X-Git-Url: https://git.madduck.net/etc/vim.git/blobdiff_plain/5e1e1a1820504b2709a7201800ff042f0abbad2c..31fbd1982f9511c5b7bce93eb133110811f90e0f:/black.py

diff --git a/black.py b/black.py
index c77166a..a1a3468 100644
--- a/black.py
+++ b/black.py
@@ -1,5 +1,3 @@
-#!/usr/bin/env python3
-
 import asyncio
 import pickle
 from asyncio.base_events import BaseEventLoop
@@ -26,6 +24,7 @@ from typing import (
     List,
     Optional,
     Pattern,
+    Sequence,
     Set,
     Tuple,
     Type,
@@ -43,8 +42,10 @@ from blib2to3 import pygram, pytree
 from blib2to3.pgen2 import driver, token
 from blib2to3.pgen2.parse import ParseError
 
-__version__ = "18.4a2"
+
+__version__ = "18.4a6"
 DEFAULT_LINE_LENGTH = 88
+
 # types
 syms = pygram.python_symbols
 FileContent = str
@@ -88,11 +89,11 @@ 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)
 
 
@@ -191,43 +192,39 @@ def main(
         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:
-        return_code = reformat_one(
-            sources[0], line_length, fast, quiet, write_back, check
-        )
+        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, write_back, fast, quiet, loop, executor, check
+                    sources, line_length, fast, write_back, report, loop, executor
                 )
             )
         finally:
             shutdown(loop)
-    ctx.exit(return_code)
+        if not quiet:
+            out("All done! ✨ 🍰 ✨")
+            click.echo(str(report))
+    ctx.exit(report.return_code)
 
 
 def reformat_one(
-    src: Path,
-    line_length: int,
-    fast: bool,
-    quiet: bool,
-    write_back: WriteBack,
-    check: bool,
-) -> int:
+    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`.
     """
-    report = Report(check=check, quiet=quiet)
     try:
         changed = Changed.NO
         if not src.is_file() and str(src) == "-":
@@ -238,35 +235,30 @@ def reformat_one(
         else:
             cache: Cache = {}
             if write_back != WriteBack.DIFF:
-                cache = read_cache()
+                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
-                )
+            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.DIFF and changed is not Changed.NO:
-                write_cache(cache, [src])
+            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))
-    return report.return_code
 
 
 async def schedule_formatting(
     sources: List[Path],
     line_length: int,
-    write_back: WriteBack,
     fast: bool,
-    quiet: bool,
+    write_back: WriteBack,
+    report: "Report",
     loop: BaseEventLoop,
     executor: Executor,
-    check: bool,
-) -> int:
+) -> None:
     """Run formatting of `sources` in parallel using the provided `executor`.
 
     (Use ProcessPoolExecutors for actual parallelism.)
@@ -274,10 +266,9 @@ async def schedule_formatting(
     `line_length`, `write_back`, and `fast` options are passed to
     :func:`format_file_in_place`.
     """
-    report = Report(check=check, quiet=quiet)
     cache: Cache = {}
     if write_back != WriteBack.DIFF:
-        cache = read_cache()
+        cache = read_cache(line_length)
         sources, cached = filter_cached(cache, sources)
         for src in cached:
             report.done(src, Changed.CACHED)
@@ -291,43 +282,33 @@ async def schedule_formatting(
             manager = Manager()
             lock = manager.Lock()
         tasks = {
-            src: loop.run_in_executor(
+            loop.run_in_executor(
                 executor, format_file_in_place, src, line_length, fast, write_back, lock
-            )
-            for src in sources
+            ): src
+            for src in sorted(sources)
         }
-        _task_values = list(tasks.values())
+        pending: Iterable[asyncio.Task] = tasks.keys()
         try:
-            loop.add_signal_handler(signal.SIGINT, cancel, _task_values)
-            loop.add_signal_handler(signal.SIGTERM, cancel, _task_values)
+            loop.add_signal_handler(signal.SIGINT, cancel, pending)
+            loop.add_signal_handler(signal.SIGTERM, cancel, pending)
         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)
-
+        while pending:
+            done, _ = await asyncio.wait(pending, return_when=asyncio.FIRST_COMPLETED)
+            for task in done:
+                src = tasks.pop(task)
+                if task.cancelled():
+                    cancelled.append(task)
+                elif task.exception():
+                    report.failed(src, str(task.exception()))
+                else:
+                    formatted.append(src)
+                    report.done(src, Changed.YES if task.result() else Changed.NO)
     if cancelled:
         await asyncio.gather(*cancelled, loop=loop, return_exceptions=True)
-    elif not quiet:
-        out("All done! ✨ 🍰 ✨")
-    if not quiet:
-        click.echo(str(report))
-
-    if write_back != WriteBack.DIFF and formatted:
-        write_cache(cache, formatted)
-
-    return report.return_code
+    if write_back == WriteBack.YES and formatted:
+        write_cache(cache, formatted, line_length)
 
 
 def format_file_in_place(
@@ -342,12 +323,13 @@ def format_file_in_place(
     If `write_back` is True, write reformatted code back to stdout.
     `line_length` and `fast` options are passed to :func:`format_file_contents`.
     """
+    is_pyi = src.suffix == ".pyi"
 
