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Line breaks before logical operators (#36)
[etc/vim.git] / black.py
index e2c427cb536c752b6e5d0e00c07ca2b2a2a6a23e..10346a9143c4e490d7fb6a8f314cc00a94e9664a 100644 (file)
--- a/black.py
+++ b/black.py
@@ -7,11 +7,12 @@ import keyword
 import os
 from pathlib import Path
 import tokenize
+import sys
 from typing import (
     Dict, Generic, Iterable, Iterator, List, Optional, Set, Tuple, TypeVar, Union
 )
 
-from attr import attrib, dataclass, Factory
+from attr import dataclass, Factory
 import click
 
 # lib2to3 fork
@@ -20,7 +21,7 @@ from blib2to3 import pygram, pytree
 from blib2to3.pgen2 import driver, token
 from blib2to3.pgen2.parse import ParseError
 
-__version__ = "18.3a1"
+__version__ = "18.3a2"
 DEFAULT_LINE_LENGTH = 88
 # types
 syms = pygram.python_symbols
@@ -192,6 +193,7 @@ def format_str(src_contents: str, line_length: int) -> FileContent:
     comments: List[Line] = []
     lines = LineGenerator()
     elt = EmptyLineTracker()
+    py36 = is_python36(src_node)
     empty_line = Line()
     after = 0
     for current_line in lines.visit(src_node):
@@ -204,7 +206,7 @@ def format_str(src_contents: str, line_length: int) -> FileContent:
             for comment in comments:
                 dst_contents += str(comment)
             comments = []
-            for line in split_line(current_line, line_length=line_length):
+            for line in split_line(current_line, line_length=line_length, py36=py36):
                 dst_contents += str(line)
         else:
             comments.append(current_line)
@@ -263,7 +265,7 @@ class Visitor(Generic[T]):
 
 @dataclass
 class DebugVisitor(Visitor[T]):
-    tree_depth: int = attrib(default=0)
+    tree_depth: int = 0
 
     def visit_default(self, node: LN) -> Iterator[T]:
         indent = ' ' * (2 * self.tree_depth)
@@ -333,10 +335,10 @@ MATH_PRIORITY = 1
 
 @dataclass
 class BracketTracker:
-    depth: int = attrib(default=0)
-    bracket_match: Dict[Tuple[Depth, NodeType], Leaf] = attrib(default=Factory(dict))
-    delimiters: Dict[LeafID, Priority] = attrib(default=Factory(dict))
-    previous: Optional[Leaf] = attrib(default=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:
         if leaf.type == token.COMMENT:
@@ -345,8 +347,8 @@ class BracketTracker:
         if leaf.type in CLOSING_BRACKETS:
             self.depth -= 1
             opening_bracket = self.bracket_match.pop((self.depth, leaf.type))
-            leaf.opening_bracket = opening_bracket  # type: ignore
-        leaf.bracket_depth = self.depth  # type: ignore
+            leaf.opening_bracket = opening_bracket
+        leaf.bracket_depth = self.depth
         if self.depth == 0:
             delim = is_delimiter(leaf)
             if delim:
@@ -355,19 +357,25 @@ class BracketTracker:
                 if leaf.type == token.STRING and self.previous.type == token.STRING:
                     self.delimiters[id(self.previous)] = STRING_PRIORITY
                 elif (
-                    leaf.type == token.NAME and
-                    leaf.value == 'for' and
-                    leaf.parent and
-                    leaf.parent.type in {syms.comp_for, syms.old_comp_for}
+                    leaf.type == token.NAME
+                    and leaf.value == 'for'
+                    and leaf.parent
+                    and leaf.parent.type in {syms.comp_for, syms.old_comp_for}
                 ):
                     self.delimiters[id(self.previous)] = COMPREHENSION_PRIORITY
                 elif (
-                    leaf.type == token.NAME and
-                    leaf.value == 'if' and
-                    leaf.parent and
-                    leaf.parent.type in {syms.comp_if, syms.old_comp_if}
+                    leaf.type == token.NAME
+                    and leaf.value == 'if'
+                    and leaf.parent
+                    and leaf.parent.type in {syms.comp_if, syms.old_comp_if}
                 ):
                     self.delimiters[id(self.previous)] = COMPREHENSION_PRIORITY
+                elif (
+                    leaf.type == token.NAME
+                    and leaf.value in LOGIC_OPERATORS
+                    and leaf.parent
+                ):
+                    self.delimiters[id(self.previous)] = LOGIC_PRIORITY
         if leaf.type in OPENING_BRACKETS:
             self.bracket_match[self.depth, BRACKET[leaf.type]] = leaf
             self.depth += 1
@@ -387,11 +395,13 @@ class BracketTracker:
 
