Show __str__ in UnformattedLines

[etc/vim.git] / black.py

diff --git a/black.py b/black.py
index d3e0761e273a58fe4b7318827e12bbd853f53eea..de861564a44bc7d05f89007e58ce687653c69dd8 100644 (file)
--- a/black.py
+++ b/black.py
@@ -10,7 +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


@@ -22,7 +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__ = "18.3a3"
+__version__ = "18.3a4"

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


@@ -38,16 +38,44 @@ err = partial(click.secho, fg='red', err=True)
 
 
 class NothingChanged(UserWarning):
 
 
 class NothingChanged(UserWarning):

-    """Raised by `format_file` when the reformatted code is the same as source."""
+    """Raised by `format_file()` when the reformatted code is the same as source."""

 
 
 class CannotSplit(Exception):
     """A readable split that fits the allotted line length is impossible.
 
 
 
 class CannotSplit(Exception):
     """A readable split that fits the allotted line length is impossible.
 

-    Raised by `left_hand_split()` and `right_hand_split()`.
+    Raised by `left_hand_split()`, `right_hand_split()`, and `delimiter_split()`.

     """
 
 
     """
 
 

+class FormatError(Exception):
+    """Base exception for `# fmt: on` and `# fmt: off` handling.
+
+    It holds the number of bytes of the prefix consumed before the format
+    control comment appeared.
+    """
+
+    def __init__(self, consumed: int) -> None:
+        super().__init__(consumed)
+        self.consumed = consumed
+
+    def trim_prefix(self, leaf: Leaf) -> None:
+        leaf.prefix = leaf.prefix[self.consumed:]
+
+    def leaf_from_consumed(self, leaf: Leaf) -> Leaf:
+        """Returns a new Leaf from the consumed part of the prefix."""
+        unformatted_prefix = leaf.prefix[:self.consumed]
+        return Leaf(token.NEWLINE, unformatted_prefix)
+
+
+class FormatOn(FormatError):
+    """Found a comment like `# fmt: on` in the file."""
+
+
+class FormatOff(FormatError):
+    """Found a comment like `# fmt: off` in the file."""
+
+

 @click.command()
 @click.option(
     '-l',
 @click.command()
 @click.option(
     '-l',


@@ -62,7 +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."
     ),
 )


@@ -100,7 +128,7 @@ 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) == '-':
                 changed = format_stdin_to_stdout(
         try:
             if not p.is_file() and str(p) == '-':
                 changed = format_stdin_to_stdout(


@@ -137,6 +165,13 @@ async def schedule_formatting(
     loop: BaseEventLoop,
     executor: Executor,
 ) -> int:
     loop: BaseEventLoop,
     executor: Executor,
 ) -> int:

+    """Run formatting of `sources` in parallel using the provided `executor`.
+
+    (Use ProcessPoolExecutors for actual parallelism.)
+
+    `line_length`, `write_back`, and `fast` options are passed to
+    :func:`format_file_in_place`.
+    """

     tasks = {
         src: loop.run_in_executor(
             executor, format_file_in_place, src, line_length, fast, write_back
     tasks = {
         src: loop.run_in_executor(
             executor, format_file_in_place, src, line_length, fast, write_back


@@ -165,7 +200,11 @@ async def schedule_formatting(
 def format_file_in_place(
     src: Path, line_length: int, fast: bool, write_back: bool = False
 ) -> bool:
 def format_file_in_place(
     src: Path, line_length: int, fast: bool, write_back: bool = False
 ) -> bool:

-    """Format the file and rewrite if changed. Return True if changed."""
+    """Format file under `src` path. Return True if changed.
+
+    If `write_back` is True, write reformatted code back to stdout.
+    `line_length` and `fast` options are passed to :func:`format_file_contents`.
+    """

     with tokenize.open(src) as src_buffer:
         src_contents = src_buffer.read()
     try:
     with tokenize.open(src) as src_buffer:
         src_contents = src_buffer.read()
     try:


@@ -184,7 +223,11 @@ def format_file_in_place(
 def format_stdin_to_stdout(
     line_length: int, fast: bool, write_back: bool = False
 ) -> 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."""
+    """Format file on stdin. Return True if changed.
+
+    If `write_back` is True, write reformatted code back to stdout.
+    `line_length` and `fast` arguments are passed to :func:`format_file_contents`.
+    """

     contents = sys.stdin.read()
     try:
         contents = format_file_contents(contents, line_length=line_length, fast=fast)
     contents = sys.stdin.read()
     try:
         contents = format_file_contents(contents, line_length=line_length, fast=fast)


@@ -201,7 +244,12 @@ def format_stdin_to_stdout(
 def format_file_contents(
     src_contents: str, line_length: int, fast: bool
 ) -> FileContent:
 def format_file_contents(
     src_contents: str, line_length: int, fast: bool
 ) -> FileContent:

-    """Reformats a file and returns its contents and encoding."""
+    """Reformats a file and returns its contents and encoding.
+
+    If `fast` is False, additionally confirm that the reformatted code is
+    valid by calling :func:`assert_equivalent` and :func:`assert_stable` on it.
+    `line_length` is passed to :func:`format_str`.
+    """

     if src_contents.strip() == '':
         raise NothingChanged
 
     if src_contents.strip() == '':
         raise NothingChanged
 


@@ -216,7 +264,10 @@ def format_file_contents(
 
 
 def format_str(src_contents: str, line_length: int) -> FileContent:
 
 
 def format_str(src_contents: str, line_length: int) -> FileContent:

-    """Reformats a string and returns new contents."""
+    """Reformats a string and returns new contents.
+
+    `line_length` determines how many characters per line are allowed.
+    """

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


@@ -235,23 +286,36 @@ def format_str(src_contents: str, line_length: int) -> FileContent:
     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])


@@ -268,9 +332,18 @@ T = TypeVar('T')
 
 
 class Visitor(Generic[T]):
 
 
 class Visitor(Generic[T]):

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

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

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

         if node.type < 256:
             name = token.tok_name[node.type]
         else:
         if node.type < 256:
             name = token.tok_name[node.type]
         else:


@@ -278,6 +351,7 @@ class Visitor(Generic[T]):
         yield from getattr(self, f'visit_{name}', self.visit_default)(node)
 
     def visit_default(self, node: LN) -> Iterator[T]:
         yield from getattr(self, f'visit_{name}', self.visit_default)(node)
 
     def visit_default(self, node: LN) -> Iterator[T]:

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

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


@@ -307,6 +381,15 @@ 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}


@@ -356,12 +439,28 @@ MATH_PRIORITY = 1
 
 @dataclass
 class BracketTracker:
 
 @dataclass
 class BracketTracker:

+    """Keeps track of brackets on a line."""
+

     depth: int = 0
     bracket_match: Dict[Tuple[Depth, NodeType], Leaf] = Factory(dict)
     delimiters: Dict[LeafID, Priority] = Factory(dict)
     previous: Optional[Leaf] = None
 
     def mark(self, leaf: Leaf) -> None:
     depth: int = 0
     bracket_match: Dict[Tuple[Depth, NodeType], Leaf] = Factory(dict)
     delimiters: Dict[LeafID, Priority] = Factory(dict)
     previous: Optional[Leaf] = None
 
     def mark(self, leaf: Leaf) -> None:

+        """Marks `leaf` with bracket-related metadata. Keeps track of delimiters.
+
+        All leaves receive an int `bracket_depth` field that stores how deep
+        within brackets a given leaf is. 0 means there are no enclosing brackets
+        that started on this line.
+
+        If a leaf is itself a closing bracket, it receives an `opening_bracket`
+        field that it forms a pair with. This is a one-directional link to
+        avoid reference cycles.
+
+        If a leaf is a delimiter (a token on which Black can split the line if
+        needed) and it's on depth 0, its `id()` is stored in the tracker's
+        `delimiters` field.
+        """

         if leaf.type == token.COMMENT:
             return
 
         if leaf.type == token.COMMENT:
             return
 


@@ -406,7 +505,7 @@ 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_delimiter_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.