     with tokenize.open(src) as src_buffer:
         src_contents = src_buffer.read()
     try:
         dst_contents = format_file_contents(
-            src_contents, line_length=line_length, fast=fast
+            src_contents, line_length=line_length, fast=fast, is_pyi=is_pyi
         )
     except NothingChanged:
         return False
@@ -356,8 +338,8 @@ def format_file_in_place(
         with open(src, "w", encoding=src_buffer.encoding) as f:
             f.write(dst_contents)
     elif write_back == write_back.DIFF:
-        src_name = f"{src.name}  (original)"
-        dst_name = f"{src.name}  (formatted)"
+        src_name = f"{src}  (original)"
+        dst_name = f"{src}  (formatted)"
         diff_contents = diff(src_contents, dst_contents, src_name, dst_name)
         if lock:
             lock.acquire()
@@ -396,7 +378,7 @@ def format_stdin_to_stdout(
 
 
 def format_file_contents(
-    src_contents: str, line_length: int, fast: bool
+    src_contents: str, *, line_length: int, fast: bool, is_pyi: bool = False
 ) -> FileContent:
     """Reformat contents a file and return new contents.
 
@@ -407,26 +389,33 @@ def format_file_contents(
     if src_contents.strip() == "":
         raise NothingChanged
 
-    dst_contents = format_str(src_contents, line_length=line_length)
+    dst_contents = format_str(src_contents, line_length=line_length, is_pyi=is_pyi)
     if src_contents == dst_contents:
         raise NothingChanged
 
     if not fast:
         assert_equivalent(src_contents, dst_contents)
-        assert_stable(src_contents, dst_contents, line_length=line_length)
+        assert_stable(
+            src_contents, dst_contents, line_length=line_length, is_pyi=is_pyi
+        )
     return dst_contents
 
 
-def format_str(src_contents: str, line_length: int) -> FileContent:
+def format_str(
+    src_contents: str, line_length: int, *, is_pyi: bool = False
+) -> FileContent:
     """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 = ""
-    lines = LineGenerator()
-    elt = EmptyLineTracker()
+    future_imports = get_future_imports(src_node)
+    elt = EmptyLineTracker(is_pyi=is_pyi)
     py36 = is_python36(src_node)
+    lines = LineGenerator(
+        remove_u_prefix=py36 or "unicode_literals" in future_imports, is_pyi=is_pyi
+    )
     empty_line = Line()
     after = 0
     for current_line in lines.visit(src_node):
@@ -443,7 +432,6 @@ 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,
 ]
 
@@ -569,19 +557,20 @@ 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 = {
@@ -597,12 +586,62 @@ UNPACKING_PARENTS = {
     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: 9,
+    token.CIRCUMFLEX: 8,
+    token.AMPER: 7,
+    token.LEFTSHIFT: 6,
+    token.RIGHTSHIFT: 6,
+    token.PLUS: 5,
+    token.MINUS: 5,
+    token.STAR: 4,
+    token.SLASH: 4,
+    token.DOUBLESLASH: 4,
+    token.PERCENT: 4,
+    token.AT: 4,
+    token.TILDE: 3,
+    token.DOUBLESTAR: 2,
+}
+DOT_PRIORITY = 1
 
 
 @dataclass
@@ -613,8 +652,8 @@ class BracketTracker:
     bracket_match: Dict[Tuple[Depth, NodeType], Leaf] = Factory(dict)
     delimiters: Dict[LeafID, Priority] = Factory(dict)
     previous: Optional[Leaf] = None
-    _for_loop_variable: bool = False
-    _lambda_arguments: bool = False
+    _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.
@@ -668,6 +707,17 @@ class BracketTracker:
         """
         return max(v for k, v in self.delimiters.items() if k not in exclude)
 
+    def delimiter_count_with_priority(self, priority: int = 0) -> int:
+        """Return the number of delimiters with the given `priority`.
+
+        If no `priority` is passed, defaults to max priority on the line.
+        """
+        if not self.delimiters:
+            return 0
+
+        priority = priority or self.max_delimiter_priority()
+        return sum(1 for p in self.delimiters.values() if p == priority)
+
     def maybe_increment_for_loop_variable(self, leaf: Leaf) -> bool:
         """In a for loop, or comprehension, the variables are often unpacks.
 
