X-Git-Url: https://git.madduck.net/etc/vim.git/blobdiff_plain/c55d08d0b96c8de8bd867ca315e380d9e9d2d7ec..32e2f20e9a7932b05aa74f83347ca0fa6ffe78ba:/black.py?ds=inline

diff --git a/black.py b/black.py
index 6499b22..b2cf543 100644
--- a/black.py
+++ b/black.py
@@ -1,22 +1,29 @@
-#!/usr/bin/env python3
-
 import asyncio
+import pickle
 from asyncio.base_events import BaseEventLoop
 from concurrent.futures import Executor, ProcessPoolExecutor
+from enum import Enum
 from functools import partial, wraps
 import keyword
+import logging
+from multiprocessing import Manager
 import os
 from pathlib import Path
+import re
 import tokenize
+import signal
 import sys
 from typing import (
+    Any,
     Callable,
+    Collection,
     Dict,
     Generic,
     Iterable,
     Iterator,
     List,
     Optional,
+    Pattern,
     Set,
     Tuple,
     Type,
@@ -24,6 +31,7 @@ from typing import (
     Union,
 )
 
+from appdirs import user_cache_dir
 from attr import dataclass, Factory
 import click
 
@@ -33,8 +41,9 @@ from blib2to3 import pygram, pytree
 from blib2to3.pgen2 import driver, token
 from blib2to3.pgen2.parse import ParseError
 
-__version__ = "18.3a4"
+__version__ = "18.4a6"
 DEFAULT_LINE_LENGTH = 88
+
 # types
 syms = pygram.python_symbols
 FileContent = str
@@ -45,9 +54,13 @@ LeafID = int
 Priority = int
 Index = int
 LN = Union[Leaf, Node]
-SplitFunc = Callable[['Line', bool], Iterator['Line']]
+SplitFunc = Callable[["Line", bool], Iterator["Line"]]
+Timestamp = float
+FileSize = int
+CacheInfo = Tuple[Timestamp, FileSize]
+Cache = Dict[Path, CacheInfo]
 out = partial(click.secho, bold=True, err=True)
-err = partial(click.secho, fg='red', err=True)
+err = partial(click.secho, fg="red", err=True)
 
 
 class NothingChanged(UserWarning):
@@ -74,11 +87,11 @@ class FormatError(Exception):
         self.consumed = consumed
 
     def trim_prefix(self, leaf: Leaf) -> None:
-        leaf.prefix = leaf.prefix[self.consumed:]
+        leaf.prefix = leaf.prefix[self.consumed :]
 
     def leaf_from_consumed(self, leaf: Leaf) -> Leaf:
         """Returns a new Leaf from the consumed part of the prefix."""
-        unformatted_prefix = leaf.prefix[:self.consumed]
+        unformatted_prefix = leaf.prefix[: self.consumed]
         return Leaf(token.NEWLINE, unformatted_prefix)
 
 
@@ -90,32 +103,58 @@ class FormatOff(FormatError):
     """Found a comment like `# fmt: off` in the file."""
 
 
+class WriteBack(Enum):
+    NO = 0
+    YES = 1
+    DIFF = 2
+
+
+class Changed(Enum):
+    NO = 0
+    CACHED = 1
+    YES = 2
+
+
 @click.command()
 @click.option(
-    '-l',
-    '--line-length',
+    "-l",
+    "--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(
-    '--check',
+    "--check",
     is_flag=True,
     help=(
-        "Don't write back the files, just return the status.  Return code 0 "
+        "Don't write the files back, just return the status.  Return code 0 "
         "means nothing would change.  Return code 1 means some files would be "
         "reformatted.  Return code 123 means there was an internal error."
     ),
 )
 @click.option(
-    '--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,
-    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(
-    'src',
+    "src",
     nargs=-1,
     type=click.Path(
         exists=True, file_okay=True, dir_okay=True, readable=True, allow_dash=True
@@ -123,7 +162,13 @@ class FormatOff(FormatError):
 )
 @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] = []
@@ -134,51 +179,87 @@ 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('-'))
+        elif s == "-":
+            sources.append(Path("-"))
         else:
-            err(f'invalid path: {s}')
+            err(f"invalid path: {s}")
+
+    if check and not diff:
+        write_back = WriteBack.NO
+    elif diff:
+        write_back = WriteBack.DIFF
+    else:
+        write_back = WriteBack.YES
+    report = Report(check=check, quiet=quiet)
     if len(sources) == 0:
+        out("No paths given. Nothing to do 😴")
         ctx.exit(0)
+        return
+
     elif len(sources) == 1:
-        p = sources[0]
-        report = Report(check=check)
-        try:
-            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))
-        ctx.exit(report.return_code)
+        reformat_one(sources[0], line_length, fast, write_back, report)
     else:
         loop = asyncio.get_event_loop()
         executor = ProcessPoolExecutor(max_workers=os.cpu_count())
-        return_code = 1
         try:
-            return_code = loop.run_until_complete(
+            loop.run_until_complete(
                 schedule_formatting(
-                    sources, line_length, not check, fast, loop, executor
+                    sources, line_length, fast, write_back, report, loop, executor
                 )
             )
         finally:
-            loop.close()
-            ctx.exit(return_code)
+            shutdown(loop)
+        if not quiet:
+            out("All done! ✨ 🍰 ✨")
+            click.echo(str(report))
+    ctx.exit(report.return_code)
+
+
+def reformat_one(
+    src: Path, line_length: int, fast: bool, write_back: WriteBack, report: "Report"
+) -> None:
+    """Reformat a single file under `src` without spawning child processes.
+
+    If `quiet` is True, non-error messages are not output. `line_length`,
+    `write_back`, and `fast` options are passed to :func:`format_file_in_place`.
+    """
+    try:
+        changed = Changed.NO
+        if not src.is_file() and str(src) == "-":
+            if format_stdin_to_stdout(
+                line_length=line_length, fast=fast, write_back=write_back
+            ):
+                changed = Changed.YES
+        else:
+            cache: Cache = {}
+            if write_back != WriteBack.DIFF:
+                cache = read_cache(line_length)
+                src = src.resolve()
+                if src in cache and cache[src] == get_cache_info(src):
+                    changed = Changed.CACHED
+            if (
+                changed is not Changed.CACHED
+                and format_file_in_place(
+                    src, line_length=line_length, fast=fast, write_back=write_back
+                )
+            ):
+                changed = Changed.YES
+            if write_back == WriteBack.YES and changed is not Changed.NO:
+                write_cache(cache, [src], line_length)
+        report.done(src, changed)
+    except Exception as exc:
+        report.failed(src, str(exc))
 
 
 async def schedule_formatting(
     sources: List[Path],
     line_length: int,
-    write_back: bool,
     fast: bool,
+    write_back: WriteBack,
+    report: "Report",
     loop: BaseEventLoop,
     executor: Executor,
-) -> int:
+) -> None:
     """Run formatting of `sources` in parallel using the provided `executor`.
 
