STANDALONE_COMMENT
+        """Is this line a 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:

+        """Is this line a decorator?"""

         return bool(self) and self.leaves[0].type == token.AT
 
     @property
     def is_import(self) -> bool:
         return bool(self) and self.leaves[0].type == token.AT
 
     @property
     def is_import(self) -> bool:

+        """Is this an import line?"""

         return bool(self) and is_import(self.leaves[0])
 
     @property
     def is_class(self) -> bool:
         return bool(self) and is_import(self.leaves[0])
 
     @property
     def is_class(self) -> bool:

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

         return (
         return (

-            bool(self) and
-            self.leaves[0].type == token.NAME and
-            self.leaves[0].value == 'class'
+            bool(self)
+            and self.leaves[0].type == token.NAME
+            and self.leaves[0].value == "class"

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

-        """Also returns True for async defs."""
+        """Is this a function definition? (Also returns True for async defs.)"""

         try:
             first_leaf = self.leaves[0]
         except IndexError:
         try:
             first_leaf = self.leaves[0]
         except IndexError:


@@ -444,50 +681,70 @@ class Line:
         except IndexError:
             second_leaf = None
         return (
         except IndexError:
             second_leaf = None
         return (

-            (first_leaf.type == token.NAME and first_leaf.value == 'def') or
-            (
-                first_leaf.type == token.NAME and
-                first_leaf.value == 'async' and
-                second_leaf is not None and
-                second_leaf.type == token.NAME and
-                second_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
+                and second_leaf.value == "def"

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

+        """Is this line a flow control statement?
+
+        Those are `return`, `raise`, `break`, and `continue`.
+        """

         return (
         return (

-            bool(self) and
-            self.leaves[0].type == token.NAME and
-            self.leaves[0].value in FLOW_CONTROL
+            bool(self)
+            and self.leaves[0].type == token.NAME
+            and self.leaves[0].value in FLOW_CONTROL

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

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

         return (
         return (

-            bool(self) and
-            self.leaves[0].type == token.NAME and
-            self.leaves[0].value == 'yield'
+            bool(self)
+            and self.leaves[0].type == token.NAME
+            and self.leaves[0].value == "yield"

         )
 
         )
 

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

     def maybe_remove_trailing_comma(self, closing: Leaf) -> bool:
     def maybe_remove_trailing_comma(self, closing: Leaf) -> bool:

+        """Remove trailing comma if there is one and it's safe."""

         if not (
         if not (

-            self.leaves and
-            self.leaves[-1].type == token.COMMA and
-            closing.type in CLOSING_BRACKETS
+            self.leaves
+            and self.leaves[-1].type == token.COMMA
+            and closing.type in CLOSING_BRACKETS

         ):
             return False
 
         ):
             return False
 

-        if closing.type == token.RSQB or closing.type == token.RBRACE:
-            self.leaves.pop()
+        if closing.type == token.RBRACE:
+            self.remove_trailing_comma()

             return True
 
             return True
 

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

         # For parens let's check if it's safe to remove the comma.  If the
         # trailing one is the only one, we might mistakenly change a tuple
         # into a different type by removing the comma.
         # For parens let's check if it's safe to remove the comma.  If the
         # trailing one is the only one, we might mistakenly change a tuple
         # into a different type by removing the comma.

-        depth = closing.bracket_depth + 1  # type: ignore
+        depth = closing.bracket_depth + 1

         commas = 0
         commas = 0

-        opening = closing.opening_bracket  # type: ignore
+        opening = closing.opening_bracket

         for _opening_index, leaf in enumerate(self.leaves):
             if leaf is opening:
                 break
         for _opening_index, leaf in enumerate(self.leaves):
             if leaf is opening:
                 break


@@ -499,107 +756,200 @@ class Line:
             if leaf is closing:
                 break
 
             if leaf is closing:
                 break
 

-            bracket_depth = leaf.bracket_depth  # type: ignore
+            bracket_depth = leaf.bracket_depth

             if bracket_depth == depth and leaf.type == token.COMMA:
                 commas += 1
             if bracket_depth == depth and leaf.type == token.COMMA:
                 commas += 1

+                if leaf.parent and leaf.parent.type == syms.arglist:
+                    commas += 1
+                    break
+

         if commas > 1:
         if commas > 1:

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

             return True
 
         return 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.
+    def maybe_increment_for_loop_variable(self, leaf: Leaf) -> bool:
+        """In a for loop, or comprehension, the variables are often unpacks.

 
 

-        This is not perfect, the line to which the standalone comment gets
-        appended will appear "too long" when splitting.
+        To avoid splitting on the comma in this situation, increase the depth of
+        tokens between `for` and `in`.

         """
         """

-        if not (
-            comment.type == STANDALONE_COMMENT and
-            self.bracket_tracker.any_open_brackets()
-        ):
-            return False
+        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

 
 

-        comment.type = token.COMMENT
-        comment.prefix = '\n' + '    ' * (self.depth + 1)
-        return self.append_comment(comment)
+    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:
 
     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.prefix = ""
+            return False
+

         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:
-        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
+
+        for index, comment_after in self.comments:
+            if _leaf_index == index:
+                yield comment_after

 
 

-        raise LookupError("No non-delimiters found")
+    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:
 
     def __str__(self) -> str:

+        """Render the line."""

         if not self:
         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:

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

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

+class UnformattedLines(Line):
+    """Just like :class:`Line` but stores lines which aren't reformatted."""
+
+    def append(self, leaf: Leaf, preformatted: bool = True) -> None:
+        """Just add a new `leaf` to the end of the lines.
+
+        The `preformatted` argument is ignored.
+
+        Keeps track of indentation `depth`, which is useful when the user
+        says `# fmt: on`. Otherwise, doesn't do anything with the `leaf`.
+        """
+        try:
+            list(generate_comments(leaf))
+        except FormatOn as f_on:
+            self.leaves.append(f_on.leaf_from_consumed(leaf))
+            raise
+
+        self.leaves.append(leaf)
+        if leaf.type == token.INDENT:
+            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:
+        """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:
+        """Does nothing and returns False."""
+        return False
+
+    def maybe_increment_for_loop_variable(self, leaf: Leaf) -> bool:
+        """Does nothing and returns False."""
+        return False
+
+

 @dataclass
 class EmptyLineTracker:
     """Provides a stateful method that returns the number of potential extra
     empty lines needed before and after the currently processed line.
 
