Coverage reporting

[etc/vim.git] / black.py

diff --git a/black.py b/black.py
index 6bfef500d6d44ea79d8d1eca2030127c7433339d..dc02128313154690de62de810abf399efcdb3433 100644 (file)
--- a/black.py
+++ b/black.py
@@ -1,4 +1,5 @@
 #!/usr/bin/env python3
 #!/usr/bin/env python3

+

 import asyncio
 from asyncio.base_events import BaseEventLoop
 from concurrent.futures import Executor, ProcessPoolExecutor
 import asyncio
 from asyncio.base_events import BaseEventLoop
 from concurrent.futures import Executor, ProcessPoolExecutor


@@ -9,7 +10,7 @@ from pathlib import Path
 import tokenize
 import sys
 from typing import (
 import tokenize
 import sys
 from typing import (

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

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


@@ -21,7 +22,7 @@ from blib2to3 import pygram, pytree
 from blib2to3.pgen2 import driver, token
 from blib2to3.pgen2.parse import ParseError
 
 from blib2to3.pgen2 import driver, token
 from blib2to3.pgen2.parse import ParseError
 

-__version__ = "18.3a2"
+__version__ = "18.3a3"

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


@@ -43,9 +44,37 @@ class NothingChanged(UserWarning):
 class CannotSplit(Exception):
     """A readable split that fits the allotted line length is impossible.
 
 class CannotSplit(Exception):
     """A readable split that fits the allotted line length is impossible.
 

-    Raised by `left_hand_split()` and `right_hand_split()`.
+    Raised by `left_hand_split()`, `right_hand_split()`, and `delimiter_split()`.
+    """
+
+
+class FormatError(Exception):
+    """Base fmt: on/off error.
+
+    It holds the number of bytes of the prefix consumed before the format
+    control comment appeared.

     """
 
     """
 

+    def __init__(self, consumed: int) -> None:
+        super().__init__(consumed)
+        self.consumed = consumed
+
+    def trim_prefix(self, leaf: Leaf) -> None:
+        leaf.prefix = leaf.prefix[self.consumed:]
+
+    def leaf_from_consumed(self, leaf: Leaf) -> Leaf:
+        """Returns a new Leaf from the consumed part of the prefix."""
+        unformatted_prefix = leaf.prefix[:self.consumed]
+        return Leaf(token.NEWLINE, unformatted_prefix)
+
+
+class FormatOn(FormatError):
+    """Found a comment like `# fmt: on` in the file."""
+
+
+class FormatOff(FormatError):
+    """Found a comment like `# fmt: off` in the file."""
+

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


@@ -61,7 +90,7 @@ class CannotSplit(Exception):
     is_flag=True,
     help=(
         "Don't write back the files, just return the status.  Return code 0 "
     is_flag=True,
     help=(
         "Don't write back the files, just return the status.  Return code 0 "

-        "means nothing changed.  Return code 1 means some files were "
+        "means nothing would change.  Return code 1 means some files would be "

         "reformatted.  Return code 123 means there was an internal error."
     ),
 )
         "reformatted.  Return code 123 means there was an internal error."
     ),
 )


@@ -99,10 +128,12 @@ def main(
         ctx.exit(0)
     elif len(sources) == 1:
         p = sources[0]
         ctx.exit(0)
     elif len(sources) == 1:
         p = sources[0]

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

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

-                changed = format_stdin_to_stdout(line_length=line_length, fast=fast)
+                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
             else:
                 changed = format_file_in_place(
                     p, line_length=line_length, fast=fast, write_back=not check


@@ -178,7 +209,9 @@ def format_file_in_place(
     return True
 
 
     return True
 
 

-def format_stdin_to_stdout(line_length: int, fast: bool) -> bool:
+def format_stdin_to_stdout(
+    line_length: int, fast: bool, write_back: bool = False
+) -> bool:

     """Format file on stdin and pipe output to stdout. Return True if changed."""
     contents = sys.stdin.read()
     try:
     """Format file on stdin and pipe output to stdout. Return True if changed."""
     contents = sys.stdin.read()
     try:


@@ -189,7 +222,8 @@ def format_stdin_to_stdout(line_length: int, fast: bool) -> bool:
         return False
 
     finally:
         return False
 
     finally:

-        sys.stdout.write(contents)
+        if write_back:
+            sys.stdout.write(contents)