     (Use ProcessPoolExecutors for actual parallelism.)
@@ -186,73 +267,117 @@ async def schedule_formatting(
     `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
-    }
-    await asyncio.wait(tasks.values())
+    cache: Cache = {}
+    if write_back != WriteBack.DIFF:
+        cache = read_cache(line_length)
+        sources, cached = filter_cached(cache, sources)
+        for src in cached:
+            report.done(src, Changed.CACHED)
     cancelled = []
-    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.exception():
-            report.failed(src, str(task.exception()))
-        else:
-            report.done(src, task.result())
+    formatted = []
+    if sources:
+        lock = None
+        if write_back == WriteBack.DIFF:
+            # For diff output, we need locks to ensure we don't interleave output
+            # from different processes.
+            manager = Manager()
+            lock = manager.Lock()
+        tasks = {
+            src: loop.run_in_executor(
+                executor, format_file_in_place, src, line_length, fast, write_back, lock
+            )
+            for src in sources
+        }
+        _task_values = list(tasks.values())
+        try:
+            loop.add_signal_handler(signal.SIGINT, cancel, _task_values)
+            loop.add_signal_handler(signal.SIGTERM, cancel, _task_values)
+        except NotImplementedError:
+            # There are no good alternatives for these on Windows
+            pass
+        await asyncio.wait(_task_values)
+        for src, task in tasks.items():
+            if not task.done():
+                report.failed(src, "timed out, cancelling")
+                task.cancel()
+                cancelled.append(task)
+            elif task.cancelled():
+                cancelled.append(task)
+            elif task.exception():
+                report.failed(src, str(task.exception()))
+            else:
+                formatted.append(src)
+                report.done(src, Changed.YES if task.result() else Changed.NO)
+
     if cancelled:
-        await asyncio.wait(cancelled, timeout=2)
-    out('All done! ✨ 🍰 ✨')
-    click.echo(str(report))
-    return report.return_code
+        await asyncio.gather(*cancelled, loop=loop, return_exceptions=True)
+    if write_back == WriteBack.YES and formatted:
+        write_cache(cache, formatted, line_length)
 
 
 def format_file_in_place(
-    src: Path, line_length: int, fast: bool, write_back: bool = False
+    src: Path,
+    line_length: int,
+    fast: bool,
+    write_back: WriteBack = WriteBack.NO,
+    lock: Any = None,  # multiprocessing.Manager().Lock() is some crazy proxy
 ) -> bool:
     """Format file under `src` path. Return True if changed.
 
     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 = format_file_contents(
+        dst_contents = format_file_contents(
             src_contents, line_length=line_length, fast=fast
         )
     except NothingChanged:
         return False
 
-    if write_back:
+    if write_back == write_back.YES:
         with open(src, "w", encoding=src_buffer.encoding) as f:
-            f.write(contents)
+            f.write(dst_contents)
+    elif write_back == write_back.DIFF:
+        src_name = f"{src}  (original)"
+        dst_name = f"{src}  (formatted)"
+        diff_contents = diff(src_contents, dst_contents, src_name, dst_name)
+        if lock:
+            lock.acquire()
+        try:
+            sys.stdout.write(diff_contents)
+        finally:
+            if lock:
+                lock.release()
     return True
 
 
 def format_stdin_to_stdout(
-    line_length: int, fast: bool, write_back: bool = False
+    line_length: int, fast: bool, write_back: WriteBack = WriteBack.NO
 ) -> bool:
     """Format file on stdin. Return True if changed.
 
     If `write_back` is True, write reformatted code back to stdout.
     `line_length` and `fast` arguments are passed to :func:`format_file_contents`.
     """
-    contents = sys.stdin.read()
+    src = sys.stdin.read()
+    dst = src
     try:
-        contents = format_file_contents(contents, line_length=line_length, fast=fast)
+        dst = format_file_contents(src, line_length=line_length, fast=fast)
         return True
 
     except NothingChanged:
         return False
 
     finally:
-        if write_back:
-            sys.stdout.write(contents)
+        if write_back == WriteBack.YES:
+            sys.stdout.write(dst)
+        elif write_back == WriteBack.DIFF:
+            src_name = "<stdin>  (original)"
+            dst_name = "<stdin>  (formatted)"
+            sys.stdout.write(diff(src, dst, src_name, dst_name))
 
 
 def format_file_contents(
@@ -264,7 +389,7 @@ def format_file_contents(
     valid by calling :func:`assert_equivalent` and :func:`assert_stable` on it.
     `line_length` is passed to :func:`format_str`.
     """
-    if src_contents.strip() == '':
+    if src_contents.strip() == "":
         raise NothingChanged
 
     dst_contents = format_str(src_contents, line_length=line_length)
@@ -303,7 +428,6 @@ def format_str(src_contents: str, line_length: int) -> FileContent:
 GRAMMARS = [
     pygram.python_grammar_no_print_statement_no_exec_statement,
     pygram.python_grammar_no_print_statement,
-    pygram.python_grammar_no_exec_statement,
     pygram.python_grammar,
 ]
 
@@ -311,8 +435,8 @@ GRAMMARS = [
 def lib2to3_parse(src_txt: str) -> Node:
     """Given a string with source, return the lib2to3 Node."""
     grammar = pygram.python_grammar_no_print_statement
-    if src_txt[-1] != '\n':
-        nl = '\r\n' if '\r\n' in src_txt[:1024] else '\n'
+    if src_txt[-1] != "\n":
+        nl = "\r\n" if "\r\n" in src_txt[:1024] else "\n"
         src_txt += nl
     for grammar in GRAMMARS:
         drv = driver.Driver(grammar, pytree.convert)
@@ -342,7 +466,7 @@ def lib2to3_unparse(node: Node) -> str:
     return code
 
 
-T = TypeVar('T')
+T = TypeVar("T")
 
 
 class Visitor(Generic[T]):
@@ -362,7 +486,7 @@ class Visitor(Generic[T]):
             name = token.tok_name[node.type]
         else:
             name = type_repr(node.type)
-        yield from getattr(self, f'visit_{name}', self.visit_default)(node)
+        yield from getattr(self, f"visit_{name}", self.visit_default)(node)
 
     def visit_default(self, node: LN) -> Iterator[T]:
         """Default `visit_*()` implementation. Recurses to children of `node`."""
@@ -376,24 +500,24 @@ class DebugVisitor(Visitor[T]):
     tree_depth: int = 0
 
     def visit_default(self, node: LN) -> Iterator[T]:
-        indent = ' ' * (2 * self.tree_depth)
+        indent = " " * (2 * self.tree_depth)
         if isinstance(node, Node):
             _type = type_repr(node.type)
-            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
-            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))
-            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.
-                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:
@@ -407,7 +531,7 @@ class DebugVisitor(Visitor[T]):
 
 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,
@@ -419,7 +543,7 @@ STATEMENT = {
     syms.classdef,
 }
 STANDALONE_COMMENT = 153
-LOGIC_OPERATORS = {'and', 'or'}
+LOGIC_OPERATORS = {"and", "or"}
 COMPARATORS = {
     token.LESS,
     token.GREATER,
@@ -429,26 +553,90 @@ COMPARATORS = {
     token.GREATEREQUAL,
 }
 MATH_OPERATORS = {
+    token.VBAR,
+    token.CIRCUMFLEX,
+    token.AMPER,
+    token.LEFTSHIFT,
+    token.RIGHTSHIFT,
     token.PLUS,
     token.MINUS,
     token.STAR,
     token.SLASH,
-    token.VBAR,
-    token.AMPER,
+    token.DOUBLESLASH,
     token.PERCENT,
-    token.CIRCUMFLEX,
+    token.AT,
     token.TILDE,
-    token.LEFTSHIFT,
-    token.RIGHTSHIFT,
     token.DOUBLESTAR,
-    token.DOUBLESLASH,
+}
+STARS = {token.STAR, token.DOUBLESTAR}
+VARARGS_PARENTS = {
+    syms.arglist,
+    syms.argument,  # double star in arglist
+    syms.trailer,  # single argument to call
+    syms.typedargslist,
+    syms.varargslist,  # lambdas
+}
+UNPACKING_PARENTS = {
+    syms.atom,  # single element of a list or set literal
+    syms.dictsetmaker,
+    syms.listmaker,
+    syms.testlist_gexp,
+}
+TEST_DESCENDANTS = {
+    syms.test,
+    syms.lambdef,
+    syms.or_test,
+    syms.and_test,
+    syms.not_test,
+    syms.comparison,
+    syms.star_expr,
+    syms.expr,
+    syms.xor_expr,
+    syms.and_expr,
+    syms.shift_expr,
+    syms.arith_expr,
+    syms.trailer,
+    syms.term,
+    syms.power,
+}
+ASSIGNMENTS = {
+    "=",
+    "+=",
+    "-=",
+    "*=",
+    "@=",
+    "/=",
+    "%=",
+    "&=",
+    "|=",
+    "^=",
+    "<<=",
+    ">>=",
+    "**=",
+    "//=",
 }
 COMPREHENSION_PRIORITY = 20
-COMMA_PRIORITY = 10
-LOGIC_PRIORITY = 5
-STRING_PRIORITY = 4
-COMPARATOR_PRIORITY = 3
-MATH_PRIORITY = 1
+COMMA_PRIORITY = 18
+TERNARY_PRIORITY = 16
+LOGIC_PRIORITY = 14
+STRING_PRIORITY = 12
+COMPARATOR_PRIORITY = 10
+MATH_PRIORITIES = {
+    token.VBAR: 8,
+    token.CIRCUMFLEX: 7,
+    token.AMPER: 6,
+    token.LEFTSHIFT: 5,
+    token.RIGHTSHIFT: 5,
+    token.PLUS: 4,
+    token.MINUS: 4,
+    token.STAR: 3,
+    token.SLASH: 3,
+    token.DOUBLESLASH: 3,
+    token.PERCENT: 3,
+    token.AT: 3,
+    token.TILDE: 2,
+    token.DOUBLESTAR: 1,
+}
 