 @dataclass
 class EmptyLineTracker:
     """Provides a stateful method that returns the number of potential extra
     empty lines needed before and after the currently processed line.
 

-    Note: this tracker works on lines that haven't been split yet.
+    Note: this tracker works on lines that haven't been split yet.  It assumes
+    the prefix of the first leaf consists of optional newlines.  Those newlines
+    are consumed by `maybe_empty_lines()` and included in the computation.

     """
     """

-    previous_line: Optional[Line] = attrib(default=None)
-    previous_after: int = attrib(default=0)
-    previous_defs: List[int] = attrib(default=Factory(list))
+    previous_line: Optional[Line] = None
+    previous_after: int = 0
+    previous_defs: List[int] = Factory(list)

 
     def maybe_empty_lines(self, current_line: Line) -> Tuple[int, int]:
 
     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
+

         before, after = self._maybe_empty_lines(current_line)
         before, after = self._maybe_empty_lines(current_line)

+        before -= self.previous_after

         self.previous_after = after
         self.previous_line = current_line
         return before, after
 
     def _maybe_empty_lines(self, current_line: Line) -> Tuple[int, int]:
         self.previous_after = after
         self.previous_line = current_line
         return before, after
 
     def _maybe_empty_lines(self, current_line: Line) -> Tuple[int, int]:

-        before = 0
+        max_allowed = 1
+        if current_line.depth == 0:
+            max_allowed = 2
+        if current_line.leaves:
+            # Consume the first leaf's extra newlines.
+            first_leaf = current_line.leaves[0]
+            before = first_leaf.prefix.count("\n")
+            before = min(before, max_allowed)
+            first_leaf.prefix = ""
+        else:
+            before = 0

         depth = current_line.depth
         while self.previous_defs and self.previous_defs[-1] >= depth:
             self.previous_defs.pop()
         depth = current_line.depth
         while self.previous_defs and self.previous_defs[-1] >= depth:
             self.previous_defs.pop()

-            before = (1 if depth else 2) - self.previous_after
+            before = 1 if depth else 2

         is_decorator = current_line.is_decorator
         if is_decorator or current_line.is_def or current_line.is_class:
             if not is_decorator:
         is_decorator = current_line.is_decorator
         if is_decorator or current_line.is_def or current_line.is_class:
             if not is_decorator:


@@ -615,24 +965,23 @@ class EmptyLineTracker:
             newlines = 2
             if current_line.depth:
                 newlines -= 1
             newlines = 2
             if current_line.depth:
                 newlines -= 1

-            newlines -= self.previous_after

             return newlines, 0
 
         if current_line.is_flow_control:
             return before, 1
 
         if (
             return newlines, 0
 
         if current_line.is_flow_control:
             return before, 1
 
         if (

-            self.previous_line and
-            self.previous_line.is_import and
-            not current_line.is_import and
-            depth == self.previous_line.depth
+            self.previous_line
+            and self.previous_line.is_import
+            and not current_line.is_import
+            and depth == self.previous_line.depth

         ):
             return (before or 1), 0
 
         if (
         ):
             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)
+            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 or 1), 0
 


@@ -646,10 +995,9 @@ class LineGenerator(Visitor[Line]):
     Note: destroys the tree it's visiting by mutating prefixes of its leaves
     in ways that will no longer stringify to valid Python code on the tree.
     """
     Note: destroys the tree it's visiting by mutating prefixes of its leaves
     in ways that will no longer stringify to valid Python code on the tree.
     """

-    current_line: Line = attrib(default=Factory(Line))
-    standalone_comments: List[Leaf] = attrib(default=Factory(list))
+    current_line: Line = Factory(Line)

 
 

-    def line(self, indent: int = 0) -> Iterator[Line]:
+    def line(self, indent: int = 0, type: Type[Line] = Line) -> Iterator[Line]:

         """Generate a line.
 
         If the line is empty, only emit if it makes sense.
         """Generate a line.
 
         If the line is empty, only emit if it makes sense.


@@ -658,60 +1006,95 @@ class LineGenerator(Visitor[Line]):
         If any lines were generated, set up a new current_line.
         """
         if not self.current_line:
         If any lines were generated, set up a new current_line.
         """
         if not self.current_line:

-            self.current_line.depth += indent
+            if self.current_line.__class__ == type:
+                self.current_line.depth += indent
+            else:
+                self.current_line = type(depth=self.current_line.depth + indent)

             return  # Line is empty, don't emit. Creating a new one unnecessary.
 
         complete_line = self.current_line
             return  # Line is empty, don't emit. Creating a new one unnecessary.
 
         complete_line = self.current_line

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

         yield complete_line
 
         yield complete_line
 

+    def visit(self, node: LN) -> Iterator[Line]:
+        """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)
+
+        else:
+            yield from super().visit(node)
+

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

+        """Default `visit_*()` implementation. Recurses to children of `node`."""

         if isinstance(node, Leaf):
         if isinstance(node, Leaf):

-            for comment in generate_comments(node):
-                if self.current_line.bracket_tracker.any_open_brackets():
-                    # any comment within brackets is subject to splitting
-                    self.current_line.append(comment)
-                elif comment.type == token.COMMENT:
-                    # regular trailing comment
-                    self.current_line.append(comment)
-                    yield from self.line()
+            any_open_brackets = self.current_line.bracket_tracker.any_open_brackets()
+            try:
+                for comment in generate_comments(node):
+                    if any_open_brackets:
+                        # any comment within brackets is subject to splitting
+                        self.current_line.append(comment)
+                    elif comment.type == token.COMMENT:
+                        # regular trailing comment
+                        self.current_line.append(comment)
+                        yield from self.line()
+
+                    else:
+                        # regular standalone comment
+                        yield from self.line()
+
+                        self.current_line.append(comment)
+                        yield from self.line()
+
+            except FormatOff as f_off:
+                f_off.trim_prefix(node)
+                yield from self.line(type=UnformattedLines)
+                yield from self.visit(node)
+
+            except FormatOn as f_on:
+                # This only happens here if somebody says "fmt: on" multiple
+                # times in a row.
+                f_on.trim_prefix(node)
+                yield from self.visit_default(node)

