X-Git-Url: https://git.madduck.net/etc/vim.git/blobdiff_plain/2fa31ff31469e587ea23cb86308495c4673b5ddd..4dfec562ed3332212cb938d6d4da9671b503ac93:/black.py?ds=sidebyside

diff --git a/black.py b/black.py
index b2b2db7..82fe5d1 100644
--- a/black.py
+++ b/black.py
@@ -1,15 +1,29 @@
 #!/usr/bin/env python3
+
 import asyncio
 from asyncio.base_events import BaseEventLoop
 from concurrent.futures import Executor, ProcessPoolExecutor
-from functools import partial
+from functools import partial, wraps
 import keyword
+import logging
 import os
 from pathlib import Path
 import tokenize
+import signal
 import sys
 from typing import (
-    Dict, Generic, Iterable, Iterator, List, Optional, Set, Tuple, TypeVar, Union
+    Callable,
+    Dict,
+    Generic,
+    Iterable,
+    Iterator,
+    List,
+    Optional,
+    Set,
+    Tuple,
+    Type,
+    TypeVar,
+    Union,
 )
 
 from attr import dataclass, Factory
@@ -21,7 +35,7 @@ from blib2to3 import pygram, pytree
 from blib2to3.pgen2 import driver, token
 from blib2to3.pgen2.parse import ParseError
 
-__version__ = "18.3a2"
+__version__ = "18.3a4"
 DEFAULT_LINE_LENGTH = 88
 # types
 syms = pygram.python_symbols
@@ -31,21 +45,52 @@ Depth = int
 NodeType = int
 LeafID = int
 Priority = int
+Index = int
 LN = Union[Leaf, Node]
+SplitFunc = Callable[['Line', bool], Iterator['Line']]
 out = partial(click.secho, bold=True, err=True)
 err = partial(click.secho, fg='red', err=True)
 
 
 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.
 
-    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."""
+
 
 @click.command()
 @click.option(
@@ -61,7 +106,7 @@ class CannotSplit(Exception):
     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 "
+        "means nothing would change.  Return code 1 means some files would be "
         "reformatted.  Return code 123 means there was an internal error."
     ),
 )
@@ -74,7 +119,9 @@ class CannotSplit(Exception):
 @click.argument(
     'src',
     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(
@@ -89,17 +136,24 @@ def main(
         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:
             err(f'invalid path: {s}')
     if len(sources) == 0:
         ctx.exit(0)
     elif len(sources) == 1:
         p = sources[0]
-        report = Report()
+        report = Report(check=check)
         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=not check
+                )
+            else:
+                changed = format_file_in_place(
+                    p, line_length=line_length, fast=fast, write_back=not check
+                )
             report.done(p, changed)
         except Exception as exc:
             report.failed(p, str(exc))
@@ -115,7 +169,7 @@ def main(
                 )
             )
         finally:
-            loop.close()
+            shutdown(loop)
             ctx.exit(return_code)
 
 
@@ -127,27 +181,40 @@ async def schedule_formatting(
     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`.
+    """
     tasks = {
         src: loop.run_in_executor(
             executor, format_file_in_place, src, line_length, fast, write_back
         )
         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 = []
-    report = Report()
+    report = Report(check=not write_back)
     for src, task in tasks.items():
         if not task.done():
             report.failed(src, 'timed out, cancelling')
             task.cancel()
             cancelled.append(task)
+        elif task.cancelled():
+            cancelled.append(task)
         elif task.exception():
             report.failed(src, str(task.exception()))
         else:
             report.done(src, task.result())
     if cancelled:
-        await asyncio.wait(cancelled, timeout=2)
-    out('All done! ✨ 🍰 ✨')
+        await asyncio.gather(*cancelled, loop=loop, return_exceptions=True)
+    else:
+        out('All done! ✨ 🍰 ✨')
     click.echo(str(report))
     return report.return_code
 