@@ -676,7 +726,7 @@ class BracketTracker:
         """
         if leaf.type == token.NAME and leaf.value == "for":
             self.depth += 1
-            self._for_loop_variable = True
+            self._for_loop_variable += 1
             return True
 
         return False
@@ -685,7 +735,7 @@ class BracketTracker:
         """See `maybe_increment_for_loop_variable` above for explanation."""
         if self._for_loop_variable and leaf.type == token.NAME and leaf.value == "in":
             self.depth -= 1
-            self._for_loop_variable = False
+            self._for_loop_variable -= 1
             return True
 
         return False
@@ -698,7 +748,7 @@ class BracketTracker:
         """
         if leaf.type == token.NAME and leaf.value == "lambda":
             self.depth += 1
-            self._lambda_arguments = True
+            self._lambda_arguments += 1
             return True
 
         return False
@@ -707,11 +757,15 @@ class BracketTracker:
         """See `maybe_increment_lambda_arguments` above for explanation."""
         if self._lambda_arguments and leaf.type == token.COLON:
             self.depth -= 1
-            self._lambda_arguments = False
+            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:
@@ -722,6 +776,7 @@ class Line:
     comments: List[Tuple[Index, Leaf]] = Factory(list)
     bracket_tracker: BracketTracker = Factory(BracketTracker)
     inside_brackets: bool = False
+    should_explode: bool = False
 
     def append(self, leaf: Leaf, preformatted: bool = False) -> None:
         """Add a new `leaf` to the end of the line.
@@ -737,14 +792,17 @@ class Line:
         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.bracket_tracker.mark(leaf)
             self.maybe_remove_trailing_comma(leaf)
-
         if not self.append_comment(leaf):
             self.leaves.append(leaf)
 
@@ -789,6 +847,13 @@ class Line:
             and self.leaves[0].value == "class"
         )
 
+    @property
+    def is_stub_class(self) -> bool:
+        """Is this line a class definition with a body consisting only of "..."?"""
+        return self.is_class and self.leaves[-3:] == [
+            Leaf(token.DOT, ".") for _ in range(3)
+        ]
+
     @property
     def is_def(self) -> bool:
         """Is this a function definition? (Also returns True for async defs.)"""
@@ -801,14 +866,11 @@ class Line:
             second_leaf: Optional[Leaf] = self.leaves[1]
         except IndexError:
             second_leaf = None
-        return (
-            (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"
-            )
+        return (first_leaf.type == token.NAME and first_leaf.value == "def") or (
+            first_leaf.type == token.ASYNC
+            and second_leaf is not None
+            and second_leaf.type == token.NAME
+            and second_leaf.value == "def"
         )
 
     @property
@@ -832,6 +894,22 @@ class Line:
             and self.leaves[0].value == "yield"
         )
 
+    @property
+    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:
@@ -860,9 +938,14 @@ 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
@@ -873,7 +956,7 @@ 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
 
@@ -912,17 +995,21 @@ class Line:
             self.comments.append((after, comment))
             return True
 
-    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
+    def comments_after(self, leaf: Leaf, _index: int = -1) -> Iterator[Leaf]:
+        """Generate comments that should appear directly after `leaf`.
 
-        else:
-            return
+        Provide a non-negative leaf `_index` to speed up the function.
+        """
+        if _index == -1:
+            for _index, _leaf in enumerate(self.leaves):
+                if leaf is _leaf:
+                    break
+
+            else:
+                return
 
         for index, comment_after in self.comments:
-            if _leaf_index == index:
+            if _index == index:
                 yield comment_after
 
     def remove_trailing_comma(self) -> None:
@@ -934,6 +1021,24 @@ 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:
@@ -1012,6 +1117,7 @@ class EmptyLineTracker:
     the prefix of the first leaf consists of optional newlines.  Those newlines
     are consumed by `maybe_empty_lines()` and included in the computation.
     """
+    is_pyi: bool = False
     previous_line: Optional[Line] = None
     previous_after: int = 0
     previous_defs: List[int] = Factory(list)
@@ -1035,7 +1141,7 @@ class EmptyLineTracker:
     def _maybe_empty_lines(self, current_line: Line) -> Tuple[int, int]:
         max_allowed = 1
         if current_line.depth == 0:
-            max_allowed = 2
+            max_allowed = 1 if self.is_pyi else 2
         if current_line.leaves:
             # Consume the first leaf's extra newlines.
             first_leaf = current_line.leaves[0]
@@ -1047,7 +1153,10 @@ class EmptyLineTracker:
         depth = current_line.depth
         while self.previous_defs and self.previous_defs[-1] >= depth:
             self.previous_defs.pop()
-            before = 1 if depth else 2
+            if self.is_pyi:
+                before = 0 if depth else 1
+            else:
+                before = 1 if depth else 2
         is_decorator = current_line.is_decorator
         if is_decorator or current_line.is_def or current_line.is_class:
             if not is_decorator:
@@ -1056,18 +1165,32 @@ 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
 
-            newlines = 2
-            if current_line.depth:
+            if (
+                self.previous_line.is_comment
+                and self.previous_line.depth == current_line.depth
+                and before == 0
+            ):
+                return 0, 0
+
+            if self.is_pyi:
+                if self.previous_line.depth > current_line.depth:
+                    newlines = 1
+                elif current_line.is_class or self.previous_line.is_class:
+                    if current_line.is_stub_class and self.previous_line.is_stub_class:
+                        newlines = 0
+                    else:
+                        newlines = 1
+                else:
+                    newlines = 0
+            else:
+                newlines = 2
+            if current_line.depth and newlines:
                 newlines -= 1
             return newlines, 0
 
-        if current_line.is_flow_control:
-            return before, 1
-
         if (
             self.previous_line
             and self.previous_line.is_import
@@ -1076,13 +1199,6 @@ 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
 