 
 @dataclass
@@ -459,6 +647,8 @@ class BracketTracker:
     bracket_match: Dict[Tuple[Depth, NodeType], Leaf] = Factory(dict)
     delimiters: Dict[LeafID, Priority] = Factory(dict)
     previous: Optional[Leaf] = None
+    _for_loop_variable: int = 0
+    _lambda_arguments: int = 0
 
     def mark(self, leaf: Leaf) -> None:
         """Mark `leaf` with bracket-related metadata. Keep track of delimiters.
@@ -478,42 +668,27 @@ class BracketTracker:
         if leaf.type == token.COMMENT:
             return
 
+        self.maybe_decrement_after_for_loop_variable(leaf)
+        self.maybe_decrement_after_lambda_arguments(leaf)
         if leaf.type in CLOSING_BRACKETS:
             self.depth -= 1
             opening_bracket = self.bracket_match.pop((self.depth, leaf.type))
             leaf.opening_bracket = opening_bracket
         leaf.bracket_depth = self.depth
         if self.depth == 0:
-            delim = is_delimiter(leaf)
-            if delim:
-                self.delimiters[id(leaf)] = delim
-            elif self.previous is not None:
-                if leaf.type == token.STRING and self.previous.type == token.STRING:
-                    self.delimiters[id(self.previous)] = STRING_PRIORITY
-                elif (
-                    leaf.type == token.NAME
-                    and leaf.value == 'for'
-                    and leaf.parent
-                    and leaf.parent.type in {syms.comp_for, syms.old_comp_for}
-                ):
-                    self.delimiters[id(self.previous)] = COMPREHENSION_PRIORITY
-                elif (
-                    leaf.type == token.NAME
-                    and leaf.value == 'if'
-                    and leaf.parent
-                    and leaf.parent.type in {syms.comp_if, syms.old_comp_if}
-                ):
-                    self.delimiters[id(self.previous)] = COMPREHENSION_PRIORITY
-                elif (
-                    leaf.type == token.NAME
-                    and leaf.value in LOGIC_OPERATORS
-                    and leaf.parent
-                ):
-                    self.delimiters[id(self.previous)] = LOGIC_PRIORITY
+            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
+        self.maybe_increment_lambda_arguments(leaf)
+        self.maybe_increment_for_loop_variable(leaf)
 
     def any_open_brackets(self) -> bool:
         """Return True if there is an yet unmatched open bracket on the line."""
@@ -522,10 +697,59 @@ class BracketTracker:
     def max_delimiter_priority(self, exclude: Iterable[LeafID] = ()) -> int:
         """Return the highest priority of a delimiter found on the line.
 
-        Values are consistent with what `is_delimiter()` returns.
+        Values are consistent with what `is_split_*_delimiter()` return.
+        Raises ValueError on no delimiters.
         """
         return max(v for k, v in self.delimiters.items() if k not in exclude)
 
+    def maybe_increment_for_loop_variable(self, leaf: Leaf) -> bool:
+        """In a for loop, or comprehension, the variables are often unpacks.
+
+        To avoid splitting on the comma in this situation, increase the depth of
+        tokens between `for` and `in`.
+        """
+        if leaf.type == token.NAME and leaf.value == "for":
+            self.depth += 1
+            self._for_loop_variable += 1
+            return True
+
+        return False
+
+    def maybe_decrement_after_for_loop_variable(self, leaf: Leaf) -> bool:
+        """See `maybe_increment_for_loop_variable` above for explanation."""
+        if self._for_loop_variable and leaf.type == token.NAME and leaf.value == "in":
+            self.depth -= 1
+            self._for_loop_variable -= 1
+            return True
+
+        return False
+
+    def maybe_increment_lambda_arguments(self, leaf: Leaf) -> bool:
+        """In a lambda expression, there might be more than one argument.
+
+        To avoid splitting on the comma in this situation, increase the depth of
+        tokens between `lambda` and `:`.
+        """
+        if leaf.type == token.NAME and leaf.value == "lambda":
+            self.depth += 1
+            self._lambda_arguments += 1
+            return True
+
+        return False
+
+    def maybe_decrement_after_lambda_arguments(self, leaf: Leaf) -> bool:
+        """See `maybe_increment_lambda_arguments` above for explanation."""
+        if self._lambda_arguments and leaf.type == token.COLON:
+            self.depth -= 1
+            self._lambda_arguments -= 1
+            return True
+
+        return False
+
+    def get_open_lsqb(self) -> Optional[Leaf]:
+        """Return the most recent opening square bracket (if any)."""
+        return self.bracket_match.get((self.depth - 1, token.RSQB))
+
 
 @dataclass
 class Line:
@@ -536,8 +760,6 @@ class Line:
     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.
@@ -549,20 +771,21 @@ class Line:
 
         Inline comments are put aside.
         """
-        has_value = leaf.value.strip()
+        has_value = leaf.type in BRACKETS or bool(leaf.value.strip())
         if not has_value:
             return
 
+        if token.COLON == leaf.type and self.is_class_paren_empty:
+            del self.leaves[-2:]
         if self.leaves and not preformatted:
             # Note: at this point leaf.prefix should be empty except for
             # imports, for which we only preserve newlines.
-            leaf.prefix += whitespace(leaf)
+            leaf.prefix += whitespace(
+                leaf, complex_subscript=self.is_complex_subscript(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.maybe_increment_for_loop_variable(leaf)
-
         if not self.append_comment(leaf):
             self.leaves.append(leaf)
 
@@ -604,7 +827,7 @@ class Line:
         return (
             bool(self)
             and self.leaves[0].type == token.NAME
-            and self.leaves[0].value == 'class'
+            and self.leaves[0].value == "class"
         )
 
     @property
@@ -620,12 +843,12 @@ class Line:
         except IndexError:
             second_leaf = None
         return (
-            (first_leaf.type == token.NAME and first_leaf.value == 'def')
+            (first_leaf.type == token.NAME and first_leaf.value == "def")
             or (
                 first_leaf.type == token.ASYNC
                 and second_leaf is not None
                 and second_leaf.type == token.NAME
-                and second_leaf.value == 'def'
+                and second_leaf.value == "def"
             )
         )
 