 
 
 def format_file_contents(
 
 
 def format_file_contents(


@@ -213,7 +247,6 @@ def format_str(src_contents: str, line_length: int) -> FileContent:
     """Reformats a string and returns new contents."""
     src_node = lib2to3_parse(src_contents)
     dst_contents = ""
     """Reformats a string and returns new contents."""
     src_node = lib2to3_parse(src_contents)
     dst_contents = ""

-    comments: List[Line] = []

     lines = LineGenerator()
     elt = EmptyLineTracker()
     py36 = is_python36(src_node)
     lines = LineGenerator()
     elt = EmptyLineTracker()
     py36 = is_python36(src_node)


@@ -225,41 +258,41 @@ def format_str(src_contents: str, line_length: int) -> FileContent:
         before, after = elt.maybe_empty_lines(current_line)
         for _ in range(before):
             dst_contents += str(empty_line)
         before, after = elt.maybe_empty_lines(current_line)
         for _ in range(before):
             dst_contents += str(empty_line)

-        if not current_line.is_comment:
-            for comment in comments:
-                dst_contents += str(comment)
-            comments = []
-            for line in split_line(current_line, line_length=line_length, py36=py36):
-                dst_contents += str(line)
-        else:
-            comments.append(current_line)
-    if comments:
-        if elt.previous_defs:
-            # Separate postscriptum comments from the last module-level def.
-            dst_contents += str(empty_line)
-            dst_contents += str(empty_line)
-        for comment in comments:
-            dst_contents += str(comment)
+        for line in split_line(current_line, line_length=line_length, py36=py36):
+            dst_contents += str(line)

     return dst_contents
 
 
     return dst_contents
 
 

+GRAMMARS = [
+    pygram.python_grammar_no_print_statement_no_exec_statement,
+    pygram.python_grammar_no_print_statement,
+    pygram.python_grammar_no_exec_statement,
+    pygram.python_grammar,
+]
+
+

 def lib2to3_parse(src_txt: str) -> Node:
     """Given a string with source, return the lib2to3 Node."""
     grammar = pygram.python_grammar_no_print_statement
 def lib2to3_parse(src_txt: str) -> Node:
     """Given a string with source, return the lib2to3 Node."""
     grammar = pygram.python_grammar_no_print_statement

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

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

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

         try:
         try:

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

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


@@ -315,6 +348,15 @@ class DebugVisitor(Visitor[T]):
                 out(f' {node.prefix!r}', fg='green', bold=False, nl=False)
             out(f' {node.value!r}', fg='blue', bold=False)
 
                 out(f' {node.prefix!r}', fg='green', bold=False, nl=False)
             out(f' {node.value!r}', fg='blue', bold=False)
 

+    @classmethod
+    def show(cls, code: str) -> None:
+        """Pretty-prints a given string of `code`.
+
+        Convenience method for debugging.
+        """
+        v: DebugVisitor[None] = DebugVisitor()
+        list(v.visit(lib2to3_parse(code)))
+

 
 KEYWORDS = set(keyword.kwlist)
 WHITESPACE = {token.DEDENT, token.INDENT, token.NEWLINE}
 
 KEYWORDS = set(keyword.kwlist)
 WHITESPACE = {token.DEDENT, token.INDENT, token.NEWLINE}


@@ -348,6 +390,7 @@ MATH_OPERATORS = {
     token.AMPER,
     token.PERCENT,
     token.CIRCUMFLEX,
     token.AMPER,
     token.PERCENT,
     token.CIRCUMFLEX,

+    token.TILDE,

     token.LEFTSHIFT,
     token.RIGHTSHIFT,
     token.DOUBLESTAR,
     token.LEFTSHIFT,
     token.RIGHTSHIFT,
     token.DOUBLESTAR,


@@ -413,7 +456,7 @@ class BracketTracker:
         """Returns True if there is an yet unmatched open bracket on the line."""
         return bool(self.bracket_match)
 
         """Returns True if there is an yet unmatched open bracket on the line."""
         return bool(self.bracket_match)
 

-    def max_priority(self, exclude: Iterable[LeafID] =()) -> int:
+    def max_priority(self, exclude: Iterable[LeafID] = ()) -> int:

         """Returns the highest priority of a delimiter found on the line.
 
         Values are consistent with what `is_delimiter()` returns.
         """Returns the highest priority of a delimiter found on the line.
 
         Values are consistent with what `is_delimiter()` returns.