 
 

-                else:
-                    # regular standalone comment, to be processed later (see
-                    # docstring in `generate_comments()`
-                    self.standalone_comments.append(comment)
-            normalize_prefix(node)
-            if node.type not in WHITESPACE:
-                for comment in self.standalone_comments:
-                    yield from self.line()
-
-                    self.current_line.append(comment)
-                    yield from self.line()
-
-                self.standalone_comments = []
-                self.current_line.append(node)
+            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)
 
         yield from super().visit_default(node)
 

-    def visit_suite(self, node: Node) -> Iterator[Line]:
-        """Body of a statement after a colon."""
-        children = iter(node.children)
-        # Process newline before indenting.  It might contain an inline
-        # comment that should go right after the colon.
-        newline = next(children)
-        yield from self.visit(newline)
+    def visit_INDENT(self, node: Node) -> Iterator[Line]:
+        """Increase indentation level, maybe yield a line."""
+        # In blib2to3 INDENT never holds comments.

         yield from self.line(+1)
         yield from self.line(+1)

+        yield from self.visit_default(node)

 
 

-        for child in children:
-            yield from self.visit(child)
-
+    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.

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

-    def visit_stmt(self, node: Node, keywords: Set[str]) -> Iterator[Line]:
+    def visit_stmt(
+        self, node: Node, keywords: Set[str], parens: Set[str]
+    ) -> Iterator[Line]:

         """Visit a statement.
 
         """Visit a statement.
 

-        The relevant Python language keywords for this statement are NAME leaves
-        within it.
+        This implementation is shared for `if`, `while`, `for`, `try`, `except`,
+        `def`, `with`, `class`, and `assert`.
+
+        The relevant Python language `keywords` for a given statement will be
+        NAME leaves within it. This methods puts those on a separate line.
+
+        `parens` holds pairs of nodes where invisible parentheses should be put.
+        Keys hold nodes after which opening parentheses should be put, values
+        hold nodes before which closing parentheses should be put.

         """
         """

+        normalize_invisible_parens(node, parens_after=parens)

         for child in node.children:
             if child.type == token.NAME and child.value in keywords:  # type: ignore
                 yield from self.line()
         for child in node.children:
             if child.type == token.NAME and child.value in keywords:  # type: ignore
                 yield from self.line()


@@ -719,7 +1102,7 @@ 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]:

-        """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)


@@ -731,13 +1114,14 @@ 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]:

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

         yield from self.line()
 
         children = iter(node.children)
         for child in children:
             yield from self.visit(child)
 
         yield from self.line()
 
         children = iter(node.children)
         for child in children:
             yield from self.visit(child)
 

-            if child.type == token.NAME and child.value == 'async':  # type: ignore
+            if child.type == token.ASYNC:

                 break
 
         internal_stmt = next(children)
                 break
 
         internal_stmt = next(children)


@@ -745,96 +1129,165 @@ 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]:

+        """Visit decorators."""

         for child in node.children:
             yield from self.line()
             yield from self.visit(child)
 
         for child in node.children:
             yield from self.line()
             yield from self.visit(child)
 

+    def visit_import_from(self, node: Node) -> Iterator[Line]:
+        """Visit import_from and maybe put invisible parentheses.
+
+        This is separate from `visit_stmt` because import statements don't
+        support arbitrary atoms and thus handling of parentheses is custom.
+        """
+        check_lpar = False
+        for index, child in enumerate(node.children):
+            if check_lpar:
+                if child.type == token.LPAR:
+                    # make parentheses invisible
+                    child.value = ""  # type: ignore
+                    node.children[-1].value = ""  # type: ignore
+                else:
+                    # insert invisible parentheses
+                    node.insert_child(index, Leaf(token.LPAR, ""))
+                    node.append_child(Leaf(token.RPAR, ""))
+                break
+
+            check_lpar = (
+                child.type == token.NAME and child.value == "import"  # type: ignore
+            )
+
+        for child in node.children:
+            yield from self.visit(child)
+

     def visit_SEMI(self, leaf: Leaf) -> Iterator[Line]:
     def visit_SEMI(self, leaf: Leaf) -> Iterator[Line]:

+        """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."""

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

+    def visit_unformatted(self, node: LN) -> Iterator[Line]:
+        """Used when file contained a `# fmt: off`."""
+        if isinstance(node, Node):
+            for child in node.children:
+                yield from self.visit(child)
+
+        else:
+            try:
+                self.current_line.append(node)
+            except FormatOn as f_on:
+                f_on.trim_prefix(node)
+                yield from self.line()
+                yield from self.visit(node)
+
+            if node.type == token.ENDMARKER:
+                # somebody decided not to put a final `# fmt: on`
+                yield from self.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'})
+        Ø: Set[str] = set()
+        self.visit_assert_stmt = partial(v, keywords={"assert"}, parens={"assert", ","})
+        self.visit_if_stmt = partial(v, keywords={"if", "else", "elif"}, parens={"if"})
+        self.visit_while_stmt = partial(v, keywords={"while", "else"}, parens={"while"})
+        self.visit_for_stmt = partial(v, keywords={"for", "else"}, parens={"for", "in"})
+        self.visit_try_stmt = partial(
+            v, keywords={"try", "except", "else", "finally"}, parens=Ø
+        )
+        self.visit_except_clause = partial(v, keywords={"except"}, parens=Ø)
+        self.visit_with_stmt = partial(v, keywords={"with"}, parens=Ø)
+        self.visit_funcdef = partial(v, keywords={"def"}, parens=Ø)
+        self.visit_classdef = partial(v, keywords={"class"}, parens=Ø)

         self.visit_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
 
 