@@ -647,15 +870,31 @@ class Line:
         return (
             bool(self)
             and self.leaves[0].type == token.NAME
-            and self.leaves[0].value == 'yield'
+            and self.leaves[0].value == "yield"
         )
 
     @property
-    def contains_standalone_comments(self) -> bool:
+    def is_class_paren_empty(self) -> bool:
+        """Is this a class with no base classes but using parentheses?
+
+        Those are unnecessary and should be removed.
+        """
+        return (
+            bool(self)
+            and len(self.leaves) == 4
+            and self.is_class
+            and self.leaves[2].type == token.LPAR
+            and self.leaves[2].value == "("
+            and self.leaves[3].type == token.RPAR
+            and self.leaves[3].value == ")"
+        )
+
+    def contains_standalone_comments(self, depth_limit: int = sys.maxsize) -> bool:
         """If so, needs to be split before emitting."""
         for leaf in self.leaves:
             if leaf.type == STANDALONE_COMMENT:
-                return True
+                if leaf.bracket_depth <= depth_limit:
+                    return True
 
         return False
 
@@ -678,9 +917,14 @@ class Line:
                 self.remove_trailing_comma()
                 return True
 
-        # For parens let's check if it's safe to remove the comma.  If the
-        # trailing one is the only one, we might mistakenly change a tuple
-        # into a different type by removing the comma.
+        # For parens let's check if it's safe to remove the comma.
+        # Imports are always safe.
+        if self.is_import:
+            self.remove_trailing_comma()
+            return True
+
+        # Otheriwsse, if the trailing one is the only one, we might mistakenly
+        # change a tuple into a different type by removing the comma.
         depth = closing.bracket_depth + 1
         commas = 0
         opening = closing.opening_bracket
@@ -691,7 +935,7 @@ class Line:
         else:
             return False
 
-        for leaf in self.leaves[_opening_index + 1:]:
+        for leaf in self.leaves[_opening_index + 1 :]:
             if leaf is closing:
                 break
 
@@ -708,36 +952,13 @@ class Line:
 
         return False
 
-    def maybe_increment_for_loop_variable(self, leaf: Leaf) -> bool:
-        """In a for loop, or comprehension, the variables are often unpacks.
-
-        To avoid splitting on the comma in this situation, increase the depth of
-        tokens between `for` and `in`.
-        """
-        if leaf.type == token.NAME and leaf.value == 'for':
-            self.has_for = True
-            self.bracket_tracker.depth += 1
-            self._for_loop_variable = True
-            return True
-
-        return False
-
-    def maybe_decrement_after_for_loop_variable(self, leaf: Leaf) -> bool:
-        """See `maybe_increment_for_loop_variable` above for explanation."""
-        if self._for_loop_variable and leaf.type == token.NAME and leaf.value == 'in':
-            self.bracket_tracker.depth -= 1
-            self._for_loop_variable = False
-            return True
-
-        return False
-
     def append_comment(self, comment: Leaf) -> bool:
         """Add an inline or standalone comment to the line."""
         if (
             comment.type == STANDALONE_COMMENT
             and self.bracket_tracker.any_open_brackets()
         ):
-            comment.prefix = ''
+            comment.prefix = ""
             return False
 
         if comment.type != token.COMMENT:
@@ -746,7 +967,7 @@ class Line:
         after = len(self.leaves) - 1
         if after == -1:
             comment.type = STANDALONE_COMMENT
-            comment.prefix = ''
+            comment.prefix = ""
             return False
 
         else:
@@ -775,20 +996,39 @@ class Line:
                 self.comments[i] = (comma_index - 1, comment)
         self.leaves.pop()
 
+    def is_complex_subscript(self, leaf: Leaf) -> bool:
+        """Return True iff `leaf` is part of a slice with non-trivial exprs."""
+        open_lsqb = (
+            leaf if leaf.type == token.LSQB else self.bracket_tracker.get_open_lsqb()
+        )
+        if open_lsqb is None:
+            return False
+
+        subscript_start = open_lsqb.next_sibling
+        if (
+            isinstance(subscript_start, Node)
+            and subscript_start.type == syms.subscriptlist
+        ):
+            subscript_start = child_towards(subscript_start, leaf)
+        return (
+            subscript_start is not None
+            and any(n.type in TEST_DESCENDANTS for n in subscript_start.pre_order())
+        )
+
     def __str__(self) -> str:
         """Render the line."""
         if not self:
-            return '\n'
+            return "\n"
 
-        indent = '    ' * self.depth
+        indent = "    " * self.depth
         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 _, comment in self.comments:
             res += str(comment)
-        return res + '\n'
+        return res + "\n"
 
     def __bool__(self) -> bool:
         """Return True if the line has leaves or comments."""
@@ -824,9 +1064,9 @@ class UnformattedLines(Line):
         `depth` is not used for indentation in this case.
         """
         if not self:
-            return '\n'
+            return "\n"
 
-        res = ''
+        res = ""
         for leaf in self.leaves:
             res += str(leaf)
         return res
@@ -880,9 +1120,9 @@ class EmptyLineTracker:
         if current_line.leaves:
             # Consume the first leaf's extra newlines.
             first_leaf = current_line.leaves[0]
-            before = first_leaf.prefix.count('\n')
+            before = first_leaf.prefix.count("\n")
             before = min(before, max_allowed)
-            first_leaf.prefix = ''
+            first_leaf.prefix = ""
         else:
             before = 0
         depth = current_line.depth
@@ -897,8 +1137,14 @@ class EmptyLineTracker:
                 # Don't insert empty lines before the first line in the file.
                 return 0, 0
 
-            if self.previous_line and self.previous_line.is_decorator:
-                # Don't insert empty lines between decorators.
+            if self.previous_line.is_decorator:
+                return 0, 0
+
+            if (
+                self.previous_line.is_comment
+                and self.previous_line.depth == current_line.depth
+                and before == 0
+            ):
                 return 0, 0
 
             newlines = 2
@@ -906,9 +1152,6 @@ class EmptyLineTracker:
                 newlines -= 1
             return newlines, 0
 
-        if current_line.is_flow_control:
-            return before, 1
-
         if (
             self.previous_line
             and self.previous_line.is_import
@@ -917,13 +1160,6 @@ class EmptyLineTracker:
         ):
             return (before or 1), 0
 
-        if (
-            self.previous_line
-            and self.previous_line.is_yield
-            and (not current_line.is_yield or depth != self.previous_line.depth)
-        ):
-            return (before or 1), 0
-
         return before, 0
 
 
@@ -1001,6 +1237,8 @@ class LineGenerator(Visitor[Line]):
 
             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)
@@ -1013,18 +1251,33 @@ class LineGenerator(Visitor[Line]):
 
     def visit_DEDENT(self, node: Node) -> Iterator[Line]:
         """Decrease indentation level, maybe yield a line."""
-        # DEDENT has no value. Additionally, in blib2to3 it never holds comments.
+        # The current line might still wait for trailing comments.  At DEDENT time
+        # there won't be any (they would be prefixes on the preceding NEWLINE).
+        # Emit the line then.
+        yield from self.line()
+
+        # While DEDENT has no value, its prefix may contain standalone comments
+        # that belong to the current indentation level.  Get 'em.
+        yield from self.visit_default(node)
+
+        # Finally, emit the dedent.
         yield from self.line(-1)
 
-    def visit_stmt(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.
 
         This implementation is shared for `if`, `while`, `for`, `try`, `except`,
-        `def`, `with`, and `class`.
+        `def`, `with`, `class`, `assert` and assignments.
+
+        The relevant Python language `keywords` for a given statement will be
+        NAME leaves within it. This methods puts those on a separate line.
 