 
@@ -1093,7 +1209,9 @@ class LineGenerator(Visitor[Line]):
     Note: destroys the tree it's visiting by mutating prefixes of its leaves
     in ways that will no longer stringify to valid Python code on the tree.
     """
+    is_pyi: bool = False
     current_line: Line = Factory(Line)
+    remove_u_prefix: bool = False
 
     def line(self, indent: int = 0, type: Type[Line] = Line) -> Iterator[Line]:
         """Generate a line.
@@ -1161,6 +1279,7 @@ class LineGenerator(Visitor[Line]):
             else:
                 normalize_prefix(node, inside_brackets=any_open_brackets)
                 if node.type == token.STRING:
+                    normalize_string_prefix(node, remove_u_prefix=self.remove_u_prefix)
                     normalize_string_quotes(node)
                 if node.type not in WHITESPACE:
                     self.current_line.append(node)
@@ -1174,7 +1293,16 @@ 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(
@@ -1183,14 +1311,13 @@ class LineGenerator(Visitor[Line]):
         """Visit a statement.
 
         This implementation is shared for `if`, `while`, `for`, `try`, `except`,
-        `def`, `with`, `class`, and `assert`.
+        `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.
 
-        `parens` holds pairs of nodes where invisible parentheses should be put.
-        Keys hold nodes after which opening parentheses should be put, values
-        hold nodes before which closing parentheses should be put.
+        `parens` holds a set of string leaf values immediately after which
+        invisible parens should be put.
         """
         normalize_invisible_parens(node, parens_after=parens)
         for child in node.children:
@@ -1199,16 +1326,27 @@ class LineGenerator(Visitor[Line]):
 
             yield from self.visit(child)
 
+    def visit_suite(self, node: Node) -> Iterator[Line]:
+        """Visit a suite."""
+        if self.is_pyi and is_stub_suite(node):
+            yield from self.visit(node.children[2])
+        else:
+            yield from self.visit_default(node)
+
     def visit_simple_stmt(self, node: Node) -> Iterator[Line]:
         """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)
-            yield from self.visit_default(node)
-            yield from self.line(-1)
+            if self.is_pyi and is_stub_body(node):
+                yield from self.visit_default(node)
+            else:
+                yield from self.line(+1)
+                yield from self.visit_default(node)
+                yield from self.line(-1)
 
         else:
-            yield from self.line()
+            if not self.is_pyi or not node.parent or not is_stub_suite(node.parent):
+                yield from self.line()
             yield from self.visit_default(node)
 
     def visit_async_stmt(self, node: Node) -> Iterator[Line]:
@@ -1290,7 +1428,9 @@ class LineGenerator(Visitor[Line]):
         v = self.visit_stmt
         Ø: Set[str] = set()
         self.visit_assert_stmt = partial(v, keywords={"assert"}, parens={"assert", ","})
-        self.visit_if_stmt = partial(v, keywords={"if", "else", "elif"}, parens={"if"})
+        self.visit_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(
@@ -1300,6 +1440,8 @@ class LineGenerator(Visitor[Line]):
         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
 
@@ -1312,8 +1454,12 @@ 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`."""
+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 = "  "
@@ -1327,7 +1473,11 @@ 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
@@ -1337,12 +1487,21 @@ 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:
                 if prevp.parent.type in {
-                    syms.arglist, syms.argument, syms.parameters, syms.varargslist
+                    syms.arglist,
+                    syms.argument,
+                    syms.parameters,
+                    syms.varargslist,
                 }:
                     return NO
 
@@ -1358,7 +1517,7 @@ def whitespace(leaf: Leaf) -> str:  # noqa C901
 
         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
@@ -1464,7 +1623,7 @@ 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"
@@ -1473,7 +1632,7 @@ def whitespace(leaf: Leaf) -> str:  # noqa C901
 
             return NO
 
-        else:
+        elif not complex_subscript:
             return NO
 
     elif p.type == syms.atom:
@@ -1495,10 +1654,10 @@ 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:
@@ -1543,6 +1702,14 @@ def preceding_leaf(node: Optional[LN]) -> Optional[Leaf]:
     return None
 
 
+def child_towards(ancestor: Node, descendant: LN) -> Optional[LN]:
+    """Return the child of `ancestor` that contains `descendant`."""
+    node: Optional[LN] = descendant
+    while node and node.parent != ancestor:
+        node = node.parent
+    return node
+
+
 def is_split_after_delimiter(leaf: Leaf, previous: Leaf = None) -> int:
     """Return the priority of the `leaf` delimiter, given a line break after it.
 