+IMPLICIT_TUPLE = {syms.testlist, syms.testlist_star_expr, syms.exprlist}

 BRACKET = {token.LPAR: token.RPAR, token.LSQB: token.RSQB, token.LBRACE: token.RBRACE}
 OPENING_BRACKETS = set(BRACKET.keys())
 CLOSING_BRACKETS = set(BRACKET.values())
 BRACKETS = OPENING_BRACKETS | CLOSING_BRACKETS
 BRACKET = {token.LPAR: token.RPAR, token.LSQB: token.RSQB, token.LBRACE: token.RBRACE}
 OPENING_BRACKETS = set(BRACKET.keys())
 CLOSING_BRACKETS = set(BRACKET.values())
 BRACKETS = OPENING_BRACKETS | CLOSING_BRACKETS

+ALWAYS_NO_SPACE = CLOSING_BRACKETS | {token.COMMA, STANDALONE_COMMENT}

 
 
 
 

-def whitespace(leaf: Leaf) -> str:
+def whitespace(leaf: Leaf) -> str:  # noqa C901

     """Return whitespace prefix if needed for the given `leaf`."""
     """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

-    if t == token.COLON:
-        return NO
-
-    if t == token.COMMA:
-        return NO
-
-    if t == token.RPAR:
+    if t in ALWAYS_NO_SPACE:

         return NO
 
     if t == token.COMMENT:
         return DOUBLESPACE
 
         return NO
 
     if t == token.COMMENT:
         return DOUBLESPACE
 

-    if t == STANDALONE_COMMENT:
-        return NO
-
-    if t in CLOSING_BRACKETS:
+    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}:

         return NO
 
         return NO
 

-    assert p is not None, f"INTERNAL ERROR: hand-made leaf without parent: {leaf!r}"

     prev = leaf.prev_sibling
     if not prev:
         prevp = preceding_leaf(p)
         if not prevp or prevp.type in OPENING_BRACKETS:
             return NO
 
     prev = leaf.prev_sibling
     if not prev:
         prevp = preceding_leaf(p)
         if not prevp or prevp.type in OPENING_BRACKETS:
             return NO
 

+        if t == token.COLON:
+            return SPACE if prevp.type == token.COMMA else NO
+

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

-            if prevp.parent and prevp.parent.type in {
-                syms.typedargslist,
-                syms.varargslist,
-                syms.parameters,
-                syms.arglist,
-                syms.argument,
-            }:
-                return NO
+            if prevp.parent:
+                if prevp.parent.type in {
+                    syms.arglist, syms.argument, syms.parameters, syms.varargslist
+                }:
+                    return NO
+
+                elif prevp.parent.type == syms.typedargslist:
+                    # A bit hacky: if the equal sign has whitespace, it means we
+                    # previously found it's a typed argument.  So, we're using
+                    # that, too.
+                    return prevp.prefix

 
         elif prevp.type == token.DOUBLESTAR:
 
         elif prevp.type == token.DOUBLESTAR:

-            if prevp.parent and prevp.parent.type in {
-                syms.typedargslist,
-                syms.varargslist,
-                syms.parameters,
-                syms.arglist,
-                syms.dictsetmaker,
-            }:
+            if (
+                prevp.parent
+                and prevp.parent.type in {
+                    syms.arglist,
+                    syms.argument,
+                    syms.dictsetmaker,
+                    syms.parameters,
+                    syms.typedargslist,
+                    syms.varargslist,
+                }
+            ):

                 return NO
 
         elif prevp.type == token.COLON:
                 return NO
 
         elif prevp.type == token.COLON:

-            if prevp.parent and prevp.parent.type == syms.subscript:
+            if prevp.parent and prevp.parent.type in {syms.subscript, syms.sliceop}:

                 return NO
 
                 return NO
 

-        elif prevp.parent and prevp.parent.type == syms.factor:
+        elif (
+            prevp.parent
+            and prevp.parent.type in {syms.factor, syms.star_expr}
+            and prevp.type in MATH_OPERATORS
+        ):
+            return NO
+
+        elif (
+            prevp.type == token.RIGHTSHIFT
+            and prevp.parent
+            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
+        ):
+            # Python 2 print chevron

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


@@ -848,7 +1301,7 @@ def whitespace(leaf: Leaf) -> str:
         if not prev or prev.type != token.COMMA:
             return NO
 
         if not prev or prev.type != token.COMMA:
             return NO
 

-    if p.type == syms.varargslist:
+    elif p.type == syms.varargslist:

         # lambdas
         if t == token.RPAR:
             return NO
         # lambdas
         if t == token.RPAR:
             return NO


@@ -931,7 +1384,14 @@ def whitespace(leaf: Leaf) -> str:
 
     elif p.type == syms.subscript:
         # indexing
 
     elif p.type == syms.subscript:
         # indexing

-        if not prev or prev.type == token.COLON:
+        if not prev:
+            assert p.parent is not None, "subscripts are always parented"
+            if p.parent.type == syms.subscriptlist:
+                return SPACE
+
+            return NO
+
+        else:

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


@@ -940,9 +1400,9 @@ def whitespace(leaf: Leaf) -> str:
             return NO
 
     elif (
             return NO
 
     elif (

-        p.type == syms.listmaker or
-        p.type == syms.testlist_gexp or
-        p.type == syms.subscriptlist
+        p.type == syms.listmaker
+        or p.type == syms.testlist_gexp
+        or p.type == syms.subscriptlist

     ):
         # list interior, including unpacking
         if not prev:
     ):
         # list interior, including unpacking
         if not prev:


@@ -956,7 +1416,7 @@ def whitespace(leaf: Leaf) -> str:
         if prev.type == token.DOUBLESTAR:
             return NO
 
         if prev.type == token.DOUBLESTAR:
             return NO
 

-    elif p.type == syms.factor or p.type == syms.star_expr:
+    elif p.type in {syms.factor, syms.star_expr}:

         # unary ops
         if not prev:
             prevp = preceding_leaf(p)
         # unary ops
         if not prev:
             prevp = preceding_leaf(p)