-        The relevant Python language `keywords` for a given statement will be NAME
-        leaves within it. This methods puts those on a separate line.
+        `parens` holds a set of string leaf values immeditely after which
+        invisible parens should be put.
         """
+        normalize_invisible_parens(node, parens_after=parens)
         for child in node.children:
             if child.type == token.NAME and child.value in keywords:  # type: ignore
                 yield from self.line()
@@ -1064,6 +1317,32 @@ class LineGenerator(Visitor[Line]):
             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]:
         """Remove a semicolon and put the other statement on a separate line."""
         yield from self.line()
@@ -1087,21 +1366,34 @@ class LineGenerator(Visitor[Line]):
                 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
-        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", "elif"}
+        )
+        self.visit_while_stmt = partial(v, keywords={"while", "else"}, parens={"while"})
+        self.visit_for_stmt = partial(v, keywords={"for", "else"}, parens={"for", "in"})
+        self.visit_try_stmt = partial(
+            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_expr_stmt = partial(v, keywords=Ø, parens=ASSIGNMENTS)
+        self.visit_return_stmt = partial(v, keywords={"return"}, parens={"return"})
         self.visit_async_funcdef = self.visit_async_stmt
         self.visit_decorated = self.visit_decorators
 
 
+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())
@@ -1109,11 +1401,15 @@ BRACKETS = OPENING_BRACKETS | CLOSING_BRACKETS
 ALWAYS_NO_SPACE = CLOSING_BRACKETS | {token.COMMA, STANDALONE_COMMENT}
 
 
-def whitespace(leaf: Leaf) -> str:  # noqa C901
-    """Return whitespace prefix if needed for the given `leaf`."""
-    NO = ''
-    SPACE = ' '
-    DOUBLESPACE = '  '
+def whitespace(leaf: Leaf, *, complex_subscript: bool) -> str:  # noqa C901
+    """Return whitespace prefix if needed for the given `leaf`.
+
+    `complex_subscript` signals whether the given leaf is part of a subscription
+    which has non-trivial arguments, like arithmetic expressions or function calls.
+    """
+    NO = ""
+    SPACE = " "
+    DOUBLESPACE = "  "
     t = leaf.type
     p = leaf.parent
     v = leaf.value
@@ -1124,7 +1420,10 @@ def whitespace(leaf: Leaf) -> str:  # noqa C901
         return DOUBLESPACE
 
     assert p is not None, f"INTERNAL ERROR: hand-made leaf without parent: {leaf!r}"
-    if t == token.COLON and p.type not in {syms.subscript, syms.subscriptlist}:
+    if (
+        t == token.COLON
+        and p.type not in {syms.subscript, syms.subscriptlist, syms.sliceop}
+    ):
         return NO
 
     prev = leaf.prev_sibling
@@ -1134,7 +1433,13 @@ def whitespace(leaf: Leaf) -> str:  # noqa C901
             return NO
 
         if t == token.COLON:
-            return SPACE if prevp.type == token.COMMA else NO
+            if prevp.type == token.COLON:
+                return NO
+
+            elif prevp.type != token.COMMA and not complex_subscript:
+                return NO
+
+            return SPACE
 
         if prevp.type == token.EQUAL:
             if prevp.parent:
@@ -1149,24 +1454,17 @@ def whitespace(leaf: Leaf) -> str:  # noqa C901
                     # that, too.
                     return prevp.prefix
 
-        elif prevp.type == token.DOUBLESTAR:
-            if prevp.parent and prevp.parent.type in {
-                syms.arglist,
-                syms.argument,
-                syms.dictsetmaker,
-                syms.parameters,
-                syms.typedargslist,
-                syms.varargslist,
-            }:
+        elif prevp.type in STARS:
+            if is_vararg(prevp, within=VARARGS_PARENTS | UNPACKING_PARENTS):
                 return NO
 
         elif prevp.type == token.COLON:
             if prevp.parent and prevp.parent.type in {syms.subscript, syms.sliceop}:
-                return NO
+                return SPACE if complex_subscript else NO
 
         elif (
             prevp.parent
-            and prevp.parent.type in {syms.factor, syms.star_expr}
+            and prevp.parent.type == syms.factor
             and prevp.type in MATH_OPERATORS
         ):
             return NO
@@ -1177,7 +1475,7 @@ def whitespace(leaf: Leaf) -> str:  # noqa C901
             and prevp.parent.type == syms.shift_expr
             and prevp.prev_sibling
             and prevp.prev_sibling.type == token.NAME
-            and prevp.prev_sibling.value == 'print'  # type: ignore
+            and prevp.prev_sibling.value == "print"  # type: ignore
         ):
             # Python 2 print chevron
             return NO
@@ -1187,17 +1485,11 @@ def whitespace(leaf: Leaf) -> str:  # noqa C901
 
     if p.type in {syms.parameters, syms.arglist}:
         # untyped function signatures or calls
-        if t == token.RPAR:
-            return NO
-
         if not prev or prev.type != token.COMMA:
             return NO
 
     elif p.type == syms.varargslist:
         # lambdas
-        if t == token.RPAR:
-            return NO
-
         if prev and prev.type != token.COMMA:
             return NO
 
@@ -1252,7 +1544,7 @@ def whitespace(leaf: Leaf) -> str:  # noqa C901
             if not prevp or prevp.type == token.LPAR:
                 return NO
 
-        elif prev.type == token.EQUAL or prev.type == token.DOUBLESTAR:
+        elif prev.type in {token.EQUAL} | STARS:
             return NO
 
     elif p.type == syms.decorator:
@@ -1274,7 +1566,7 @@ def whitespace(leaf: Leaf) -> str:  # noqa C901
         if prev and prev.type == token.LPAR:
             return NO
 
-    elif p.type == syms.subscript:
+    elif p.type in {syms.subscript, syms.sliceop}:
         # indexing
         if not prev:
             assert p.parent is not None, "subscripts are always parented"
@@ -1283,7 +1575,7 @@ def whitespace(leaf: Leaf) -> str:  # noqa C901
 
             return NO
 
-        else:
+        elif not complex_subscript:
             return NO
 
     elif p.type == syms.atom:
@@ -1291,21 +1583,9 @@ def whitespace(leaf: Leaf) -> str:  # noqa C901
             # dots, but not the first one.
             return NO
 
-    elif (
-        p.type == syms.listmaker
-        or p.type == syms.testlist_gexp
-        or p.type == syms.subscriptlist
-    ):
-        # list interior, including unpacking
-        if not prev:
-            return NO
-
     elif p.type == syms.dictsetmaker:
-        # dict and set interior, including unpacking
-        if not prev:
-            return NO
-
-        if prev.type == token.DOUBLESTAR:
+        # dict unpacking
+        if prev and prev.type == token.DOUBLESTAR:
             return NO
 
     elif p.type in {syms.factor, syms.star_expr}:
@@ -1317,9 +1597,10 @@ def whitespace(leaf: Leaf) -> str:  # noqa C901
 
             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:
@@ -1334,7 +1615,7 @@ def whitespace(leaf: Leaf) -> str:  # noqa C901
                 return NO
 
         elif t == token.NAME:
-            if v == 'import':
+            if v == "import":
                 return SPACE
 
             if prev and prev.type == token.DOT:
@@ -1364,23 +1645,84 @@ def preceding_leaf(node: Optional[LN]) -> Optional[Leaf]:
     return None
 