@@ -155,42 +222,76 @@ async def schedule_formatting(
 def format_file_in_place(
     src: Path, line_length: int, fast: bool, write_back: bool = False
 ) -> 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:
-        contents, encoding = format_file(src, line_length=line_length, fast=fast)
+        contents = format_file_contents(
+            src_contents, line_length=line_length, fast=fast
+        )
     except NothingChanged:
         return False
 
     if write_back:
-        with open(src, "w", encoding=encoding) as f:
+        with open(src, "w", encoding=src_buffer.encoding) as f:
             f.write(contents)
     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()
+def format_stdin_to_stdout(
+    line_length: int, fast: bool, write_back: bool = False
+) -> bool:
+    """Format file on stdin. Return True if changed.
+
+    If `write_back` is True, write reformatted code back to stdout.
+    `line_length` and `fast` arguments are passed to :func:`format_file_contents`.
+    """
+    contents = sys.stdin.read()
+    try:
+        contents = format_file_contents(contents, line_length=line_length, fast=fast)
+        return True
+
+    except NothingChanged:
+        return False
+
+    finally:
+        if write_back:
+            sys.stdout.write(contents)
+
+
+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(src)
+        raise NothingChanged
 
     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)
-    return dst_contents, src_buffer.encoding
+    return dst_contents
 
 
 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 = ""
-    comments: List[Line] = []
     lines = LineGenerator()
     elt = EmptyLineTracker()
     py36 = is_python36(src_node)
@@ -202,41 +303,41 @@ 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)
-        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, py36=py36):
-                dst_contents += str(line)
-        else:
-            comments.append(current_line)
-    if comments:
-        if elt.previous_defs:
-            # Separate postscriptum comments from the last module-level def.
-            dst_contents += str(empty_line)
-            dst_contents += str(empty_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
 
 
+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
-    drv = driver.Driver(grammar, pytree.convert)
     if src_txt[-1] != '\n':
         nl = '\r\n' if '\r\n' in src_txt[:1024] else '\n'
         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:
-            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])
@@ -253,9 +354,18 @@ T = TypeVar('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]:
+        """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:
@@ -263,6 +373,7 @@ class Visitor(Generic[T]):
         yield from getattr(self, f'visit_{name}', self.visit_default)(node)
 
     def visit_default(self, node: LN) -> Iterator[T]:
+        """Default `visit_*()` implementation. Recurses to children of `node`."""
         if isinstance(node, Node):
             for child in node.children:
                 yield from self.visit(child)
@@ -292,6 +403,15 @@ class DebugVisitor(Visitor[T]):
                 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}
@@ -325,6 +445,7 @@ MATH_OPERATORS = {
     token.AMPER,
     token.PERCENT,
     token.CIRCUMFLEX,
+    token.TILDE,
     token.LEFTSHIFT,
     token.RIGHTSHIFT,
     token.DOUBLESTAR,
@@ -340,12 +461,28 @@ MATH_PRIORITY = 1
 
 @dataclass
 class BracketTracker:
+    """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:
+        """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
 
@@ -387,11 +524,11 @@ class BracketTracker:
         self.previous = leaf
 
     def any_open_brackets(self) -> bool:
-        """Returns True if there is an yet unmatched open bracket on the line."""
+        """Return True if there is an yet unmatched open bracket on the line."""
         return bool(self.bracket_match)
 
-    def max_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.
         """
@@ -400,15 +537,26 @@ class BracketTracker:
 
 @dataclass
 class Line:
+    """Holds leaves and comments. Can be printed with `str(line)`."""
+
     depth: int = 0
     leaves: List[Leaf] = Factory(list)
-    comments: Dict[LeafID, Leaf] = Factory(dict)
+    comments: List[Tuple[Index, Leaf]] = Factory(list)
     bracket_tracker: BracketTracker = Factory(BracketTracker)
     inside_brackets: bool = False
     has_for: bool = False
     _for_loop_variable: bool = False
 
     def append(self, leaf: Leaf, preformatted: bool = False) -> None:
+        """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.value.strip()
         if not has_value:
             return
@@ -422,26 +570,45 @@ class Line:
             self.bracket_tracker.mark(leaf)
             self.maybe_remove_trailing_comma(leaf)
             self.maybe_increment_for_loop_variable(leaf)
-            if self.maybe_adapt_standalone_comment(leaf):
-                return
 
         if not self.append_comment(leaf):
             self.leaves.append(leaf)
 
+    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:
-        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:
+        """Is this line a decorator?"""
         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:
+        """Is this line a class definition?"""
         return (
             bool(self)
             and self.leaves[0].type == token.NAME
@@ -450,7 +617,7 @@ class Line:
 