@@ -965,9 +1425,10 @@ def whitespace(leaf: Leaf) -> str:
 
             prevp_parent = prevp.parent
             assert prevp_parent is not None
 
             prevp_parent = prevp.parent
             assert prevp_parent is not None

-            if prevp.type == token.COLON and prevp_parent.type in {
-                syms.subscript, syms.sliceop
-            }:
+            if (
+                prevp.type == token.COLON
+                and prevp_parent.type in {syms.subscript, syms.sliceop}
+            ):

                 return NO
 
             elif prevp.type == token.EQUAL and prevp_parent.type == syms.argument:
                 return NO
 
             elif prevp.type == token.EQUAL and prevp_parent.type == syms.argument:


@@ -982,7 +1443,7 @@ def whitespace(leaf: Leaf) -> str:
                 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:


@@ -995,7 +1456,7 @@ def whitespace(leaf: Leaf) -> str:
 
 
 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:


@@ -1012,32 +1473,89 @@ 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 == token.NAME and leaf.value in LOGIC_OPERATORS:
-        return LOGIC_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 VARARGS
+        and leaf.parent
+        and leaf.parent.type in {syms.argument, syms.typedargslist, syms.dictsetmaker}
+    ):
+        # * and ** might also be MATH_OPERATORS but in this case they are not.
+        # Don't treat them as a delimiter.
+        return 0
+
+    if (
+        leaf.type in MATH_OPERATORS
+        and leaf.parent
+        and leaf.parent.type not in {syms.factor, syms.star_expr}
+    ):
+        return MATH_PRIORITY

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

-        leaf.type in MATH_OPERATORS and
-        leaf.parent and
-        leaf.parent.type not in {syms.factor, syms.star_expr}
+        leaf.type == token.STRING
+        and previous is not None
+        and previous.type == token.STRING

     ):
     ):

-        return MATH_PRIORITY
+        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


@@ -1055,53 +1573,94 @@ def generate_comments(leaf: Leaf) -> Iterator[Leaf]:
     Inline comments are emitted as regular token.COMMENT leaves.  Standalone
     are emitted with a fake STANDALONE_COMMENT token identifier.
     """
     Inline comments are emitted as regular token.COMMENT leaves.  Standalone
     are emitted with a fake STANDALONE_COMMENT token identifier.
     """

-    if not leaf.prefix:
+    p = leaf.prefix
+    if not p:

         return
 
         return
 

-    if '#' not in leaf.prefix:
+    if "#" not in p:

         return
 
         return
 

-    before_comment, content = leaf.prefix.split('#', 1)
-    content = content.rstrip()
-    if content and (content[0] not in {' ', '!', '#'}):
-        content = ' ' + content
-    is_standalone_comment = (
-        '\n' in before_comment or '\n' in content or leaf.type == token.DEDENT
-    )
-    if not is_standalone_comment:
-        # simple trailing comment
-        yield Leaf(token.COMMENT, value='#' + content)
-        return
-
-    for line in ('#' + content).split('\n'):
+    consumed = 0
+    nlines = 0
+    for index, line in enumerate(p.split("\n")):
+        consumed += len(line) + 1  # adding the length of the split '\n'

         line = line.lstrip()
         line = line.lstrip()

-        if not line.startswith('#'):
+        if not line:
+            nlines += 1
+        if not line.startswith("#"):

             continue
 
             continue
 

-        yield Leaf(STANDALONE_COMMENT, line)
+        if index == 0 and leaf.type != token.ENDMARKER:
+            comment_type = token.COMMENT  # simple trailing comment
+        else:
+            comment_type = STANDALONE_COMMENT
+        comment = make_comment(line)
+        yield Leaf(comment_type, comment, prefix="\n" * nlines)
+
+        if comment in {"# fmt: on", "# yapf: enable"}:
+            raise FormatOn(consumed)

 
 

+        if comment in {"# fmt: off", "# yapf: disable"}:
+            if comment_type == STANDALONE_COMMENT:
+                raise FormatOff(consumed)

 
 

-def split_line(line: Line, line_length: int, inner: bool = False) -> Iterator[Line]:
-    """Splits a `line` into potentially many lines.
+            prev = preceding_leaf(leaf)
+            if not prev or prev.type in WHITESPACE:  # standalone comment in disguise
+                raise FormatOff(consumed)
+
+        nlines = 0
+
+
+def make_comment(content: str) -> str:
+    """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.
+
+    If `content` didn't start with a hash sign, one is provided.
+    """
+    content = content.rstrip()
+    if not content:
+        return "#"
+
+    if content[0] == "#":
+        content = content[1:]
+    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]:
+    """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
     current `line`, possibly transitively. This means we can fallback to splitting
     by delimiters if the LHS/RHS don't yield any results.
 
     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
     current `line`, possibly transitively. This means we can fallback to splitting
     by delimiters if the LHS/RHS don't yield any results.

+
+    If `py36` is True, splitting may generate syntax that is only compatible
+    with Python 3.6 and later.

     """
     """

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


@@ -1110,11 +1669,13 @@ def split_line(line: Line, line_length: int, inner: bool = False) -> Iterator[Li
         # split altogether.
         result: List[Line] = []
         try:
         # split altogether.
         result: List[Line] = []
         try:

-            for l in split_func(line):
-                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")
 
                     raise CannotSplit("Split function returned an unchanged result")
 

-                result.extend(split_line(l, line_length=line_length, inner=True))
+                result.extend(
+                    split_line(l, line_length=line_length, inner=True, py36=py36)
+                )

         except CannotSplit as cs:
             continue
 
         except CannotSplit as cs:
             continue
 


@@ -1126,7 +1687,7 @@ def split_line(line: Line, line_length: int, inner: bool = False) -> Iterator[Li
         yield line
 
 
         yield line
 
 

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

     """Split line into many lines, starting with the first matching bracket pair.
 
     Note: this usually looks weird, only use this for function definitions.
     """Split line into many lines, starting with the first matching bracket pair.
 
     Note: this usually looks weird, only use this for function definitions.