 
-def is_delimiter(leaf: Leaf) -> int:
-    """Return the priority of the `leaf` delimiter. Return 0 if not delimiter.
+def child_towards(ancestor: Node, descendant: LN) -> Optional[LN]:
+    """Return the child of `ancestor` that contains `descendant`."""
+    node: Optional[LN] = descendant
+    while node and node.parent != ancestor:
+        node = node.parent
+    return node
+
+
+def is_split_after_delimiter(leaf: Leaf, previous: Leaf = None) -> int:
+    """Return the priority of the `leaf` delimiter, given a line break after it.
+
+    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
 
-    if leaf.type in COMPARATORS:
-        return COMPARATOR_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 is_vararg(leaf, within=VARARGS_PARENTS | UNPACKING_PARENTS):
+        # * and ** might also be MATH_OPERATORS but in this case they are not.
+        # Don't treat them as a delimiter.
+        return 0
 
     if (
         leaf.type in MATH_OPERATORS
         and leaf.parent
         and leaf.parent.type not in {syms.factor, syms.star_expr}
     ):
-        return MATH_PRIORITY
+        return MATH_PRIORITIES[leaf.type]
+
+    if leaf.type in COMPARATORS:
+        return COMPARATOR_PRIORITY
+
+    if (
+        leaf.type == token.STRING
+        and previous is not None
+        and previous.type == token.STRING
+    ):
+        return STRING_PRIORITY
+
+    if (
+        leaf.type == token.NAME
+        and leaf.value == "for"
+        and leaf.parent
+        and leaf.parent.type in {syms.comp_for, syms.old_comp_for}
+    ):
+        return COMPREHENSION_PRIORITY
+
+    if (
+        leaf.type == token.NAME
+        and leaf.value == "if"
+        and leaf.parent
+        and leaf.parent.type in {syms.comp_if, syms.old_comp_if}
+    ):
+        return COMPREHENSION_PRIORITY
+
+    if (
+        leaf.type == token.NAME
+        and leaf.value in {"if", "else"}
+        and leaf.parent
+        and leaf.parent.type == syms.test
+    ):
+        return TERNARY_PRIORITY
+
+    if leaf.type == token.NAME and leaf.value in LOGIC_OPERATORS and leaf.parent:
+        return LOGIC_PRIORITY
 
     return 0
 
@@ -1408,17 +1750,17 @@ def generate_comments(leaf: Leaf) -> Iterator[Leaf]:
     if not p:
         return
 
-    if '#' not in p:
+    if "#" not in p:
         return
 
     consumed = 0
     nlines = 0
-    for index, line in enumerate(p.split('\n')):
+    for index, line in enumerate(p.split("\n")):
         consumed += len(line) + 1  # adding the length of the split '\n'
         line = line.lstrip()
         if not line:
             nlines += 1
-        if not line.startswith('#'):
+        if not line.startswith("#"):
             continue
 
         if index == 0 and leaf.type != token.ENDMARKER:
@@ -1426,13 +1768,18 @@ def generate_comments(leaf: Leaf) -> Iterator[Leaf]:
         else:
             comment_type = STANDALONE_COMMENT
         comment = make_comment(line)
-        yield Leaf(comment_type, comment, prefix='\n' * nlines)
+        yield Leaf(comment_type, comment, prefix="\n" * nlines)
 
-        if comment in {'# fmt: on', '# yapf: enable'}:
+        if comment in {"# fmt: on", "# yapf: enable"}:
             raise FormatOn(consumed)
 
-        if comment in {'# fmt: off', '# yapf: disable'}:
-            raise FormatOff(consumed)
+        if comment in {"# fmt: off", "# yapf: disable"}:
+            if comment_type == STANDALONE_COMMENT:
+                raise FormatOff(consumed)
+
+            prev = preceding_leaf(leaf)
+            if not prev or prev.type in WHITESPACE:  # standalone comment in disguise
+                raise FormatOff(consumed)
 
         nlines = 0
 
@@ -1447,13 +1794,13 @@ def make_comment(content: str) -> str:
     """
     content = content.rstrip()
     if not content:
-        return '#'
+        return "#"
 
-    if content[0] == '#':
+    if content[0] == "#":
         content = content[1:]
-    if content and content[0] not in ' !:#':
-        content = ' ' + content
-    return '#' + content
+    if content and content[0] not in " !:#":
+        content = " " + content
+    return "#" + content
 
 
 def split_line(
@@ -1473,11 +1820,11 @@ def split_line(
         yield line
         return
 
-    line_str = str(line).strip('\n')
+    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
+        and "\n" not in line_str  # multiline strings
+        and not line.contains_standalone_comments()
     ):
         yield line
         return
@@ -1485,6 +1832,8 @@ def split_line(
     split_funcs: List[SplitFunc]
     if line.is_def:
         split_funcs = [left_hand_split]
+    elif line.is_import:
+        split_funcs = [explode_split]
     elif line.inside_brackets:
         split_funcs = [delimiter_split, standalone_comment_split, right_hand_split]
     else:
@@ -1496,7 +1845,7 @@ def split_line(
         result: List[Line] = []
         try:
             for l in split_func(line, py36):
-                if str(l).strip('\n') == line_str:
+                if str(l).strip("\n") == line_str:
                     raise CannotSplit("Split function returned an unchanged result")
 
                 result.extend(
@@ -1517,7 +1866,8 @@ def left_hand_split(line: Line, py36: bool = False) -> Iterator[Line]:
     """Split line into many lines, starting with the first matching bracket pair.
 
     Note: this usually looks weird, only use this for function definitions.
-    Prefer RHS otherwise.
+    Prefer RHS otherwise.  This is why this function is not symmetrical with
+    :func:`right_hand_split` which also handles optional parentheses.
     """
     head = Line(depth=line.depth)
     body = Line(depth=line.depth + 1, inside_brackets=True)
@@ -1543,9 +1893,7 @@ def left_hand_split(line: Line, py36: bool = False) -> Iterator[Line]:
     if body_leaves:
         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 result, leaves in (head, head_leaves), (body, body_leaves), (tail, tail_leaves):
         for leaf in leaves:
             result.append(leaf, preformatted=True)
             for comment_after in line.comments_after(leaf):
@@ -1556,8 +1904,13 @@ def left_hand_split(line: Line, py36: bool = False) -> Iterator[Line]:
             yield result
 