@@ -486,8 +529,7 @@ class Line:
         return (
             (first_leaf.type == token.NAME and first_leaf.value == 'def')
             or (
         return (
             (first_leaf.type == token.NAME and first_leaf.value == 'def')
             or (

-                first_leaf.type == token.NAME
-                and first_leaf.value == 'async'
+                first_leaf.type == token.ASYNC

                 and second_leaf is not None
                 and second_leaf.type == token.NAME
                 and second_leaf.value == 'def'
                 and second_leaf is not None
                 and second_leaf.type == token.NAME
                 and second_leaf.value == 'def'


@@ -518,10 +560,16 @@ class Line:
         ):
             return False
 
         ):
             return False
 

-        if closing.type == token.RSQB or closing.type == token.RBRACE:
+        if closing.type == token.RBRACE:

             self.leaves.pop()
             return True
 
             self.leaves.pop()
             return True
 

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

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


@@ -638,6 +686,43 @@ class Line:
         return bool(self.leaves or self.comments)
 
 
         return bool(self.leaves or self.comments)
 
 

+class UnformattedLines(Line):
+
+    def append(self, leaf: Leaf, preformatted: bool = False) -> None:
+        try:
+            list(generate_comments(leaf))
+        except FormatOn as f_on:
+            self.leaves.append(f_on.leaf_from_consumed(leaf))
+            raise
+
+        self.leaves.append(leaf)
+        if leaf.type == token.INDENT:
+            self.depth += 1
+        elif leaf.type == token.DEDENT:
+            self.depth -= 1
+
+    def append_comment(self, comment: Leaf) -> bool:
+        raise NotImplementedError("Unformatted lines don't store comments separately.")
+
+    def maybe_remove_trailing_comma(self, closing: Leaf) -> bool:
+        return False
+
+    def maybe_increment_for_loop_variable(self, leaf: Leaf) -> bool:
+        return False
+
+    def maybe_adapt_standalone_comment(self, comment: Leaf) -> bool:
+        return False
+
+    def __str__(self) -> str:
+        if not self:
+            return '\n'
+
+        res = ''
+        for leaf in self.leaves:
+            res += str(leaf)
+        return res
+
+

 @dataclass
 class EmptyLineTracker:
     """Provides a stateful method that returns the number of potential extra
 @dataclass
 class EmptyLineTracker:
     """Provides a stateful method that returns the number of potential extra


@@ -658,8 +743,7 @@ class EmptyLineTracker:
         (two on module-level), as well as providing an extra empty line after flow
         control keywords to make them more prominent.
         """
         (two on module-level), as well as providing an extra empty line after flow
         control keywords to make them more prominent.
         """

-        if current_line.is_comment:
-            # Don't count standalone comments towards previous empty lines.
+        if isinstance(current_line, UnformattedLines):

             return 0, 0
 
         before, after = self._maybe_empty_lines(current_line)
             return 0, 0
 
         before, after = self._maybe_empty_lines(current_line)


@@ -669,10 +753,14 @@ class EmptyLineTracker:
         return before, after
 
     def _maybe_empty_lines(self, current_line: Line) -> Tuple[int, int]:
         return before, after
 
     def _maybe_empty_lines(self, current_line: Line) -> Tuple[int, int]:

+        max_allowed = 1
+        if current_line.is_comment and current_line.depth == 0:
+            max_allowed = 2

         if current_line.leaves:
             # Consume the first leaf's extra newlines.
             first_leaf = current_line.leaves[0]
         if current_line.leaves:
             # Consume the first leaf's extra newlines.
             first_leaf = current_line.leaves[0]

-            before = int('\n' in first_leaf.prefix)
+            before = first_leaf.prefix.count('\n')
+            before = min(before, max(before, max_allowed))

             first_leaf.prefix = ''
         else:
             before = 0
             first_leaf.prefix = ''
         else:
             before = 0


@@ -726,9 +814,8 @@ class LineGenerator(Visitor[Line]):
     in ways that will no longer stringify to valid Python code on the tree.
     """
     current_line: Line = Factory(Line)
     in ways that will no longer stringify to valid Python code on the tree.
     """
     current_line: Line = Factory(Line)

-    standalone_comments: List[Leaf] = Factory(list)

 
 

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

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


@@ -737,52 +824,68 @@ class LineGenerator(Visitor[Line]):
         If any lines were generated, set up a new current_line.
         """
         if not self.current_line:
         If any lines were generated, set up a new current_line.
         """
         if not self.current_line:

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

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

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

         yield complete_line
 
         yield complete_line
 

+    def visit(self, node: LN) -> Iterator[Line]:
+        """High-level entry point to the visitor."""
+        if isinstance(self.current_line, UnformattedLines):
+            # File contained `# fmt: off`
+            yield from self.visit_unformatted(node)
+
+        else:
+            yield from super().visit(node)
+

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

-            for comment in generate_comments(node):
-                if self.current_line.bracket_tracker.any_open_brackets():
-                    # any comment within brackets is subject to splitting
-                    self.current_line.append(comment)
-                elif comment.type == token.COMMENT:
-                    # regular trailing comment
-                    self.current_line.append(comment)
-                    yield from self.line()
-
-                else:
-                    # regular standalone comment, to be processed later (see
-                    # docstring in `generate_comments()`
-                    self.standalone_comments.append(comment)
-            normalize_prefix(node)
-            if node.type not in WHITESPACE:
-                for comment in self.standalone_comments:
-                    yield from self.line()
-
-                    self.current_line.append(comment)
-                    yield from self.line()
-
-                self.standalone_comments = []
-                self.current_line.append(node)
+            any_open_brackets = self.current_line.bracket_tracker.any_open_brackets()
+            try:
+                for comment in generate_comments(node):
+                    if any_open_brackets:
+                        # any comment within brackets is subject to splitting
+                        self.current_line.append(comment)
+                    elif comment.type == token.COMMENT:
+                        # regular trailing comment
+                        self.current_line.append(comment)
+                        yield from self.line()
+
+                    else:
+                        # regular standalone comment
+                        yield from self.line()
+
+                        self.current_line.append(comment)
+                        yield from self.line()
+
+            except FormatOff as f_off:
+                f_off.trim_prefix(node)
+                yield from self.line(type=UnformattedLines)
+                yield from self.visit(node)
+
+            except FormatOn as f_on:
+                # This only happens here if somebody says "fmt: on" multiple
+                # times in a row.
+                f_on.trim_prefix(node)
+                yield from self.visit_default(node)
+
+            else:
+                normalize_prefix(node, inside_brackets=any_open_brackets)
+                if node.type not in WHITESPACE:
+                    self.current_line.append(node)

         yield from super().visit_default(node)
 
         yield from super().visit_default(node)
 

-    def visit_suite(self, node: Node) -> Iterator[Line]:
-        """Body of a statement after a colon."""
-        children = iter(node.children)
-        # Process newline before indenting.  It might contain an inline
-        # comment that should go right after the colon.
-        newline = next(children)
-        yield from self.visit(newline)
+    def visit_INDENT(self, node: Node) -> Iterator[Line]:

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

+        yield from self.visit_default(node)

 
 

-        for child in children:
-            yield from self.visit(child)
-
+    def visit_DEDENT(self, node: Node) -> Iterator[Line]:
+        # DEDENT has no value. Additionally, in blib2to3 it never holds comments.

         yield from self.line(-1)
 
     def visit_stmt(self, node: Node, keywords: Set[str]) -> Iterator[Line]:
         yield from self.line(-1)
 
     def visit_stmt(self, node: Node, keywords: Set[str]) -> Iterator[Line]:


@@ -816,7 +919,7 @@ class LineGenerator(Visitor[Line]):
         for child in children:
             yield from self.visit(child)
 
         for child in children:
             yield from self.visit(child)
 

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

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


@@ -835,6 +938,19 @@ class LineGenerator(Visitor[Line]):
         yield from self.visit_default(leaf)
         yield from self.line()
 
         yield from self.visit_default(leaf)
         yield from self.line()
 

+    def visit_unformatted(self, node: LN) -> Iterator[Line]:
+        if isinstance(node, Node):
+            for child in node.children:
+                yield from self.visit(child)
+
+        else:
+            try:
+                self.current_line.append(node)
+            except FormatOn as f_on:
+                f_on.trim_prefix(node)
+                yield from self.line()
+                yield from self.visit(node)
+

     def __attrs_post_init__(self) -> None:
         """You are in a twisty little maze of passages."""
         v = self.visit_stmt
     def __attrs_post_init__(self) -> None:
         """You are in a twisty little maze of passages."""
         v = self.visit_stmt


@@ -872,7 +988,7 @@ def whitespace(leaf: Leaf) -> str:  # noqa C901
         return DOUBLESPACE
 
     assert p is not None, f"INTERNAL ERROR: hand-made leaf without parent: {leaf!r}"
         return DOUBLESPACE
 
     assert p is not None, f"INTERNAL ERROR: hand-made leaf without parent: {leaf!r}"