@@ -416,6 +515,8 @@ class BracketTracker:
 
 @dataclass
 class Line:
 
 @dataclass
 class Line:

+    """Holds leaves and comments. Can be printed with `str(line)`."""
+

     depth: int = 0
     leaves: List[Leaf] = Factory(list)
     comments: Dict[LeafID, Leaf] = Factory(dict)
     depth: int = 0
     leaves: List[Leaf] = Factory(list)
     comments: Dict[LeafID, Leaf] = Factory(dict)


@@ -425,6 +526,15 @@ class Line:
     _for_loop_variable: bool = False
 
     def append(self, leaf: Leaf, preformatted: bool = False) -> None:
     _for_loop_variable: bool = False
 
     def append(self, leaf: Leaf, preformatted: bool = False) -> None:

+        """Add a new `leaf` to the end of the line.
+
+        Unless `preformatted` is True, the `leaf` will receive a new consistent
+        whitespace prefix and metadata applied by :class:`BracketTracker`.
+        Trailing commas are maybe removed, unpacked for loop variables are
+        demoted from being delimiters.
+
+        Inline comments are put aside.
+        """

         has_value = leaf.value.strip()
         if not has_value:
             return
         has_value = leaf.value.strip()
         if not has_value:
             return


@@ -446,18 +556,22 @@ class Line:
 
     @property
     def is_comment(self) -> bool:
 
     @property
     def is_comment(self) -> bool:

+        """Is this line a standalone comment?"""

         return bool(self) and self.leaves[0].type == STANDALONE_COMMENT
 
     @property
     def is_decorator(self) -> bool:
         return bool(self) and self.leaves[0].type == STANDALONE_COMMENT
 
     @property
     def is_decorator(self) -> bool:

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

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

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

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

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

         return (
             bool(self)
             and self.leaves[0].type == token.NAME
         return (
             bool(self)
             and self.leaves[0].type == token.NAME


@@ -466,7 +580,7 @@ class Line:
 
     @property
     def is_def(self) -> bool:
 
     @property
     def is_def(self) -> bool:

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

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


@@ -488,6 +602,10 @@ class Line:
 
     @property
     def is_flow_control(self) -> bool:
 
     @property
     def is_flow_control(self) -> bool:

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

         return (
             bool(self)
             and self.leaves[0].type == token.NAME
         return (
             bool(self)
             and self.leaves[0].type == token.NAME


@@ -496,6 +614,7 @@ class Line:
 
     @property
     def is_yield(self) -> bool:
 
     @property
     def is_yield(self) -> bool:

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

         return (
             bool(self)
             and self.leaves[0].type == token.NAME
         return (
             bool(self)
             and self.leaves[0].type == token.NAME


@@ -503,6 +622,7 @@ class Line:
         )
 
     def maybe_remove_trailing_comma(self, closing: Leaf) -> bool:
         )
 
     def maybe_remove_trailing_comma(self, closing: Leaf) -> bool:

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

         if not (
             self.leaves
             and self.leaves[-1].type == token.COMMA
         if not (
             self.leaves
             and self.leaves[-1].type == token.COMMA


@@ -510,10 +630,16 @@ 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.


@@ -559,7 +685,7 @@ class Line:
         return False
 
     def maybe_decrement_after_for_loop_variable(self, leaf: Leaf) -> bool:
         return False
 
     def maybe_decrement_after_for_loop_variable(self, leaf: Leaf) -> bool:

-        # See `maybe_increment_for_loop_variable` above for explanation.
+        """See `maybe_increment_for_loop_variable` above for explanation."""

         if self._for_loop_variable and leaf.type == token.NAME and leaf.value == 'in':
             self.bracket_tracker.depth -= 1
             self._for_loop_variable = False
         if self._for_loop_variable and leaf.type == token.NAME and leaf.value == 'in':
             self.bracket_tracker.depth -= 1
             self._for_loop_variable = False


@@ -587,6 +713,7 @@ class Line:
         return self.append_comment(comment)
 
     def append_comment(self, comment: Leaf) -> bool:
         return self.append_comment(comment)
 
     def append_comment(self, comment: Leaf) -> bool:

+        """Add an inline comment to the line."""