@@ -1570,12 +1737,20 @@ def is_split_before_delimiter(leaf: Leaf, previous: Leaf = None) -> int:
         # Don't treat them as a delimiter.
         return 0
 
+    if (
+        leaf.type == token.DOT
+        and leaf.parent
+        and leaf.parent.type not in {syms.import_from, syms.dotted_name}
+        and (previous is None or previous.type in CLOSING_BRACKETS)
+    ):
+        return DOT_PRIORITY
+
     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
@@ -1587,23 +1762,54 @@ def is_split_before_delimiter(leaf: Leaf, previous: Leaf = None) -> int:
     ):
         return STRING_PRIORITY
 
+    if leaf.type != token.NAME:
+        return 0
+
     if (
-        leaf.type == token.NAME
-        and leaf.value == "for"
+        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"
+        leaf.value == "if"
         and leaf.parent
         and leaf.parent.type in {syms.comp_if, syms.old_comp_if}
     ):
         return COMPREHENSION_PRIORITY
 
-    if leaf.type == token.NAME and leaf.value in LOGIC_OPERATORS and leaf.parent:
+    if leaf.value in {"if", "else"} and leaf.parent and leaf.parent.type == syms.test:
+        return TERNARY_PRIORITY
+
+    if leaf.value == "is":
+        return COMPARATOR_PRIORITY
+
+    if (
+        leaf.value == "in"
+        and leaf.parent
+        and leaf.parent.type in {syms.comp_op, syms.comparison}
+        and not (
+            previous is not None
+            and previous.type == token.NAME
+            and previous.value == "not"
+        )
+    ):
+        return COMPARATOR_PRIORITY
+
+    if (
+        leaf.value == "not"
+        and leaf.parent
+        and leaf.parent.type == syms.comp_op
+        and not (
+            previous is not None
+            and previous.type == token.NAME
+            and previous.value == "is"
+        )
+    ):
+        return COMPARATOR_PRIORITY
+
+    if leaf.value in LOGIC_OPERATORS and leaf.parent:
         return LOGIC_PRIORITY
 
     return 0
@@ -1703,10 +1909,8 @@ def split_line(
         return
 
     line_str = str(line).strip("\n")
-    if (
-        len(line_str) <= line_length
-        and "\n" not in line_str  # multiline strings
-        and not line.contains_standalone_comments()
+    if not line.should_explode and is_line_short_enough(
+        line, line_length=line_length, line_str=line_str
     ):
         yield line
         return
@@ -1714,10 +1918,22 @@ def split_line(
     split_funcs: List[SplitFunc]
     if line.is_def:
         split_funcs = [left_hand_split]
-    elif line.inside_brackets:
-        split_funcs = [delimiter_split, standalone_comment_split, right_hand_split]
     else:
-        split_funcs = [right_hand_split]
+
+        def rhs(line: Line, py36: bool = False) -> Iterator[Line]:
+            for omit in generate_trailers_to_omit(line, line_length):
+                lines = list(right_hand_split(line, py36, omit=omit))
+                if is_line_short_enough(lines[0], line_length=line_length):
+                    yield from lines
+                    return
+
+            # All splits failed, best effort split with no omits.
+            yield from right_hand_split(line, py36)
+
+        if line.inside_brackets:
+            split_funcs = [delimiter_split, standalone_comment_split, rhs]
+        else:
+            split_funcs = [rhs]
     for split_func in split_funcs:
         # We are accumulating lines in `result` because we might want to abort
         # mission and return the original line in the end, or attempt a different
@@ -1746,7 +1962,8 @@ 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)
@@ -1786,7 +2003,14 @@ def left_hand_split(line: Line, py36: bool = False) -> Iterator[Line]:
 def right_hand_split(
     line: Line, py36: bool = False, omit: Collection[LeafID] = ()
 ) -> Iterator[Line]:
-    """Split line into many lines, starting with the last matching bracket pair."""
+    """Split line into many lines, starting with the last matching bracket pair.
+
+    If the split was by optional parentheses, attempt splitting without them, too.
+    `omit` is a collection of closing bracket IDs that shouldn't be considered for
+    this split.
+
+    Note: running this function modifies `bracket_depth` on the leaves of `line`.
+    """
     head = Line(depth=line.depth)
     body = Line(depth=line.depth + 1, inside_brackets=True)
     tail = Line(depth=line.depth)
@@ -1812,8 +2036,9 @@ def right_hand_split(
     # 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.
+    if not head_leaves:
+        # No `head` means the split failed. Either `tail` has all content or
+        # the matching `opening_bracket` wasn't available on `line` anymore.
         raise CannotSplit("No brackets found")
 
     # Build the new lines.
@@ -1825,23 +2050,37 @@ def right_hand_split(
     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 standalone comments in the body
+        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
     ):
-        # These parens were optional. If there aren't any delimiters or standalone
-        # comments in the body, they were unnecessary and another split without
-        # them should be attempted.
-        if not (
-            body.bracket_tracker.delimiters or line.contains_standalone_comments(0)
+        omit = {id(closing_bracket), *omit}
+        delimiter_count = body.bracket_tracker.delimiter_count_with_priority()
+        if (
+            delimiter_count == 0
+            or delimiter_count == 1
+            and (
+                body.leaves[0].type in OPENING_BRACKETS
+                or body.leaves[-1].type in CLOSING_BRACKETS
+            )
         ):
-            omit = {id(closing_bracket), *omit}
-            yield from right_hand_split(line, py36=py36, omit=omit)
-            return
+            try:
+                yield from right_hand_split(line, py36=py36, omit=omit)
+                return
+            except CannotSplit:
+                pass
 
     ensure_visible(opening_bracket)
     ensure_visible(closing_bracket)
+    body.should_explode = should_explode(body, opening_bracket)
     for result in (head, body, tail):
         if result:
             yield result
@@ -1900,14 +2139,16 @@ def delimiter_split(line: Line, py36: bool = False) -> Iterator[Line]:
     except IndexError:
         raise CannotSplit("Line empty")
 