@@ -1142,11 +1703,11 @@ def left_hand_split(line: Line) -> Iterator[Line]:
     matching_bracket = None
     for leaf in line.leaves:
         if (
     matching_bracket = None
     for leaf in line.leaves:
         if (

-            current_leaves is body_leaves and
-            leaf.type in CLOSING_BRACKETS and
-            leaf.opening_bracket is matching_bracket  # type: ignore
+            current_leaves is body_leaves
+            and leaf.type in CLOSING_BRACKETS
+            and leaf.opening_bracket is matching_bracket

         ):
         ):

-            current_leaves = tail_leaves
+            current_leaves = tail_leaves if body_leaves else head_leaves

         current_leaves.append(leaf)
         if current_leaves is head_leaves:
             if leaf.type in OPENING_BRACKETS:
         current_leaves.append(leaf)
         if current_leaves is head_leaves:
             if leaf.type in OPENING_BRACKETS:


@@ -1154,34 +1715,22 @@ def left_hand_split(line: Line) -> Iterator[Line]:
                 current_leaves = body_leaves
     # Since body is a new indent level, remove spurious leading whitespace.
     if body_leaves:
                 current_leaves = body_leaves
     # Since body is a new indent level, remove spurious leading whitespace.
     if body_leaves:

-        normalize_prefix(body_leaves[0])
+        normalize_prefix(body_leaves[0], inside_brackets=True)

     # Build the new lines.
     # Build the new lines.

-    for result, leaves in (
-        (head, head_leaves), (body, body_leaves), (tail, tail_leaves)
-    ):
+    for result, leaves in (head, head_leaves), (body, body_leaves), (tail, tail_leaves):

         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)
                 result.append(comment_after, preformatted=True)

-    # Check if the split succeeded.
-    tail_len = len(str(tail))
-    if not body:
-        if tail_len == 0:
-            raise CannotSplit("Splitting brackets produced the same line")
-
-        elif tail_len < 3:
-            raise CannotSplit(
-                f"Splitting brackets on an empty body to save "
-                f"{tail_len} characters is not worth it"
-            )
-
+    bracket_split_succeeded_or_raise(head, body, tail)

     for result in (head, body, tail):
         if result:
             yield result
 
 
     for result in (head, body, tail):
         if result:
             yield result
 
 

-def right_hand_split(line: Line) -> Iterator[Line]:
+def right_hand_split(
+    line: Line, py36: bool = False, omit: Collection[LeafID] = ()
+) -> Iterator[Line]:

     """Split line into many lines, starting with the last matching bracket pair."""
     head = Line(depth=line.depth)
     body = Line(depth=line.depth + 1, inside_brackets=True)
     """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)


@@ -1191,32 +1740,73 @@ def right_hand_split(line: Line) -> Iterator[Line]:
     head_leaves: List[Leaf] = []
     current_leaves = tail_leaves
     opening_bracket = None
     head_leaves: List[Leaf] = []
     current_leaves = tail_leaves
     opening_bracket = None

+    closing_bracket = None

     for leaf in reversed(line.leaves):
         if current_leaves is body_leaves:
             if leaf is opening_bracket:
     for leaf in reversed(line.leaves):
         if current_leaves is body_leaves:
             if leaf is opening_bracket:

-                current_leaves = head_leaves
+                current_leaves = head_leaves if body_leaves else tail_leaves

         current_leaves.append(leaf)
         if current_leaves is tail_leaves:
         current_leaves.append(leaf)
         if current_leaves is tail_leaves:

-            if leaf.type in CLOSING_BRACKETS:
-                opening_bracket = leaf.opening_bracket  # type: ignore
+            if leaf.type in CLOSING_BRACKETS and id(leaf) not in omit:
+                opening_bracket = leaf.opening_bracket
+                closing_bracket = leaf

                 current_leaves = body_leaves
     tail_leaves.reverse()
     body_leaves.reverse()
     head_leaves.reverse()
     # Since body is a new indent level, remove spurious leading whitespace.
     if body_leaves:
                 current_leaves = body_leaves
     tail_leaves.reverse()
     body_leaves.reverse()
     head_leaves.reverse()
     # Since body is a new indent level, remove spurious leading whitespace.
     if body_leaves:

-        normalize_prefix(body_leaves[0])
+        normalize_prefix(body_leaves[0], inside_brackets=True)
+    elif not head_leaves:
+        # No `head` and no `body` means the split failed. `tail` has all content.
+        raise CannotSplit("No brackets found")
+

     # Build the new lines.
     # Build the new lines.

-    for result, leaves in (
-        (head, head_leaves), (body, body_leaves), (tail, tail_leaves)
-    ):
+    for result, leaves in (head, head_leaves), (body, body_leaves), (tail, tail_leaves):

         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)
                 result.append(comment_after, preformatted=True)

-    # Check if the split succeeded.
-    tail_len = len(str(tail).strip('\n'))
+    bracket_split_succeeded_or_raise(head, body, tail)
+    assert opening_bracket and closing_bracket
+    if (
+        opening_bracket.type == token.LPAR
+        and not opening_bracket.value
+        and closing_bracket.type == token.RPAR
+        and not closing_bracket.value
+    ):
+        # These parens were optional. If there aren't any delimiters or standalone
+        # comments in the body, they were unnecessary and another split without
+        # them should be attempted.
+        if not (
+            body.bracket_tracker.delimiters or line.contains_standalone_comments(0)
+        ):
+            omit = {id(closing_bracket), *omit}
+            yield from right_hand_split(line, py36=py36, omit=omit)
+            return
+
+    ensure_visible(opening_bracket)
+    ensure_visible(closing_bracket)
+    for result in (head, body, tail):
+        if result:
+            yield result
+
+
+def bracket_split_succeeded_or_raise(head: Line, body: Line, tail: Line) -> None:
+    """Raise :exc:`CannotSplit` if the last left- or right-hand split failed.
+
+    Do nothing otherwise.
+
+    A left- or right-hand split is based on a pair of brackets. Content before
+    (and including) the opening bracket is left on one line, content inside the
+    brackets is put on a separate line, and finally content starting with and
+    following the closing bracket is put on a separate line.
+
+    Those are called `head`, `body`, and `tail`, respectively. If the split
+    produced the same line (all content in `head`) or ended up with an empty `body`
+    and the `tail` is just the closing bracket, then it's considered failed.
+    """
+    tail_len = len(str(tail).strip())

     if not body:
         if tail_len == 0:
             raise CannotSplit("Splitting brackets produced the same line")
     if not body:
         if tail_len == 0:
             raise CannotSplit("Splitting brackets produced the same line")