 
-def right_hand_split(line: Line, py36: bool = False) -> Iterator[Line]:
-    """Split line into many lines, starting with the last matching bracket pair."""
+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.
+
+    If the split was by optional parentheses, attempt splitting without them, too.
+    """
     head = Line(depth=line.depth)
     body = Line(depth=line.depth + 1, inside_brackets=True)
     tail = Line(depth=line.depth)
@@ -1566,14 +1919,16 @@ def right_hand_split(line: Line, py36: bool = False) -> Iterator[Line]:
     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:
                 current_leaves = head_leaves if body_leaves else tail_leaves
         current_leaves.append(leaf)
         if current_leaves is tail_leaves:
-            if leaf.type in CLOSING_BRACKETS:
+            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()
@@ -1581,15 +1936,41 @@ def right_hand_split(line: Line, py36: bool = False) -> Iterator[Line]:
     # Since body is a new indent level, remove spurious leading whitespace.
     if body_leaves:
         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.
-    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 comment_after in line.comments_after(leaf):
                 result.append(comment_after, preformatted=True)
     bracket_split_succeeded_or_raise(head, body, tail)
+    assert opening_bracket and closing_bracket
+    if (
+        # the opening bracket is an optional paren
+        opening_bracket.type == token.LPAR
+        and not opening_bracket.value
+        # the closing bracket is an optional paren
+        and closing_bracket.type == token.RPAR
+        and not closing_bracket.value
+        # there are no delimiters or standalone comments in the body
+        and not body.bracket_tracker.delimiters
+        and not line.contains_standalone_comments(0)
+        # and it's not an import (optional parens are the only thing we can split
+        # on in this case; attempting a split without them is a waste of time)
+        and not line.is_import
+    ):
+        omit = {id(closing_bracket), *omit}
+        try:
+            yield from right_hand_split(line, py36=py36, omit=omit)
+            return
+        except CannotSplit:
+            pass
+
+    ensure_visible(opening_bracket)
+    ensure_visible(closing_bracket)
     for result in (head, body, tail):
         if result:
             yield result
@@ -1680,8 +2061,7 @@ def delimiter_split(line: Line, py36: bool = False) -> Iterator[Line]:
         lowest_depth = min(lowest_depth, leaf.bracket_depth)
         if (
             leaf.bracket_depth == lowest_depth
-            and leaf.type == token.STAR
-            or leaf.type == token.DOUBLESTAR
+            and is_vararg(leaf, within=VARARGS_PARENTS)
         ):
             trailing_comma_safe = trailing_comma_safe and py36
         leaf_priority = delimiters.get(id(leaf))
@@ -1691,23 +2071,19 @@ def delimiter_split(line: Line, py36: bool = False) -> Iterator[Line]:
             current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)
     if current_line:
         if (
-            delimiter_priority == COMMA_PRIORITY
+            trailing_comma_safe
+            and delimiter_priority == COMMA_PRIORITY
             and current_line.leaves[-1].type != token.COMMA
-            and trailing_comma_safe
+            and current_line.leaves[-1].type != STANDALONE_COMMENT
         ):
-            current_line.append(Leaf(token.COMMA, ','))
+            current_line.append(Leaf(token.COMMA, ","))
         yield current_line
 
 
 @dont_increase_indentation
 def standalone_comment_split(line: Line, py36: bool = False) -> Iterator[Line]:
     """Split standalone comments from the rest of the line."""
-    for leaf in line.leaves:
-        if leaf.type == STANDALONE_COMMENT:
-            if leaf.bracket_depth == 0:
-                break
-
-    else:
+    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)
@@ -1733,6 +2109,26 @@ def standalone_comment_split(line: Line, py36: bool = False) -> Iterator[Line]:
         yield current_line
 
 
+def explode_split(
+    line: Line, py36: bool = False, omit: Collection[LeafID] = ()
+) -> Iterator[Line]:
+    """Split by rightmost bracket and immediately split contents by a delimiter."""
+    new_lines = list(right_hand_split(line, py36, omit))
+    if len(new_lines) != 3:
+        yield from new_lines
+        return
+
+    yield new_lines[0]
+
+    try:
+        yield from delimiter_split(new_lines[1], py36)
+
+    except CannotSplit:
+        yield new_lines[1]
+
+    yield new_lines[2]
+
+
 def is_import(leaf: Leaf) -> bool:
     """Return True if the given leaf starts an import statement."""
     p = leaf.parent
@@ -1741,8 +2137,8 @@ def is_import(leaf: Leaf) -> bool:
     return bool(
         t == token.NAME
         and (
-            (v == 'import' and p and p.type == syms.import_name)
-            or (v == 'from' and p and p.type == syms.import_from)
+            (v == "import" and p and p.type == syms.import_name)
+            or (v == "from" and p and p.type == syms.import_from)
         )
     )
 
@@ -1754,15 +2150,247 @@ def normalize_prefix(leaf: Leaf, *, inside_brackets: bool) -> None:
     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')
+        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
+            leaf.prefix = "\n" * nl_count
             return
 
-    leaf.prefix = ''
+    leaf.prefix = ""
+
+
+def normalize_string_quotes(leaf: Leaf) -> None:
+    """Prefer double quotes but only if it doesn't cause more escaping.
+
+    Adds or removes backslashes as appropriate. Doesn't parse and fix
+    strings nested in f-strings (yet).
+
+    Note: Mutates its argument.
+    """
+    value = leaf.value.lstrip("furbFURB")
+    if value[:3] == '"""':
+        return
+
+    elif value[:3] == "'''":
+        orig_quote = "'''"
+        new_quote = '"""'
+    elif value[0] == '"':
+        orig_quote = '"'
+        new_quote = "'"
+    else:
+        orig_quote = "'"
+        new_quote = '"'
+    first_quote_pos = leaf.value.find(orig_quote)
+    if first_quote_pos == -1:
+        return  # There's an internal error
+
+    prefix = leaf.value[:first_quote_pos]
+    unescaped_new_quote = re.compile(rf"(([^\\]|^)(\\\\)*){new_quote}")
+    escaped_new_quote = re.compile(rf"([^\\]|^)\\(\\\\)*{new_quote}")
+    escaped_orig_quote = re.compile(rf"([^\\]|^)\\(\\\\)*{orig_quote}")
+    body = leaf.value[first_quote_pos + len(orig_quote) : -len(orig_quote)]
+    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:
+        # remove unnecessary quotes
+        new_body = sub_twice(escaped_new_quote, rf"\1\2{new_quote}", body)
+        if body != new_body:
+            # Consider the string without unnecessary quotes as the original
+            body = new_body
+            leaf.value = f"{prefix}{orig_quote}{body}{orig_quote}"
+        new_body = sub_twice(escaped_orig_quote, rf"\1\2{orig_quote}", new_body)
+        new_body = sub_twice(unescaped_new_quote, 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.
+
+    `parens_after` is a set of string leaf values immeditely after which parens
+    should be put.
+
+    Standardizes on visible parentheses for single-element tuples, and keeps
+    existing visible parentheses for other tuples and generator expressions.
+    """
+    check_lpar = False
+    for child in list(node.children):
+        if check_lpar:
+            if child.type == syms.atom:
+                maybe_make_parens_invisible_in_atom(child)
+            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 maybe_make_parens_invisible_in_atom(node: LN) -> bool:
+    """If it's safe, make the parens in the atom `node` invisible, recusively."""
+    if (
+        node.type != syms.atom
+        or is_empty_tuple(node)
+        or is_one_tuple(node)
+        or is_yield(node)
+        or max_delimiter_priority_in_atom(node) >= COMMA_PRIORITY
+    ):
+        return False
+
+    first = node.children[0]
+    last = node.children[-1]
+    if first.type == token.LPAR and last.type == token.RPAR:
+        # make parentheses invisible
+        first.value = ""  # type: ignore
+        last.value = ""  # type: ignore
+        if len(node.children) > 1:
+            maybe_make_parens_invisible_in_atom(node.children[1])
+        return True
+
+    return False
+
+
+def is_empty_tuple(node: LN) -> bool:
+    """Return True if `node` holds an empty tuple."""
+    return (
+        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
+    )
+
+
+def is_yield(node: LN) -> bool:
+    """Return True if `node` holds a `yield` or `yield from` expression."""
+    if node.type == syms.yield_expr:
+        return True
+
+    if node.type == token.NAME and node.value == "yield":  # type: ignore
+        return True
+
+    if node.type != syms.atom:
+        return False
+
+    if len(node.children) != 3:
+        return False
+
+    lpar, expr, rpar = node.children
+    if lpar.type == token.LPAR and rpar.type == token.RPAR:
+        return is_yield(expr)
+
+    return False
+
+
+def is_vararg(leaf: Leaf, within: Set[NodeType]) -> bool:
+    """Return True if `leaf` is a star or double star in a vararg or kwarg.
+
+    If `within` includes VARARGS_PARENTS, this applies to function signatures.
+    If `within` includes UNPACKING_PARENTS, it applies to right hand-side
+    extended iterable unpacking (PEP 3132) and additional unpacking
+    generalizations (PEP 448).
+    """
+    if leaf.type not in STARS or not leaf.parent:
+        return False
+
+    p = leaf.parent
+    if p.type == syms.star_expr:
+        # Star expressions are also used as assignment targets in extended
+        # iterable unpacking (PEP 3132).  See what its parent is instead.
+        if not p.parent:
+            return False
+
+        p = p.parent
+
+    return p.type in within
+
+
+def max_delimiter_priority_in_atom(node: LN) -> int:
+    """Return maximum delimiter priority inside `node`.
+
+    This is specific to atoms with contents contained in a pair of parentheses.
+    If `node` isn't an atom or there are no enclosing parentheses, returns 0.
+    """
+    if node.type != syms.atom:
+        return 0
+
+    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:
@@ -1770,29 +2398,34 @@ def is_python36(node: Node) -> bool:
 