-    delimiters = line.bracket_tracker.delimiters
+    bt = line.bracket_tracker
     try:
-        delimiter_priority = line.bracket_tracker.max_delimiter_priority(
-            exclude={id(last_leaf)}
-        )
+        delimiter_priority = bt.max_delimiter_priority(exclude={id(last_leaf)})
     except ValueError:
         raise CannotSplit("No delimiters found")
 
+    if delimiter_priority == DOT_PRIORITY:
+        if bt.delimiter_count_with_priority(delimiter_priority) == 1:
+            raise CannotSplit("Splitting a single attribute from its owner looks wrong")
+
     current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)
     lowest_depth = sys.maxsize
     trailing_comma_safe = True
@@ -1923,19 +2164,18 @@ def delimiter_split(line: Line, py36: bool = False) -> Iterator[Line]:
             current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)
             current_line.append(leaf)
 
-    for leaf in line.leaves:
+    for index, leaf in enumerate(line.leaves):
         yield from append_to_line(leaf)
 
-        for comment_after in line.comments_after(leaf):
+        for comment_after in line.comments_after(leaf, index):
             yield from append_to_line(comment_after)
 
         lowest_depth = min(lowest_depth, leaf.bracket_depth)
-        if (
-            leaf.bracket_depth == lowest_depth
-            and is_vararg(leaf, within=VARARGS_PARENTS)
+        if leaf.bracket_depth == lowest_depth and is_vararg(
+            leaf, within=VARARGS_PARENTS
         ):
             trailing_comma_safe = trailing_comma_safe and py36
-        leaf_priority = delimiters.get(id(leaf))
+        leaf_priority = bt.delimiters.get(id(leaf))
         if leaf_priority == delimiter_priority:
             yield current_line
 
@@ -1970,10 +2210,10 @@ def standalone_comment_split(line: Line, py36: bool = False) -> Iterator[Line]:
             current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)
             current_line.append(leaf)
 
-    for leaf in line.leaves:
+    for index, leaf in enumerate(line.leaves):
         yield from append_to_line(leaf)
 
-        for comment_after in line.comments_after(leaf):
+        for comment_after in line.comments_after(leaf, index):
             yield from append_to_line(comment_after)
 
     if current_line:
@@ -2012,6 +2252,22 @@ def normalize_prefix(leaf: Leaf, *, inside_brackets: bool) -> None:
     leaf.prefix = ""
 
 
+def normalize_string_prefix(leaf: Leaf, remove_u_prefix: bool = False) -> None:
+    """Make all string prefixes lowercase.
+
+    If remove_u_prefix is given, also removes any u prefix from the string.
+
+    Note: Mutates its argument.
+    """
+    match = re.match(r"^([furbFURB]*)(.*)$", leaf.value, re.DOTALL)
+    assert match is not None, f"failed to match string {leaf.value!r}"
+    orig_prefix = match.group(1)
+    new_prefix = orig_prefix.lower()
+    if remove_u_prefix:
+        new_prefix = new_prefix.replace("u", "")
+    leaf.value = f"{new_prefix}{match.group(2)}"
+
+
 def normalize_string_quotes(leaf: Leaf) -> None:
     """Prefer double quotes but only if it doesn't cause more escaping.
 
@@ -2041,7 +2297,7 @@ def normalize_string_quotes(leaf: Leaf) -> None:
     unescaped_new_quote = re.compile(rf"(([^\\]|^)(\\\\)*){new_quote}")
     escaped_new_quote = re.compile(rf"([^\\]|^)\\(\\\\)*{new_quote}")
     escaped_orig_quote = re.compile(rf"([^\\]|^)\\(\\\\)*{orig_quote}")
-    body = leaf.value[first_quote_pos + len(orig_quote):-len(orig_quote)]
+    body = leaf.value[first_quote_pos + len(orig_quote) : -len(orig_quote)]
     if "r" in prefix.casefold():
         if unescaped_new_quote.search(body):
             # There's at least one unescaped new_quote in this raw string
@@ -2076,6 +2332,9 @@ def normalize_string_quotes(leaf: Leaf) -> None:
 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.
     """
@@ -2083,24 +2342,14 @@ def normalize_invisible_parens(node: Node, parens_after: Set[str]) -> None:
     for child in list(node.children):
         if check_lpar:
             if child.type == syms.atom:
-                if not (
-                    is_empty_tuple(child)
-                    or is_one_tuple(child)
-                    or max_delimiter_priority_in_atom(child) >= COMMA_PRIORITY
-                ):
-                    first = child.children[0]
-                    last = child.children[-1]
-                    if first.type == token.LPAR and last.type == token.RPAR:
-                        # make parentheses invisible
-                        first.value = ""  # type: ignore
-                        last.value = ""  # type: ignore
+                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:
+            elif not (isinstance(child, Leaf) and is_multiline_string(child)):
                 # wrap child in invisible parentheses
                 lpar = Leaf(token.LPAR, "")
                 rpar = Leaf(token.RPAR, "")
@@ -2110,6 +2359,30 @@ def normalize_invisible_parens(node: Node, parens_after: Set[str]) -> None:
         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 (
@@ -2143,12 +2416,33 @@ def is_one_tuple(node: LN) -> bool:
     )
 