     @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:
@@ -463,8 +630,7 @@ class Line:
         return (
             (first_leaf.type == token.NAME and first_leaf.value == 'def')
             or (
-                first_leaf.type == token.NAME
-                and first_leaf.value == 'async'
+                first_leaf.type == token.ASYNC
                 and second_leaf is not None
                 and second_leaf.type == token.NAME
                 and second_leaf.value == 'def'
@@ -473,6 +639,10 @@ class Line:
 
     @property
     def is_flow_control(self) -> bool:
+        """Is this line a flow control statement?
+
+        Those are `return`, `raise`, `break`, and `continue`.
+        """
         return (
             bool(self)
             and self.leaves[0].type == token.NAME
@@ -481,13 +651,24 @@ class Line:
 
     @property
     def is_yield(self) -> bool:
+        """Is this line a yield statement?"""
         return (
             bool(self)
             and self.leaves[0].type == token.NAME
             and self.leaves[0].value == 'yield'
         )
 
+    @property
+    def contains_standalone_comments(self) -> bool:
+        """If so, needs to be split before emitting."""
+        for leaf in self.leaves:
+            if leaf.type == STANDALONE_COMMENT:
+                return True
+
+        return False
+
     def maybe_remove_trailing_comma(self, closing: Leaf) -> bool:
+        """Remove trailing comma if there is one and it's safe."""
         if not (
             self.leaves
             and self.leaves[-1].type == token.COMMA
@@ -495,10 +676,16 @@ class Line:
         ):
             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
 
+        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.
@@ -524,7 +711,7 @@ class Line:
                     break
 
         if commas > 1:
-            self.leaves.pop()
+            self.remove_trailing_comma()
             return True
 
         return False
@@ -532,8 +719,8 @@ class Line:
     def maybe_increment_for_loop_variable(self, leaf: Leaf) -> bool:
         """In a for loop, or comprehension, the variables are often unpacks.
 
-        To avoid splitting on the comma in this situation, we will increase
-        the depth of tokens between `for` and `in`.
+        To avoid splitting on the comma in this situation, increase the depth of
+        tokens between `for` and `in`.
         """
         if leaf.type == token.NAME and leaf.value == 'for':
             self.has_for = True
@@ -544,7 +731,7 @@ class Line:
         return False
 
     def maybe_decrement_after_for_loop_variable(self, leaf: Leaf) -> bool:
-        # See `maybe_increment_for_loop_variable` above for explanation.
+        """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
@@ -552,52 +739,52 @@ class Line:
 
         return False
 
-    def maybe_adapt_standalone_comment(self, comment: Leaf) -> bool:
-        """Hack a standalone comment to act as a trailing comment for line splitting.
-
-        If this line has brackets and a standalone `comment`, we need to adapt
-        it to be able to still reformat the line.
-
-        This is not perfect, the line to which the standalone comment gets
-        appended will appear "too long" when splitting.
-        """
-        if not (
+    def append_comment(self, comment: Leaf) -> bool:
+        """Add an inline or standalone comment to the line."""
+        if (
             comment.type == STANDALONE_COMMENT
             and self.bracket_tracker.any_open_brackets()
         ):
+            comment.prefix = ''
             return False
 
-        comment.type = token.COMMENT
-        comment.prefix = '\n' + '    ' * (self.depth + 1)
-        return self.append_comment(comment)
-
-    def append_comment(self, comment: Leaf) -> bool:
         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.prefix = ''
             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
 
-    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
 
-        raise LookupError("No non-delimiters found")
+        else:
+            return
+
+        for index, comment_after in self.comments:
+            if _leaf_index == index:
+                yield comment_after
+
+    def remove_trailing_comma(self) -> None:
+        """Remove the trailing comma and moves the comments attached to it."""
+        comma_index = len(self.leaves) - 1
+        for i in range(len(self.comments)):
+            comment_index, comment = self.comments[i]
+            if comment_index == comma_index:
+                self.comments[i] = (comma_index - 1, comment)
+        self.leaves.pop()
 
     def __str__(self) -> str:
+        """Render the line."""
         if not self:
             return '\n'
 