 @dataclass
 class Line:
-    depth: int = attrib(default=0)
-    leaves: List[Leaf] = attrib(default=Factory(list))
-    comments: Dict[LeafID, Leaf] = attrib(default=Factory(dict))
-    bracket_tracker: BracketTracker = attrib(default=Factory(BracketTracker))
-    inside_brackets: bool = attrib(default=False)
+    depth: int = 0
+    leaves: List[Leaf] = Factory(list)
+    comments: Dict[LeafID, Leaf] = Factory(dict)
+    bracket_tracker: BracketTracker = Factory(BracketTracker)
+    inside_brackets: bool = False
+    has_for: bool = False
+    _for_loop_variable: bool = False
 
     def append(self, leaf: Leaf, preformatted: bool = False) -> None:
         has_value = leaf.value.strip()
@@ -403,8 +413,10 @@ class Line:
             # imports, for which we only preserve newlines.
             leaf.prefix += whitespace(leaf)
         if self.inside_brackets or not preformatted:
+            self.maybe_decrement_after_for_loop_variable(leaf)
             self.bracket_tracker.mark(leaf)
             self.maybe_remove_trailing_comma(leaf)
+            self.maybe_increment_for_loop_variable(leaf)
             if self.maybe_adapt_standalone_comment(leaf):
                 return
 
@@ -426,9 +438,9 @@ class Line:
     @property
     def is_class(self) -> bool:
         return (
-            bool(self) and
-            self.leaves[0].type == token.NAME and
-            self.leaves[0].value == 'class'
+            bool(self)
+            and self.leaves[0].type == token.NAME
+            and self.leaves[0].value == 'class'
         )
 
     @property
@@ -444,37 +456,37 @@ class Line:
         except IndexError:
             second_leaf = None
         return (
-            (first_leaf.type == token.NAME and first_leaf.value == 'def') or
-            (
-                first_leaf.type == token.NAME and
-                first_leaf.value == 'async' and
-                second_leaf is not None and
-                second_leaf.type == token.NAME and
-                second_leaf.value == 'def'
+            (first_leaf.type == token.NAME and first_leaf.value == 'def')
+            or (
+                first_leaf.type == token.NAME
+                and first_leaf.value == 'async'
+                and second_leaf is not None
+                and second_leaf.type == token.NAME
+                and second_leaf.value == 'def'
             )
         )
 
     @property
     def is_flow_control(self) -> bool:
         return (
-            bool(self) and
-            self.leaves[0].type == token.NAME and
-            self.leaves[0].value in FLOW_CONTROL
+            bool(self)
+            and self.leaves[0].type == token.NAME
+            and self.leaves[0].value in FLOW_CONTROL
         )
 
     @property
     def is_yield(self) -> bool:
         return (
-            bool(self) and
-            self.leaves[0].type == token.NAME and
-            self.leaves[0].value == 'yield'
+            bool(self)
+            and self.leaves[0].type == token.NAME
+            and self.leaves[0].value == 'yield'
         )
 
     def maybe_remove_trailing_comma(self, closing: Leaf) -> bool:
         if not (
-            self.leaves and
-            self.leaves[-1].type == token.COMMA and
-            closing.type in CLOSING_BRACKETS
+            self.leaves
+            and self.leaves[-1].type == token.COMMA
+            and closing.type in CLOSING_BRACKETS
         ):
             return False
 