 
+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 COLLECTION_LIBERALS_PARENTS, it applies to right
-    hand-side extended iterable unpacking (PEP 3132) and additional unpacking
+    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:
@@ -2166,6 +2460,41 @@ def is_vararg(leaf: Leaf, within: Set[NodeType]) -> bool:
     return p.type in within
 
 
+def is_multiline_string(leaf: Leaf) -> bool:
+    """Return True if `leaf` is a multiline string that actually spans many lines."""
+    value = leaf.value.lstrip("furbFURB")
+    return value[:3] in {'"""', "'''"} and "\n" in value
+
+
+def is_stub_suite(node: Node) -> bool:
+    """Return True if `node` is a suite with a stub body."""
+    if (
+        len(node.children) != 4
+        or node.children[0].type != token.NEWLINE
+        or node.children[1].type != token.INDENT
+        or node.children[3].type != token.DEDENT
+    ):
+        return False
+
+    return is_stub_body(node.children[2])
+
+
+def is_stub_body(node: LN) -> bool:
+    """Return True if `node` is a simple statement containing an ellipsis."""
+    if not isinstance(node, Node) or node.type != syms.simple_stmt:
+        return False
+
+    if len(node.children) != 2:
+        return False
+
+    child = node.children[0]
+    return (
+        child.type == syms.atom
+        and len(child.children) == 3
+        and all(leaf == Leaf(token.DOT, ".") for leaf in child.children)
+    )
+
+
 def max_delimiter_priority_in_atom(node: LN) -> int:
     """Return maximum delimiter priority inside `node`.
 
@@ -2206,12 +2535,23 @@ def ensure_visible(leaf: Leaf) -> None:
         leaf.value = ")"
 
 
+def should_explode(line: Line, opening_bracket: Leaf) -> bool:
+    """Should `line` immediately be split with `delimiter_split()` after RHS?"""
+    return bool(
+        opening_bracket.parent
+        and opening_bracket.parent.type in {syms.atom, syms.import_from}
+        and opening_bracket.value in "[{("
+        and line.bracket_tracker.delimiters
+        and line.bracket_tracker.max_delimiter_priority() == COMMA_PRIORITY
+    )
+
+
 def is_python36(node: Node) -> bool:
     """Return True if the current file is using Python 3.6+ features.
 
     Currently looking for:
     - f-strings; and
-    - trailing commas after * or ** in function signatures.
+    - trailing commas after * or ** in function signatures and calls.
     """
     for n in node.pre_order():
         if n.type == token.STRING:
@@ -2220,7 +2560,7 @@ def is_python36(node: Node) -> bool:
                 return True
 
         elif (
-            n.type == syms.typedargslist
+            n.type in {syms.typedargslist, syms.arglist}
             and n.children
             and n.children[-1].type == token.COMMA
         ):
@@ -2228,12 +2568,121 @@ def is_python36(node: Node) -> bool:
                 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"}
+def generate_trailers_to_omit(line: Line, line_length: int) -> Iterator[Set[LeafID]]:
+    """Generate sets of closing bracket IDs that should be omitted in a RHS.
+
+    Brackets can be omitted if the entire trailer up to and including
+    a preceding closing bracket fits in one line.
+
+    Yielded sets are cumulative (contain results of previous yields, too).  First
+    set is empty.
+    """
+
+    omit: Set[LeafID] = set()
+    yield omit
+
+    length = 4 * line.depth
+    opening_bracket = None
+    closing_bracket = None
+    optional_brackets: Set[LeafID] = set()
+    inner_brackets: Set[LeafID] = set()
+    for index, leaf in enumerate_reversed(line.leaves):
+        length += len(leaf.prefix) + len(leaf.value)
+        if length > line_length:
+            break
+
+        comment: Optional[Leaf]
+        for comment in line.comments_after(leaf, index):
+            if "\n" in comment.prefix:
+                break  # Oops, standalone comment!
+
+            length += len(comment.value)
+        else:
+            comment = None
+        if comment is not None:
+            break  # There was a standalone comment, we can't continue.
+
+        optional_brackets.discard(id(leaf))
+        if opening_bracket:
+            if leaf is opening_bracket:
+                opening_bracket = None
+            elif leaf.type in CLOSING_BRACKETS:
+                inner_brackets.add(id(leaf))
+        elif leaf.type in CLOSING_BRACKETS:
+            if not leaf.value:
+                optional_brackets.add(id(opening_bracket))
+                continue
+
+            if index > 0 and line.leaves[index - 1].type in OPENING_BRACKETS:
+                # Empty brackets would fail a split so treat them as "inner"
+                # brackets (e.g. only add them to the `omit` set if another
+                # pair of brackets was good enough.
+                inner_brackets.add(id(leaf))
+                continue
+
+            opening_bracket = leaf.opening_bracket
+            if closing_bracket:
+                omit.add(id(closing_bracket))
+                omit.update(inner_brackets)
+                inner_brackets.clear()
+                yield omit
+            closing_bracket = leaf
+
+
+def get_future_imports(node: Node) -> Set[str]:
+    """Return a set of __future__ imports in the file."""
+    imports = set()
+    for child in node.children:
+        if child.type != syms.simple_stmt:
+            break
+        first_child = child.children[0]
+        if isinstance(first_child, Leaf):
+            # Continue looking if we see a docstring; otherwise stop.
+            if (
+                len(child.children) == 2
+                and first_child.type == token.STRING
+                and child.children[1].type == token.NEWLINE
+            ):
+                continue
+            else:
+                break
+        elif first_child.type == syms.import_from:
+            module_name = first_child.children[1]
+            if not isinstance(module_name, Leaf) or module_name.value != "__future__":
+                break
+            for import_from_child in first_child.children[3:]:
+                if isinstance(import_from_child, Leaf):
+                    if import_from_child.type == token.NAME:
+                        imports.add(import_from_child.value)
+                else:
+                    assert import_from_child.type == syms.import_as_names
+                    for leaf in import_from_child.children:
+                        if isinstance(leaf, Leaf) and leaf.type == token.NAME:
+                            imports.add(leaf.value)
+        else:
+            break
+    return imports
+
+
+PYTHON_EXTENSIONS = {".py", ".pyi"}
 BLACKLISTED_DIRECTORIES = {
-    "build", "buck-out", "dist", "_build", ".git", ".hg", ".mypy_cache", ".tox", ".venv"
+    "build",
+    "buck-out",
+    "dist",
+    "_build",
+    ".git",
+    ".hg",
+    ".mypy_cache",
+    ".tox",
+    ".venv",
 }
 