-    if t == token.COLON and p.type != syms.subscript:
+    if t == token.COLON and p.type not in {syms.subscript, syms.subscriptlist}:

         return NO
 
     prev = leaf.prev_sibling
         return NO
 
     prev = leaf.prev_sibling


@@ -885,30 +1001,49 @@ def whitespace(leaf: Leaf) -> str:  # noqa C901
             return SPACE if prevp.type == token.COMMA else NO
 
         if prevp.type == token.EQUAL:
             return SPACE if prevp.type == token.COMMA else NO
 
         if prevp.type == token.EQUAL:

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

 
         elif prevp.type == token.DOUBLESTAR:
             if prevp.parent and prevp.parent.type in {
 
         elif prevp.type == token.DOUBLESTAR:
             if prevp.parent and prevp.parent.type in {

-                syms.typedargslist,
-                syms.varargslist,
-                syms.parameters,

                 syms.arglist,
                 syms.arglist,

+                syms.argument,

                 syms.dictsetmaker,
                 syms.dictsetmaker,

+                syms.parameters,
+                syms.typedargslist,
+                syms.varargslist,

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

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

                 return NO
 
                 return NO
 

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

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


@@ -922,7 +1057,7 @@ def whitespace(leaf: Leaf) -> str:  # noqa C901
         if not prev or prev.type != token.COMMA:
             return NO
 
         if not prev or prev.type != token.COMMA:
             return NO
 

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

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


@@ -1133,30 +1268,49 @@ def generate_comments(leaf: Leaf) -> Iterator[Leaf]:
     Inline comments are emitted as regular token.COMMENT leaves.  Standalone
     are emitted with a fake STANDALONE_COMMENT token identifier.
     """
     Inline comments are emitted as regular token.COMMENT leaves.  Standalone
     are emitted with a fake STANDALONE_COMMENT token identifier.
     """

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

         return
 
         return
 

-    before_comment, content = leaf.prefix.split('#', 1)
-    content = content.rstrip()
-    if content and (content[0] not in {' ', '!', '#'}):
-        content = ' ' + content
-    is_standalone_comment = (
-        '\n' in before_comment or '\n' in content or leaf.type == token.ENDMARKER
-    )
-    if not is_standalone_comment:
-        # simple trailing comment
-        yield Leaf(token.COMMENT, value='#' + content)
+    if '#' not in p:

         return
 
         return
 

-    for line in ('#' + content).split('\n'):
+    consumed = 0
+    nlines = 0
+    for index, line in enumerate(p.split('\n')):
+        consumed += len(line) + 1  # adding the length of the split '\n'

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

+        if not line:
+            nlines += 1

         if not line.startswith('#'):
             continue
 
         if not line.startswith('#'):
             continue
 

-        yield Leaf(STANDALONE_COMMENT, line)
+        if index == 0 and leaf.type != token.ENDMARKER:
+            comment_type = token.COMMENT  # simple trailing comment
+        else:
+            comment_type = STANDALONE_COMMENT
+        comment = make_comment(line)
+        yield Leaf(comment_type, comment, prefix='\n' * nlines)
+
+        if comment in {'# fmt: on', '# yapf: enable'}:
+            raise FormatOn(consumed)
+
+        if comment in {'# fmt: off', '# yapf: disable'}:
+            raise FormatOff(consumed)
+
+        nlines = 0
+
+
+def make_comment(content: str) -> str:
+    content = content.rstrip()
+    if not content:
+        return '#'
+
+    if content[0] == '#':
+        content = content[1:]
+    if content and content[0] not in ' !:#':
+        content = ' ' + content
+    return '#' + content

 
 
 def split_line(
 
 
 def split_line(


@@ -1172,6 +1326,10 @@ def split_line(
     If `py36` is True, splitting may generate syntax that is only compatible
     with Python 3.6 and later.
     """
     If `py36` is True, splitting may generate syntax that is only compatible
     with Python 3.6 and later.
     """

+    if isinstance(line, UnformattedLines):
+        yield line
+        return
+

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


@@ -1239,7 +1397,7 @@ def left_hand_split(line: Line, py36: bool = False) -> Iterator[Line]:
                 current_leaves = body_leaves
     # Since body is a new indent level, remove spurious leading whitespace.
     if body_leaves:
                 current_leaves = body_leaves
     # Since body is a new indent level, remove spurious leading whitespace.
     if body_leaves:

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

     # Build the new lines.
     for result, leaves in (
         (head, head_leaves), (body, body_leaves), (tail, tail_leaves)
     # Build the new lines.
     for result, leaves in (
         (head, head_leaves), (body, body_leaves), (tail, tail_leaves)


@@ -1279,7 +1437,7 @@ def right_hand_split(line: Line, py36: bool = False) -> Iterator[Line]:
     head_leaves.reverse()
     # Since body is a new indent level, remove spurious leading whitespace.
     if body_leaves:
     head_leaves.reverse()
     # Since body is a new indent level, remove spurious leading whitespace.
     if body_leaves:

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

     # Build the new lines.
     for result, leaves in (
         (head, head_leaves), (body, body_leaves), (tail, tail_leaves)
     # Build the new lines.
     for result, leaves in (
         (head, head_leaves), (body, body_leaves), (tail, tail_leaves)


@@ -1343,7 +1501,7 @@ def delimiter_split(line: Line, py36: bool = False) -> Iterator[Line]:
             trailing_comma_safe = trailing_comma_safe and py36
         leaf_priority = delimiters.get(id(leaf))
         if leaf_priority == delimiter_priority:
             trailing_comma_safe = trailing_comma_safe and py36
         leaf_priority = delimiters.get(id(leaf))
         if leaf_priority == delimiter_priority:

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

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


@@ -1354,7 +1512,7 @@ def delimiter_split(line: Line, py36: bool = False) -> Iterator[Line]:
             and trailing_comma_safe
         ):
             current_line.append(Leaf(token.COMMA, ','))
             and trailing_comma_safe
         ):
             current_line.append(Leaf(token.COMMA, ','))

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

         yield current_line
 
 
         yield current_line
 
 


@@ -1372,13 +1530,20 @@ def is_import(leaf: Leaf) -> bool:
     )
 
 
     )
 
 

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

 
     leaf.prefix = ''
 
 
     leaf.prefix = ''
 


@@ -1429,6 +1594,7 @@ def gen_python_files_in_dir(path: Path) -> Iterator[Path]:
 @dataclass
 class Report:
     """Provides a reformatting counter."""
 @dataclass
 class Report:
     """Provides a reformatting counter."""

+    check: bool = False

     change_count: int = 0
     same_count: int = 0
     failure_count: int = 0
     change_count: int = 0
     same_count: int = 0
     failure_count: int = 0


@@ -1436,7 +1602,8 @@ class Report:
     def done(self, src: Path, changed: bool) -> None:
         """Increment the counter for successful reformatting. Write out a message."""
         if changed:
     def done(self, src: Path, changed: bool) -> None:
         """Increment the counter for successful reformatting. Write out a message."""
         if changed:

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

             self.change_count += 1
         else:
             out(f'{src} already well formatted, good job.', bold=False)
             self.change_count += 1
         else:
             out(f'{src} already well formatted, good job.', bold=False)


@@ -1455,7 +1622,7 @@ class Report:
         if self.failure_count:
             return 123
 
         if self.failure_count:
             return 123
 

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

             return 1
 
         return 0
             return 1
 
         return 0


@@ -1465,21 +1632,27 @@ class Report:
 
         Use `click.unstyle` to remove colors.
         """
 
         Use `click.unstyle` to remove colors.
         """

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

         report = []
         if self.change_count:
             s = 's' if self.change_count > 1 else ''
             report.append(
         report = []
         if self.change_count:
             s = 's' if self.change_count > 1 else ''
             report.append(

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

             )
         if self.same_count:
             s = 's' if self.same_count > 1 else ''
             )
         if self.same_count:
             s = 's' if self.same_count > 1 else ''

-            report.append(f'{self.same_count} file{s} left unchanged')
+            report.append(f'{self.same_count} file{s} {unchanged}')

         if self.failure_count:
             s = 's' if self.failure_count > 1 else ''
             report.append(
         if self.failure_count:
             s = 's' if self.failure_count > 1 else ''
             report.append(

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

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


@@ -1521,7 +1694,12 @@ def assert_equivalent(src: str, dst: str) -> None:
     try:
         src_ast = ast.parse(src)
     except Exception as exc:
     try:
         src_ast = ast.parse(src)
     except Exception as exc:

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

 
     try:
         dst_ast = ast.parse(dst)
 
     try:
         dst_ast = ast.parse(dst)