@@ -1227,15 +1817,28 @@ def right_hand_split(line: Line) -> Iterator[Line]:
                 f"{tail_len} characters is not worth it"
             )
 
                 f"{tail_len} characters is not worth it"
             )
 

-    for result in (head, body, tail):
-        if result:
-            yield result
+
+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

 
 
 
 

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

     """Split 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 `**`.

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


@@ -1244,67 +1847,314 @@ def delimiter_split(line: Line) -> Iterator[Line]:
 
     delimiters = line.bracket_tracker.delimiters
     try:
 
     delimiters = line.bracket_tracker.delimiters
     try:

-        delimiter_priority = line.bracket_tracker.max_priority(exclude={id(last_leaf)})
+        delimiter_priority = line.bracket_tracker.max_delimiter_priority(
+            exclude={id(last_leaf)}
+        )

     except ValueError:
         raise CannotSplit("No delimiters found")
 
     current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)
     except ValueError:
         raise CannotSplit("No delimiters found")
 
     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
+            and leaf.type == token.STAR
+            or leaf.type == token.DOUBLESTAR
+        ):
+            trailing_comma_safe = trailing_comma_safe and py36

         leaf_priority = delimiters.get(id(leaf))
         if leaf_priority == delimiter_priority:
         leaf_priority = delimiters.get(id(leaf))
         if leaf_priority == delimiter_priority:

-            normalize_prefix(current_line.leaves[0])

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

-            delimiter_priority == COMMA_PRIORITY and
-            current_line.leaves[-1].type != token.COMMA
+            trailing_comma_safe
+            and delimiter_priority == COMMA_PRIORITY
+            and current_line.leaves[-1].type != token.COMMA
+            and current_line.leaves[-1].type != STANDALONE_COMMENT

         ):
         ):

-            current_line.append(Leaf(token.COMMA, ','))
-        normalize_prefix(current_line.leaves[0])
+            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."""
+    if not line.contains_standalone_comments(0):
+        raise CannotSplit("Line does not have any standalone comments")
+
+    current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)
+
+    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(
     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)
+        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)

         )
     )
 
 
         )
     )
 
 

-def normalize_prefix(leaf: Leaf) -> None:
-    """Leave existing extra newlines for imports.  Remove everything else."""
-    if is_import(leaf):
-        spl = leaf.prefix.split('#', 1)
-        nl_count = spl[0].count('\n')
-        if len(spl) > 1:
-            # Skip one newline since it was for a standalone comment.
-            nl_count -= 1
-        leaf.prefix = '\n' * nl_count
+def normalize_prefix(leaf: Leaf, *, inside_brackets: bool) -> None:
+    """Leave existing extra newlines if not `inside_brackets`. Remove everything
+    else.
+
+    Note: don't use backslashes for formatting or you'll lose your voting rights.
+    """
+    if not inside_brackets:
+        spl = leaf.prefix.split("#")
+        if "\\" not in spl[0]:
+            nl_count = spl[-1].count("\n")
+            if len(spl) > 1:
+                nl_count -= 1
+            leaf.prefix = "\n" * nl_count
+            return
+
+    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
 
         return
 

-    leaf.prefix = ''
+    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
+
+    prefix = leaf.value[:first_quote_pos]
+    body = leaf.value[first_quote_pos + len(orig_quote):-len(orig_quote)]
+    unescaped_new_quote = re.compile(rf"(([^\\]|^)(\\\\)*){new_quote}")
+    escaped_orig_quote = re.compile(rf"\\(\\\\)*{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
+            # so converting is impossible
+            return
+
+        # Do not introduce or remove backslashes in raw strings
+        new_body = body
+    else:
+        new_body = escaped_orig_quote.sub(rf"\1{orig_quote}", body)
+        new_body = unescaped_new_quote.sub(rf"\1\\{new_quote}", new_body)
+        # Add escapes again for consecutive occurences of new_quote (sub
+        # doesn't match overlapping substrings).
+        new_body = unescaped_new_quote.sub(rf"\1\\{new_quote}", new_body)
+    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
+
+    leaf.value = f"{prefix}{new_quote}{new_body}{new_quote}"
+
+
+def normalize_invisible_parens(node: Node, parens_after: Set[str]) -> None:
+    """Make existing optional parentheses invisible or create new ones.
+
+    Standardizes on visible parentheses for single-element tuples, and keeps
+    existing visible parentheses for other tuples and generator expressions.
+    """
+    check_lpar = False
+    for child in list(node.children):
+        if check_lpar:
+            if child.type == syms.atom:
+                if not (
+                    is_empty_tuple(child)
+                    or is_one_tuple(child)
+                    or max_delimiter_priority_in_atom(child) >= COMMA_PRIORITY
+                ):
+                    first = child.children[0]
+                    last = child.children[-1]
+                    if first.type == token.LPAR and last.type == token.RPAR:
+                        # make parentheses invisible
+                        first.value = ""  # type: ignore
+                        last.value = ""  # type: ignore
+            elif is_one_tuple(child):
+                # wrap child in visible parentheses
+                lpar = Leaf(token.LPAR, "(")
+                rpar = Leaf(token.RPAR, ")")
+                index = child.remove() or 0
+                node.insert_child(index, Node(syms.atom, [lpar, child, rpar]))
+            else:
+                # wrap child in invisible parentheses
+                lpar = Leaf(token.LPAR, "")
+                rpar = Leaf(token.RPAR, "")
+                index = child.remove() or 0
+                node.insert_child(index, Node(syms.atom, [lpar, child, rpar]))
+
+        check_lpar = isinstance(child, Leaf) and child.value in parens_after
+
+
+def is_empty_tuple(node: LN) -> bool:
+    """Return True if `node` holds an empty tuple."""
+    return (
+        node.type == syms.atom
+        and len(node.children) == 2
+        and node.children[0].type == token.LPAR
+        and node.children[1].type == token.RPAR
+    )
+
+
+def is_one_tuple(node: LN) -> bool:
+    """Return True if `node` holds a tuple with one element, with or without parens."""
+    if node.type == syms.atom:
+        if len(node.children) != 3:
+            return False