 
@@ -2248,7 +2697,7 @@ def gen_python_files_in_dir(path: Path) -> Iterator[Path]:
 
             yield from gen_python_files_in_dir(child)
 
-        elif child.suffix in PYTHON_EXTENSIONS:
+        elif child.is_file() and child.suffix in PYTHON_EXTENSIONS:
             yield child
 
 
@@ -2394,9 +2843,9 @@ def assert_equivalent(src: str, dst: str) -> None:
         ) from None
 
 
-def assert_stable(src: str, dst: str, line_length: int) -> None:
+def assert_stable(src: str, dst: str, line_length: int, is_pyi: bool = False) -> None:
     """Raise AssertionError if `dst` reformats differently the second time."""
-    newdst = format_str(dst, line_length=line_length)
+    newdst = format_str(dst, line_length=line_length, is_pyi=is_pyi)
     if dst != newdst:
         log = dump_to_file(
             diff(src, dst, "source", "first pass"),
@@ -2435,7 +2884,7 @@ def diff(a: str, b: str, a_name: str, b_name: str) -> str:
     )
 
 
-def cancel(tasks: List[asyncio.Task]) -> None:
+def cancel(tasks: Iterable[asyncio.Task]) -> None:
     """asyncio signal handler that cancels all `tasks` and reports to stderr."""
     err("Aborted!")
     for task in tasks:
@@ -2473,19 +2922,45 @@ def sub_twice(regex: Pattern[str], replacement: str, original: str) -> str:
     return regex.sub(replacement, regex.sub(replacement, original))
 
 
+def enumerate_reversed(sequence: Sequence[T]) -> Iterator[Tuple[Index, T]]:
+    """Like `reversed(enumerate(sequence))` if that were possible."""
+    index = len(sequence) - 1
+    for element in reversed(sequence):
+        yield (index, element)
+        index -= 1
+
+
+def is_line_short_enough(line: Line, *, line_length: int, line_str: str = "") -> bool:
+    """Return True if `line` is no longer than `line_length`.
+
+    Uses the provided `line_str` rendering, if any, otherwise computes a new one.
+    """
+    if not line_str:
+        line_str = str(line).strip("\n")
+    return (
+        len(line_str) <= line_length
+        and "\n" not in line_str  # multiline strings
+        and not line.contains_standalone_comments()
+    )
+
+
 CACHE_DIR = Path(user_cache_dir("black", version=__version__))
-CACHE_FILE = CACHE_DIR / "cache.pickle"
 
 
-def read_cache() -> Cache:
+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.
     """
-    if not CACHE_FILE.exists():
+    cache_file = get_cache_file(line_length)
+    if not cache_file.exists():
         return {}
 
-    with CACHE_FILE.open("rb") as fobj:
+    with cache_file.open("rb") as fobj:
         try:
             cache: Cache = pickle.load(fobj)
         except pickle.UnpicklingError:
@@ -2518,13 +2993,14 @@ def filter_cached(
     return todo, done
 
 
-def write_cache(cache: Cache, sources: List[Path]) -> None:
+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:
+        with cache_file.open("wb") as fobj:
             pickle.dump(new_cache, fobj, protocol=pickle.HIGHEST_PROTOCOL)
     except OSError:
         pass