@@ -607,14 +794,64 @@ class Line:
         res = f'{first.prefix}{indent}{first.value}'
         for leaf in leaves:
             res += str(leaf)
-        for comment in self.comments.values():
+        for _, comment in self.comments:
             res += str(comment)
         return res + '\n'
 
     def __bool__(self) -> bool:
+        """Return True if the line has leaves or 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
@@ -629,14 +866,13 @@ class EmptyLineTracker:
     previous_defs: List[int] = Factory(list)
 
     def maybe_empty_lines(self, current_line: Line) -> Tuple[int, int]:
-        """Returns the number of extra empty lines before and after the `current_line`.
+        """Return the number of extra empty lines before and after the `current_line`.
 
-        This is for separating `def`, `async def` and `class` with extra empty lines
-        (two on module-level), as well as providing an extra empty line after flow
-        control keywords to make them more prominent.
+        This is for separating `def`, `async def` and `class` with extra empty
+        lines (two on module-level), as well as providing an extra empty line
+        after flow control keywords to make them more prominent.
         """
-        if current_line.is_comment:
-            # Don't count standalone comments towards previous empty lines.
+        if isinstance(current_line, UnformattedLines):
             return 0, 0
 
         before, after = self._maybe_empty_lines(current_line)
@@ -646,10 +882,14 @@ class EmptyLineTracker:
         return before, after
 
     def _maybe_empty_lines(self, current_line: Line) -> Tuple[int, int]:
+        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 = int('\n' in first_leaf.prefix)
+            before = first_leaf.prefix.count('\n')
+            before = min(before, max_allowed)
             first_leaf.prefix = ''
         else:
             before = 0
@@ -703,9 +943,8 @@ class LineGenerator(Visitor[Line]):
     in ways that will no longer stringify to valid Python code on the tree.
     """
     current_line: Line = Factory(Line)
-    standalone_comments: List[Leaf] = Factory(list)
 
-    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.
@@ -714,59 +953,85 @@ class LineGenerator(Visitor[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
-        self.current_line = Line(depth=complete_line.depth + indent)
+        self.current_line = type(depth=complete_line.depth + indent)
         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]:
+        """Default `visit_*()` implementation. Recurses to children of `node`."""
         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()
-
-                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)
+            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:
+                normalize_prefix(node, inside_brackets=any_open_brackets)
+                if node.type not in WHITESPACE:
+                    self.current_line.append(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.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)
 
     def visit_stmt(self, node: Node, keywords: Set[str]) -> Iterator[Line]:
         """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`, and `class`.
+
+        The relevant Python language `keywords` for a given statement will be NAME
+        leaves within it. This methods puts those on a separate line.
         """
         for child in node.children:
             if child.type == token.NAME and child.value in keywords:  # type: ignore
@@ -775,7 +1040,7 @@ class LineGenerator(Visitor[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)
@@ -787,13 +1052,14 @@ class LineGenerator(Visitor[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)
 
-            if child.type == token.NAME and child.value == 'async':  # type: ignore
+            if child.type == token.ASYNC:
                 break
 
         internal_stmt = next(children)
@@ -801,17 +1067,34 @@ class LineGenerator(Visitor[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)
 
     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]:
+        """End of file. Process outstanding comments and end with a newline."""
         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)
+
     def __attrs_post_init__(self) -> None:
         """You are in a twisty little maze of passages."""
         v = self.visit_stmt
@@ -849,7 +1132,7 @@ def whitespace(leaf: Leaf) -> str:  # noqa C901
         return DOUBLESPACE
 
     assert p is not None, f"INTERNAL ERROR: hand-made leaf without parent: {leaf!r}"
-    if t == token.COLON and p.type != syms.subscript:
+    if t == token.COLON and p.type not in {syms.subscript, syms.subscriptlist}:
         return NO
 
     prev = leaf.prev_sibling
@@ -862,30 +1145,49 @@ def whitespace(leaf: Leaf) -> str:  # noqa C901
             return SPACE if prevp.type == token.COMMA else NO
 
         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:
             if prevp.parent and prevp.parent.type in {
-                syms.typedargslist,
-                syms.varargslist,
-                syms.parameters,
                 syms.arglist,
+                syms.argument,
                 syms.dictsetmaker,
+                syms.parameters,
+                syms.typedargslist,
+                syms.varargslist,
             }:
                 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
 