         if comment.type != token.COMMENT:
             return False
 
         if comment.type != token.COMMENT:
             return False
 


@@ -605,6 +732,7 @@ class Line:
             return True
 
     def last_non_delimiter(self) -> Leaf:
             return True
 
     def last_non_delimiter(self) -> Leaf:

+        """Returns the last non-delimiter on the line. Raises LookupError otherwise."""

         for i in range(len(self.leaves)):
             last = self.leaves[-i - 1]
             if not is_delimiter(last):
         for i in range(len(self.leaves)):
             last = self.leaves[-i - 1]
             if not is_delimiter(last):


@@ -613,6 +741,7 @@ class Line:
         raise LookupError("No non-delimiters found")
 
     def __str__(self) -> str:
         raise LookupError("No non-delimiters found")
 
     def __str__(self) -> str:

+        """Render the line."""

         if not self:
             return '\n'
 
         if not self:
             return '\n'
 


@@ -627,9 +756,63 @@ class Line:
         return res + '\n'
 
     def __bool__(self) -> bool:
         return res + '\n'
 
     def __bool__(self) -> bool:

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

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

+class UnformattedLines(Line):
+    """Just like :class:`Line` but stores lines which aren't reformatted."""
+
+    def append(self, leaf: Leaf, preformatted: bool = True) -> None:
+        """Just add a new `leaf` to the end of the lines.
+
+        The `preformatted` argument is ignored.
+
+        Keeps track of indentation `depth`, which is useful when the user
+        says `# fmt: on`. Otherwise, doesn't do anything with the `leaf`.
+        """
+        try:
+            list(generate_comments(leaf))
+        except FormatOn as f_on:
+            self.leaves.append(f_on.leaf_from_consumed(leaf))
+            raise
+
+        self.leaves.append(leaf)
+        if leaf.type == token.INDENT:
+            self.depth += 1
+        elif leaf.type == token.DEDENT:
+            self.depth -= 1
+
+    def append_comment(self, comment: Leaf) -> bool:
+        """Not implemented in this class."""
+        raise NotImplementedError("Unformatted lines don't store comments separately.")
+
+    def maybe_remove_trailing_comma(self, closing: Leaf) -> bool:
+        """Does nothing and returns False."""
+        return False
+
+    def maybe_increment_for_loop_variable(self, leaf: Leaf) -> bool:
+        """Does nothing and returns False."""
+        return False
+
+    def maybe_adapt_standalone_comment(self, comment: Leaf) -> bool:
+        """Does nothing and returns False."""
+        return False
+
+    def __str__(self) -> str:
+        """Renders unformatted lines from leaves which were added with `append()`.
+
+        `depth` is not used for indentation in this case.
+        """
+        if not self:
+            return '\n'
+
+        res = ''
+        for leaf in self.leaves:
+            res += str(leaf)
+        return res
+
+

 @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


@@ -650,6 +833,9 @@ 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 isinstance(current_line, UnformattedLines):
+            return 0, 0
+

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


@@ -658,13 +844,13 @@ class EmptyLineTracker:
 
     def _maybe_empty_lines(self, current_line: Line) -> Tuple[int, int]:
         max_allowed = 1
 
     def _maybe_empty_lines(self, current_line: Line) -> Tuple[int, int]:
         max_allowed = 1

-        if current_line.is_comment and current_line.depth == 0:
+        if current_line.depth == 0:

             max_allowed = 2
         if current_line.leaves:
             # Consume the first leaf's extra newlines.
             first_leaf = current_line.leaves[0]
             before = first_leaf.prefix.count('\n')
             max_allowed = 2
         if current_line.leaves:
             # Consume the first leaf's extra newlines.
             first_leaf = current_line.leaves[0]
             before = first_leaf.prefix.count('\n')

-            before = min(before, max(before, max_allowed))
+            before = min(before, max_allowed)

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


@@ -719,7 +905,7 @@ class LineGenerator(Visitor[Line]):
     """
     current_line: Line = Factory(Line)
 
     """
     current_line: Line = Factory(Line)
 

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

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


@@ -728,49 +914,82 @@ class LineGenerator(Visitor[Line]):
         If any lines were generated, set up a new current_line.
         """
         if not self.current_line:
         If any lines were generated, set up a new current_line.
         """
         if not self.current_line:

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

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

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

         yield complete_line
 
         yield complete_line
 

+    def visit(self, node: LN) -> Iterator[Line]:
+        """Main method to start the visit process. Yields :class:`Line` objects."""
+        if isinstance(self.current_line, UnformattedLines):
+            # File contained `# fmt: off`
+            yield from self.visit_unformatted(node)
+
+        else:
+            yield from super().visit(node)
+

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

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

         if isinstance(node, Leaf):
             any_open_brackets = self.current_line.bracket_tracker.any_open_brackets()
         if isinstance(node, Leaf):
             any_open_brackets = self.current_line.bracket_tracker.any_open_brackets()

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

         yield from super().visit_default(node)
 
     def visit_INDENT(self, node: Node) -> Iterator[Line]:
         yield from super().visit_default(node)
 
     def visit_INDENT(self, node: Node) -> Iterator[Line]:

+        """Increases indentation level, maybe yields a line."""
+        # In blib2to3 INDENT never holds comments.

         yield from self.line(+1)
         yield from self.visit_default(node)
 
     def visit_DEDENT(self, node: Node) -> Iterator[Line]:
         yield from self.line(+1)
         yield from self.visit_default(node)
 
     def visit_DEDENT(self, node: Node) -> Iterator[Line]:

+        """Decreases indentation level, maybe yields a line."""
+        # DEDENT has no value. Additionally, in blib2to3 it never holds comments.

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

-        """Visit a statement.
+        """Visits a statement.
+
+        This implementation is shared for `if`, `while`, `for`, `try`, `except`,
+        `def`, `with`, and `class`.

 
 

-        The relevant Python language keywords for this statement are NAME leaves
-        within it.
+        The relevant Python language `keywords` for a given statement will be NAME
+        leaves within it. This methods puts those on a separate line.

         """
         for child in node.children:
             if child.type == token.NAME and child.value in keywords:  # type: ignore
         """
         for child in node.children:
             if child.type == token.NAME and child.value in keywords:  # type: ignore


@@ -779,7 +998,7 @@ class LineGenerator(Visitor[Line]):
             yield from self.visit(child)
 
     def visit_simple_stmt(self, node: Node) -> Iterator[Line]:
             yield from self.visit(child)
 
     def visit_simple_stmt(self, node: Node) -> Iterator[Line]:

-        """A statement without nested statements."""
+        """Visits a statement without nested statements."""

         is_suite_like = node.parent and node.parent.type in STATEMENT
         if is_suite_like:
             yield from self.line(+1)
         is_suite_like = node.parent and node.parent.type in STATEMENT
         if is_suite_like:
             yield from self.line(+1)


@@ -791,6 +1010,7 @@ class LineGenerator(Visitor[Line]):
             yield from self.visit_default(node)
 
     def visit_async_stmt(self, node: Node) -> Iterator[Line]:
             yield from self.visit_default(node)
 
     def visit_async_stmt(self, node: Node) -> Iterator[Line]:

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

         yield from self.line()
 
         children = iter(node.children)
         yield from self.line()
 
         children = iter(node.children)


@@ -805,17 +1025,40 @@ class LineGenerator(Visitor[Line]):
             yield from self.visit(child)
 
     def visit_decorators(self, node: Node) -> Iterator[Line]:
             yield from self.visit(child)
 
     def visit_decorators(self, node: Node) -> Iterator[Line]:

+        """Visits decorators."""

         for child in node.children:
             yield from self.line()
             yield from self.visit(child)
 
     def visit_SEMI(self, leaf: Leaf) -> Iterator[Line]:
         for child in node.children:
             yield from self.line()
             yield from self.visit(child)
 
     def visit_SEMI(self, leaf: Leaf) -> Iterator[Line]:

+        """Semicolons are always removed.
+
+        Statements between them are put on separate lines.
+        """

         yield from self.line()
 
     def visit_ENDMARKER(self, leaf: Leaf) -> Iterator[Line]:
         yield from self.line()
 
     def visit_ENDMARKER(self, leaf: Leaf) -> Iterator[Line]:

+        """End of file.
+
+        Process outstanding comments and end with a newline.
+        """

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

+    def visit_unformatted(self, node: LN) -> Iterator[Line]:
+        """Used when file contained a `# fmt: off`."""
+        if isinstance(node, Node):
+            for child in node.children:
+                yield from self.visit(child)
+
+        else:
+            try:
+                self.current_line.append(node)
+            except FormatOn as f_on:
+                f_on.trim_prefix(node)
+                yield from self.line()
+                yield from self.visit(node)
+

     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


@@ -866,14 +1109,17 @@ 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.arglist,
-                syms.argument,
-                syms.parameters,
-                syms.typedargslist,
-                syms.varargslist,
-            }:
-                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 {


@@ -897,6 +1143,17 @@ def whitespace(leaf: Leaf) -> str:  # noqa C901
         ):
             return NO
 
         ):
             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:
         return NO
 


@@ -908,7 +1165,7 @@ 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


@@ -1126,8 +1383,10 @@ def generate_comments(leaf: Leaf) -> Iterator[Leaf]:
     if '#' not in p:
         return
 
     if '#' not in p:
         return
 

+    consumed = 0

     nlines = 0
     for index, line in enumerate(p.split('\n')):
     nlines = 0
     for index, line in enumerate(p.split('\n')):

+        consumed += len(line) + 1  # adding the length of the split '\n'

         line = line.lstrip()
         if not line:
             nlines += 1
         line = line.lstrip()
         if not line:
             nlines += 1


@@ -1138,19 +1397,33 @@ def generate_comments(leaf: Leaf) -> Iterator[Leaf]:
             comment_type = token.COMMENT  # simple trailing comment
         else:
             comment_type = STANDALONE_COMMENT
             comment_type = token.COMMENT  # simple trailing comment
         else:
             comment_type = STANDALONE_COMMENT

-        yield Leaf(comment_type, make_comment(line), prefix='\n' * nlines)
+        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:
 
         nlines = 0
 
 
 def make_comment(content: str) -> str:

+    """Returns a consistently formatted comment from the given `content` string.
+
+    All comments (except for "##", "#!", "#:") should have a single space between
+    the hash sign and the content.
+
+    If `content` didn't start with a hash sign, one is provided.
+    """

     content = content.rstrip()
     if not content:
         return '#'
 
     if content[0] == '#':
         content = content[1:]
     content = content.rstrip()
     if not content:
         return '#'
 
     if content[0] == '#':
         content = content[1:]

-    if content and content[0] not in {' ', '!', '#'}:
+    if content and content[0] not in ' !:#':

         content = ' ' + content
     return '#' + content
 
         content = ' ' + content
     return '#' + content
 


@@ -1168,6 +1441,10 @@ 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


@@ -1208,7 +1485,7 @@ def split_line(
 
 
 def left_hand_split(line: Line, py36: bool = False) -> Iterator[Line]:
 
 
 def left_hand_split(line: Line, py36: bool = False) -> Iterator[Line]:

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


@@ -1245,14 +1522,14 @@ def left_hand_split(line: Line, py36: bool = False) -> Iterator[Line]:
             comment_after = line.comments.get(id(leaf))
             if comment_after:
                 result.append(comment_after, preformatted=True)
             comment_after = line.comments.get(id(leaf))
             if comment_after:
                 result.append(comment_after, preformatted=True)

-    split_succeeded_or_raise(head, body, tail)
+    bracket_split_succeeded_or_raise(head, body, tail)

     for result in (head, body, tail):
         if result:
             yield result
 
 
 def right_hand_split(line: Line, py36: bool = False) -> Iterator[Line]:
     for result in (head, body, tail):
         if result:
             yield result
 
 
 def right_hand_split(line: Line, py36: bool = False) -> Iterator[Line]:

-    """Split line into many lines, starting with the last matching bracket pair."""
+    """Splits line into many lines, starting with the last matching bracket pair."""

     head = Line(depth=line.depth)
     body = Line(depth=line.depth + 1, inside_brackets=True)
     tail = Line(depth=line.depth)
     head = Line(depth=line.depth)
     body = Line(depth=line.depth + 1, inside_brackets=True)
     tail = Line(depth=line.depth)