 
 

+        lpar, gexp, rpar = node.children
+        if not (
+            lpar.type == token.LPAR
+            and gexp.type == syms.testlist_gexp
+            and rpar.type == token.RPAR
+        ):
+            return False
+
+        return len(gexp.children) == 2 and gexp.children[1].type == token.COMMA
+
+    return (
+        node.type in IMPLICIT_TUPLE
+        and len(node.children) == 2
+        and node.children[1].type == token.COMMA
+    )

 
 

-PYTHON_EXTENSIONS = {'.py'}
+
+def max_delimiter_priority_in_atom(node: LN) -> int:
+    if node.type != syms.atom:
+        return 0
+
+    first = node.children[0]
+    last = node.children[-1]
+    if not (first.type == token.LPAR and last.type == token.RPAR):
+        return 0
+
+    bt = BracketTracker()
+    for c in node.children[1:-1]:
+        if isinstance(c, Leaf):
+            bt.mark(c)
+        else:
+            for leaf in c.leaves():
+                bt.mark(leaf)
+    try:
+        return bt.max_delimiter_priority()
+
+    except ValueError:
+        return 0
+
+
+def ensure_visible(leaf: Leaf) -> None:
+    """Make sure parentheses are visible.
+
+    They could be invisible as part of some statements (see
+    :func:`normalize_invible_parens` and :func:`visit_import_from`).
+    """
+    if leaf.type == token.LPAR:
+        leaf.value = "("
+    elif leaf.type == token.RPAR:
+        leaf.value = ")"
+
+
+def is_python36(node: Node) -> bool:
+    """Return True if the current file is using Python 3.6+ features.
+
+    Currently looking for:
+    - f-strings; and
+    - trailing commas after * or ** in function signatures.
+    """
+    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"}:
+                return True
+
+        elif (
+            n.type == syms.typedargslist
+            and n.children
+            and n.children[-1].type == token.COMMA
+        ):
+            for ch in n.children:
+                if ch.type == token.STAR or ch.type == token.DOUBLESTAR:
+                    return True
+
+    return False
+
+
+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]:

+    """Generate all files under `path` which aren't under BLACKLISTED_DIRECTORIES
+    and have one of the PYTHON_EXTENSIONS.
+    """

     for child in path.iterdir():
         if child.is_dir():
             if child.name in BLACKLISTED_DIRECTORIES:
     for child in path.iterdir():
         if child.is_dir():
             if child.name in BLACKLISTED_DIRECTORIES:


@@ -1318,67 +2168,81 @@ def gen_python_files_in_dir(path: Path) -> Iterator[Path]:
 
 @dataclass
 class Report:
 
 @dataclass
 class Report:

-    """Provides a reformatting counter."""
-    change_count: int = attrib(default=0)
-    same_count: int = attrib(default=0)
-    failure_count: int = attrib(default=0)
+    """Provides a reformatting counter. Can be rendered with `str(report)`."""
+    check: bool = False
+    quiet: bool = False
+    change_count: int = 0
+    same_count: int = 0
+    failure_count: int = 0

 
     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:

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

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

-            out(f'{src} already well formatted, good job.', bold=False)
+            if not self.quiet:
+                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:
             return 123
 
         # 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:
             return 123
 

-        elif self.change_count:
+        elif self.change_count and self.check:

             return 1
 
         return 0
 
     def __str__(self) -> str:
             return 1
 
         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.
         """

+        if self.check:
+            reformatted = "would be reformatted"
+            unchanged = "would be left unchanged"
+            failed = "would fail to reformat"
+        else:
+            reformatted = "reformatted"
+            unchanged = "left unchanged"
+            failed = "failed to reformat"

         report = []
         if self.change_count:
         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} left 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 to reformat', 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


@@ -1411,72 +2275,104 @@ def assert_equivalent(src: str, dst: str) -> None:
     try:
         src_ast = ast.parse(src)
     except Exception as exc:
     try:
         src_ast = ast.parse(src)
     except Exception as exc:

-        raise AssertionError(f"cannot parse source: {exc}") from None
+        major, minor = sys.version_info[:2]
+        raise AssertionError(
+            f"cannot use --safe with this file; failed to parse source file "
+            f"with Python {major}.{minor}'s builtin AST. Re-run with --fast "
+            f"or stop using deprecated Python 2 syntax. AST error message: {exc}"
+        )

 
     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.  "
         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}",
+            f"This invalid output might be helpful: {log}"

         ) from None
 
         ) 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.  "
             f"Please report a bug on https://github.com/ambv/black/issues.  "
         raise AssertionError(
             f"INTERNAL ERROR: Black produced code that is not equivalent to "
             f"the source.  "
             f"Please report a bug on https://github.com/ambv/black/issues.  "

-            f"This diff might be helpful: {log}",
+            f"This diff might be helpful: {log}"

         ) from None
 
 
 def assert_stable(src: str, dst: str, line_length: int) -> None:
         ) from 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 "
             f"of the formatter.  "
             f"Please report a bug on https://github.com/ambv/black/issues.  "
         )
         raise AssertionError(
             f"INTERNAL ERROR: Black produced different code on the second pass "
             f"of the formatter.  "
             f"Please report a bug on https://github.com/ambv/black/issues.  "

-            f"This diff might be helpful: {log}",
+            f"This diff might be helpful: {log}"

         ) from None
 
 
 def dump_to_file(*output: str) -> str:
         ) from None
 
 
 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()