-        elif prevp.parent and prevp.parent.type in {syms.factor, syms.star_expr}:
+        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:
@@ -899,7 +1201,7 @@ def whitespace(leaf: Leaf) -> str:  # noqa C901
         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
@@ -1053,7 +1355,7 @@ def whitespace(leaf: Leaf) -> str:  # noqa C901
 
 
 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:
@@ -1071,7 +1373,7 @@ def preceding_leaf(node: Optional[LN]) -> Optional[Leaf]:
 
 
 def is_delimiter(leaf: Leaf) -> int:
-    """Returns the priority of the `leaf` delimiter. Returns 0 if not delimiter.
+    """Return the priority of the `leaf` delimiter. Return 0 if not delimiter.
 
     Higher numbers are higher priority.
     """
@@ -1092,7 +1394,7 @@ def is_delimiter(leaf: Leaf) -> int:
 
 
 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
@@ -1110,36 +1412,62 @@ 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.
     """
-    if not leaf.prefix:
+    p = leaf.prefix
+    if not p:
         return
 
-    if '#' not in leaf.prefix:
-        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.ENDMARKER
-    )
-    if not is_standalone_comment:
-        # simple trailing comment
-        yield Leaf(token.COMMENT, value='#' + content)
+    if '#' not in p:
         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()
+        if not line:
+            nlines += 1
         if not line.startswith('#'):
             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'}:
+            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]:
-    """Splits a `line` into potentially many lines.
+    """Split a `line` into potentially many lines.
 
     They should fit in the allotted `line_length` but might not be able to.
     `inner` signifies that there were a pair of brackets somewhere around the
@@ -1149,19 +1477,24 @@ def split_line(
     If `py36` is True, splitting may generate syntax that is only compatible
     with Python 3.6 and later.
     """
+    if isinstance(line, UnformattedLines) or line.is_comment:
+        yield line
+        return
+
     line_str = str(line).strip('\n')
-    if len(line_str) <= line_length and '\n' not in line_str:
+    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:
-        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:
@@ -1170,7 +1503,7 @@ def split_line(
         # split altogether.
         result: List[Line] = []
         try:
-            for l in split_func(line, py36=py36):
+            for l in split_func(line, py36):
                 if str(l).strip('\n') == line_str:
                     raise CannotSplit("Split function returned an unchanged result")
 
@@ -1216,17 +1549,16 @@ def left_hand_split(line: Line, py36: bool = False) -> Iterator[Line]:
                 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.
     for result, leaves in (
         (head, head_leaves), (body, body_leaves), (tail, tail_leaves)
     ):
         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)
-    split_succeeded_or_raise(head, body, tail)
+    bracket_split_succeeded_or_raise(head, body, tail)
     for result in (head, body, tail):
         if result:
             yield result
@@ -1256,23 +1588,35 @@ def right_hand_split(line: Line, py36: bool = False) -> Iterator[Line]:
     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)
     # Build the new lines.
     for result, leaves in (
         (head, head_leaves), (body, body_leaves), (tail, tail_leaves)
     ):
         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)
-    split_succeeded_or_raise(head, body, tail)
+    bracket_split_succeeded_or_raise(head, body, tail)
     for result in (head, body, tail):
         if result:
             yield result
 
 
-def split_succeeded_or_raise(head: Line, body: Line, tail: Line) -> None:
+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:
@@ -1285,12 +1629,27 @@ def split_succeeded_or_raise(head: Line, body: Line, tail: Line) -> None:
             )
 
 
+def dont_increase_indentation(split_func: SplitFunc) -> SplitFunc:
+    """Normalize prefix of the first leaf in every line returned by `split_func`.
+
+    This is a decorator over relevant split functions.
+    """
+
+    @wraps(split_func)
+    def split_wrapper(line: Line, py36: bool = False) -> Iterator[Line]:
+        for l in split_func(line, py36):
+            normalize_prefix(l.leaves[0], inside_brackets=True)
+            yield l
+
+    return split_wrapper
+
+
+@dont_increase_indentation
 def delimiter_split(line: Line, py36: bool = False) -> Iterator[Line]:
     """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 **.
+    signatures that contain `*` and `**`.
     """
     try:
         last_leaf = line.leaves[-1]
@@ -1299,18 +1658,33 @@ def delimiter_split(line: Line, py36: bool = False) -> Iterator[Line]:
 