@@ -1285,13 +1562,26 @@ def right_hand_split(line: Line, py36: bool = False) -> Iterator[Line]:
             comment_after = line.comments.get(id(leaf))
             if comment_after:
                 result.append(comment_after, preformatted=True)
             comment_after = line.comments.get(id(leaf))
             if comment_after:
                 result.append(comment_after, preformatted=True)

-    split_succeeded_or_raise(head, body, tail)
+    bracket_split_succeeded_or_raise(head, body, tail)

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

-def split_succeeded_or_raise(head: Line, body: Line, tail: Line) -> None:
+def bracket_split_succeeded_or_raise(head: Line, body: Line, tail: Line) -> None:
+    """Raise :exc:`CannotSplit` if the last left- or right-hand split failed.
+
+    Do nothing otherwise.
+
+    A left- or right-hand split is based on a pair of brackets. Content before
+    (and including) the opening bracket is left on one line, content inside the
+    brackets is put on a separate line, and finally content starting with and
+    following the closing bracket is put on a separate line.
+
+    Those are called `head`, `body`, and `tail`, respectively. If the split
+    produced the same line (all content in `head`) or ended up with an empty `body`
+    and the `tail` is just the closing bracket, then it's considered failed.
+    """

     tail_len = len(str(tail).strip())
     if not body:
         if tail_len == 0:
     tail_len = len(str(tail).strip())
     if not body:
         if tail_len == 0:


@@ -1305,11 +1595,11 @@ def split_succeeded_or_raise(head: Line, body: Line, tail: Line) -> None:
 
 
 def delimiter_split(line: Line, py36: bool = False) -> Iterator[Line]:
 
 
 def delimiter_split(line: Line, py36: bool = False) -> Iterator[Line]:

-    """Split according to delimiters of the highest priority.
+    """Splits according to delimiters of the highest priority.

 
     This kind of split doesn't increase indentation.
     If `py36` is True, the split will add trailing commas also in function
 
     This kind of split doesn't increase indentation.
     If `py36` is True, the split will add trailing commas also in function

-    signatures that contain * and **.
+    signatures that contain `*` and `**`.

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


@@ -1318,7 +1608,9 @@ def delimiter_split(line: Line, py36: bool = False) -> Iterator[Line]:
 
     delimiters = line.bracket_tracker.delimiters
     try:
 
     delimiters = line.bracket_tracker.delimiters
     try:

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

     except ValueError:
         raise CannotSplit("No delimiters found")
 
     except ValueError:
         raise CannotSplit("No delimiters found")
 


@@ -1369,9 +1661,9 @@ def is_import(leaf: Leaf) -> bool:
 
 
 def normalize_prefix(leaf: Leaf, *, inside_brackets: bool) -> None:
 
 
 def normalize_prefix(leaf: Leaf, *, inside_brackets: bool) -> None:

-    """Leave existing extra newlines if not `inside_brackets`.
+    """Leaves existing extra newlines if not `inside_brackets`.

 
 

-    Remove everything else.  Note: don't use backslashes for formatting or
+    Removes everything else.  Note: don't use backslashes for formatting or

     you'll lose your voting rights.
     """
     if not inside_brackets:
     you'll lose your voting rights.
     """
     if not inside_brackets:


@@ -1418,6 +1710,9 @@ BLACKLISTED_DIRECTORIES = {
 
 
 def gen_python_files_in_dir(path: Path) -> Iterator[Path]:
 
 
 def gen_python_files_in_dir(path: Path) -> Iterator[Path]:

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

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


@@ -1431,7 +1726,8 @@ def gen_python_files_in_dir(path: Path) -> Iterator[Path]:
 
 @dataclass
 class Report:
 
 @dataclass
 class Report:

-    """Provides a reformatting counter."""
+    """Provides a reformatting counter. Can be rendered with `str(report)`."""
+    check: bool = False

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


@@ -1439,7 +1735,8 @@ 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)


@@ -1458,7 +1755,7 @@ 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


@@ -1468,21 +1765,27 @@ 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) + '.'
 


@@ -1524,7 +1827,12 @@ 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)