     Currently looking for:
     - f-strings; and
-    - trailing commas after * or ** in function signatures.
+    - trailing commas after * or ** in function signatures and calls.
     """
     for n in node.pre_order():
         if n.type == token.STRING:
             value_head = n.value[:2]  # type: ignore
-            if value_head in {'f"', 'F"', "f'", "F'", 'rf', 'fr', 'RF', 'FR'}:
+            if value_head in {'f"', 'F"', "f'", "F'", "rf", "fr", "RF", "FR"}:
                 return True
 
         elif (
-            n.type == syms.typedargslist
+            n.type in {syms.typedargslist, syms.arglist}
             and n.children
             and n.children[-1].type == token.COMMA
         ):
             for ch in n.children:
-                if ch.type == token.STAR or ch.type == token.DOUBLESTAR:
+                if ch.type in STARS:
                     return True
 
+                if ch.type == syms.argument:
+                    for argch in ch.children:
+                        if argch.type in STARS:
+                            return True
+
     return False
 
 
-PYTHON_EXTENSIONS = {'.py'}
+PYTHON_EXTENSIONS = {".py"}
 BLACKLISTED_DIRECTORIES = {
-    'build', 'buck-out', 'dist', '_build', '.git', '.hg', '.mypy_cache', '.tox', '.venv'
+    "build", "buck-out", "dist", "_build", ".git", ".hg", ".mypy_cache", ".tox", ".venv"
 }
 
 
@@ -1815,23 +2448,30 @@ def gen_python_files_in_dir(path: Path) -> Iterator[Path]:
 class Report:
     """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:
+    def done(self, src: Path, changed: Changed) -> None:
         """Increment the counter for successful reformatting. Write out a message."""
-        if changed:
-            reformatted = 'would reformat' if self.check else 'reformatted'
-            out(f'{reformatted} {src}')
+        if changed is Changed.YES:
+            reformatted = "would reformat" if self.check else "reformatted"
+            if not self.quiet:
+                out(f"{reformatted} {src}")
             self.change_count += 1
         else:
-            out(f'{src} already well formatted, good job.', bold=False)
+            if not self.quiet:
+                if changed is Changed.NO:
+                    msg = f"{src} already well formatted, good job."
+                else:
+                    msg = f"{src} wasn't modified on disk since last run."
+                out(msg, bold=False)
             self.same_count += 1
 
     def failed(self, src: Path, message: str) -> None:
         """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
@@ -1868,19 +2508,19 @@ class Report:
             failed = "failed to reformat"
         report = []
         if self.change_count:
-            s = 's' if self.change_count > 1 else ''
+            s = "s" if self.change_count > 1 else ""
             report.append(
-                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} {unchanged}')
+            s = "s" if self.same_count > 1 else ""
+            report.append(f"{self.same_count} file{s} {unchanged}")
         if self.failure_count:
-            s = 's' if self.failure_count > 1 else ''
+            s = "s" if self.failure_count > 1 else ""
             report.append(
-                click.style(f'{self.failure_count} file{s} {failed}', fg='red')
+                click.style(f"{self.failure_count} file{s} {failed}", fg="red")
             )
-        return ', '.join(report) + '.'
+        return ", ".join(report) + "."
 
 
 def assert_equivalent(src: str, dst: str) -> None:
@@ -1927,17 +2567,17 @@ def assert_equivalent(src: str, dst: str) -> None:
     try:
         dst_ast = ast.parse(dst)
     except Exception as exc:
-        log = dump_to_file(''.join(traceback.format_tb(exc.__traceback__)), dst)
+        log = dump_to_file("".join(traceback.format_tb(exc.__traceback__)), dst)
         raise AssertionError(
             f"INTERNAL ERROR: Black produced invalid code: {exc}. "
             f"Please report a bug on https://github.com/ambv/black/issues.  "
             f"This invalid output might be helpful: {log}"
         ) from None
 
-    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:
-        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.  "
@@ -1951,8 +2591,8 @@ def assert_stable(src: str, dst: str, line_length: int) -> None:
     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 "
@@ -1967,11 +2607,12 @@ def dump_to_file(*output: str) -> str:
     import tempfile
 
     with tempfile.NamedTemporaryFile(
-        mode='w', prefix='blk_', suffix='.log', delete=False
+        mode="w", prefix="blk_", suffix=".log", delete=False, encoding="utf8"
     ) as f:
         for lines in output:
             f.write(lines)
-            f.write('\n')
+            if lines and lines[-1] != "\n":
+                f.write("\n")
     return f.name
 
 
@@ -1979,12 +2620,112 @@ def diff(a: str, b: str, a_name: str, b_name: str) -> str:
     """Return a unified diff string between strings `a` and `b`."""
     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)
     )
 
 
-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()
+
+
+def sub_twice(regex: Pattern[str], replacement: str, original: str) -> str:
+    """Replace `regex` with `replacement` twice on `original`.
+
+    This is used by string normalization to perform replaces on
+    overlapping matches.
+    """
+    return regex.sub(replacement, regex.sub(replacement, original))
+
+
+CACHE_DIR = Path(user_cache_dir("black", version=__version__))
+
+
+def get_cache_file(line_length: int) -> Path:
+    return CACHE_DIR / f"cache.{line_length}.pickle"
+
+
+def read_cache(line_length: int) -> Cache:
+    """Read the cache if it exists and is well formed.
+
+    If it is not well formed, the call to write_cache later should resolve the issue.
+    """
+    cache_file = get_cache_file(line_length)
+    if not cache_file.exists():
+        return {}
+
+    with cache_file.open("rb") as fobj:
+        try:
+            cache: Cache = pickle.load(fobj)
+        except pickle.UnpicklingError:
+            return {}
+
+    return cache
+
+
+def get_cache_info(path: Path) -> CacheInfo:
+    """Return the information used to check if a file is already formatted or not."""
+    stat = path.stat()
+    return stat.st_mtime, stat.st_size
+
+
+def filter_cached(
+    cache: Cache, sources: Iterable[Path]
+) -> Tuple[List[Path], List[Path]]:
+    """Split a list of paths into two.
+
+    The first list contains paths of files that modified on disk or are not in the
+    cache. The other list contains paths to non-modified files.
+    """
+    todo, done = [], []
+    for src in sources:
+        src = src.resolve()
+        if cache.get(src) != get_cache_info(src):
+            todo.append(src)
+        else:
+            done.append(src)
+    return todo, done
+
+
+def write_cache(cache: Cache, sources: List[Path], line_length: int) -> None:
+    """Update the cache file."""
+    cache_file = get_cache_file(line_length)
+    try:
+        if not CACHE_DIR.exists():
+            CACHE_DIR.mkdir(parents=True)
+        new_cache = {**cache, **{src.resolve(): get_cache_info(src) for src in sources}}
+        with cache_file.open("wb") as fobj:
+            pickle.dump(new_cache, fobj, protocol=pickle.HIGHEST_PROTOCOL)
+    except OSError:
+        pass
+
+
+if __name__ == "__main__":
     main()