     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)
     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:
-        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
@@ -1320,7 +1694,6 @@ def delimiter_split(line: Line, py36: bool = False) -> Iterator[Line]:
             trailing_comma_safe = trailing_comma_safe and py36
         leaf_priority = delimiters.get(id(leaf))
         if leaf_priority == delimiter_priority:
-            normalize_prefix(current_line.leaves[0])
             yield current_line
 
             current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)
@@ -1331,12 +1704,45 @@ def delimiter_split(line: Line, py36: bool = False) -> Iterator[Line]:
             and trailing_comma_safe
         ):
             current_line.append(Leaf(token.COMMA, ','))
-        normalize_prefix(current_line.leaves[0])
+        yield current_line
+
+
+@dont_increase_indentation
+def standalone_comment_split(line: Line, py36: bool = False) -> Iterator[Line]:
+    """Split standalone comments from the rest of the line."""
+    for leaf in line.leaves:
+        if leaf.type == STANDALONE_COMMENT:
+            if leaf.bracket_depth == 0:
+                break
+
+    else:
+        raise CannotSplit("Line does not have any standalone comments")
+
+    current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)
+
+    def append_to_line(leaf: Leaf) -> Iterator[Line]:
+        """Append `leaf` to current line or to new line if appending impossible."""
+        nonlocal current_line
+        try:
+            current_line.append_safe(leaf, preformatted=True)
+        except ValueError as ve:
+            yield current_line
+
+            current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)
+            current_line.append(leaf)
+
+    for leaf in line.leaves:
+        yield from append_to_line(leaf)
+
+        for comment_after in line.comments_after(leaf):
+            yield from append_to_line(comment_after)
+
+    if current_line:
         yield current_line
 
 
 def is_import(leaf: Leaf) -> bool:
-    """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
@@ -1349,19 +1755,26 @@ def is_import(leaf: Leaf) -> bool:
     )
 
 
-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')
-        leaf.prefix = '\n' * nl_count
-        return
+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 is_python36(node: Node) -> bool:
-    """Returns True if the current file is using Python 3.6+ features.
+    """Return True if the current file is using Python 3.6+ features.
 
     Currently looking for:
     - f-strings; and
@@ -1392,6 +1805,9 @@ BLACKLISTED_DIRECTORIES = {
 
 
 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:
@@ -1405,7 +1821,8 @@ def gen_python_files_in_dir(path: Path) -> Iterator[Path]:
 
 @dataclass
 class Report:
-    """Provides a reformatting counter."""
+    """Provides a reformatting counter. Can be rendered with `str(report)`."""
+    check: bool = False
     change_count: int = 0
     same_count: int = 0
     failure_count: int = 0
@@ -1413,7 +1830,8 @@ class Report:
     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'
+            out(f'{reformatted} {src}')
             self.change_count += 1
         else:
             out(f'{src} already well formatted, good job.', bold=False)
@@ -1426,46 +1844,55 @@ class Report:
 
     @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
 
-        elif self.change_count:
+        elif self.change_count and self.check:
             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.
         """
+        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:
             s = 's' if self.change_count > 1 else ''
             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:
             s = 's' if self.same_count > 1 else ''
-            report.append(f'{self.same_count} file{s} left unchanged')
+            report.append(f'{self.same_count} file{s} {unchanged}')
         if self.failure_count:
             s = 's' if self.failure_count > 1 else ''
             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) + '.'
 
 
 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
@@ -1498,7 +1925,12 @@ def assert_equivalent(src: str, dst: str) -> None:
     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)
@@ -1523,10 +1955,7 @@ def assert_equivalent(src: str, dst: str) -> 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(
@@ -1542,7 +1971,7 @@ def assert_stable(src: str, dst: str, line_length: int) -> 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(
@@ -1555,7 +1984,7 @@ def dump_to_file(*output: str) -> str:
 
 
 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
 
     a_lines = [line + '\n' for line in a.split('\n')]
@@ -1565,5 +1994,34 @@ def diff(a: str, b: str, a_name: str, b_name: str) -> str:
     )
 
 
+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()