@@ -485,9 +497,9 @@ class Line:
         # 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.
-        depth = closing.bracket_depth + 1  # type: ignore
+        depth = closing.bracket_depth + 1
         commas = 0
-        opening = closing.opening_bracket  # type: ignore
+        opening = closing.opening_bracket
         for _opening_index, leaf in enumerate(self.leaves):
             if leaf is opening:
                 break
@@ -499,15 +511,42 @@ class Line:
             if leaf is closing:
                 break
 
-            bracket_depth = leaf.bracket_depth  # type: ignore
+            bracket_depth = leaf.bracket_depth
             if bracket_depth == depth and leaf.type == token.COMMA:
                 commas += 1
+                if leaf.parent and leaf.parent.type == syms.arglist:
+                    commas += 1
+                    break
+
         if commas > 1:
             self.leaves.pop()
             return True
 
         return False
 
+    def maybe_increment_for_loop_variable(self, leaf: Leaf) -> bool:
+        """In a for loop, or comprehension, the variables are often unpacks.
+
+        To avoid splitting on the comma in this situation, we will increase
+        the depth of tokens between `for` and `in`.
+        """
+        if leaf.type == token.NAME and leaf.value == 'for':
+            self.has_for = True
+            self.bracket_tracker.depth += 1
+            self._for_loop_variable = True
+            return True
+
+        return False
+
+    def maybe_decrement_after_for_loop_variable(self, leaf: Leaf) -> bool:
+        # See `maybe_increment_for_loop_variable` above for explanation.
+        if self._for_loop_variable and leaf.type == token.NAME and leaf.value == 'in':
+            self.bracket_tracker.depth -= 1
+            self._for_loop_variable = False
+            return True
+
+        return False
+
     def maybe_adapt_standalone_comment(self, comment: Leaf) -> bool:
         """Hack a standalone comment to act as a trailing comment for line splitting.
 
@@ -518,8 +557,8 @@ class Line:
         appended will appear "too long" when splitting.
         """
         if not (
-            comment.type == STANDALONE_COMMENT and
-            self.bracket_tracker.any_open_brackets()
+            comment.type == STANDALONE_COMMENT
+            and self.bracket_tracker.any_open_brackets()
         ):
             return False
 
@@ -578,9 +617,9 @@ class EmptyLineTracker:
 
     Note: this tracker works on lines that haven't been split yet.
     """
-    previous_line: Optional[Line] = attrib(default=None)
-    previous_after: int = attrib(default=0)
-    previous_defs: List[int] = attrib(default=Factory(list))
+    previous_line: Optional[Line] = None
+    previous_after: int = 0
+    previous_defs: List[int] = Factory(list)
 
     def maybe_empty_lines(self, current_line: Line) -> Tuple[int, int]:
         """Returns the number of extra empty lines before and after the `current_line`.
@@ -622,17 +661,17 @@ class EmptyLineTracker:
             return before, 1
 
         if (
-            self.previous_line and
-            self.previous_line.is_import and
-            not current_line.is_import and
-            depth == self.previous_line.depth
+            self.previous_line
+            and self.previous_line.is_import
+            and not current_line.is_import
+            and depth == self.previous_line.depth
         ):
             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)
+            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
 
@@ -646,8 +685,8 @@ 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.
     """
-    current_line: Line = attrib(default=Factory(Line))
-    standalone_comments: List[Leaf] = attrib(default=Factory(list))
+    current_line: Line = Factory(Line)
+    standalone_comments: List[Leaf] = Factory(list)
 
     def line(self, indent: int = 0) -> Iterator[Line]:
         """Generate a line.
@@ -775,9 +814,10 @@ BRACKET = {token.LPAR: token.RPAR, token.LSQB: token.RSQB, token.LBRACE: token.R
 OPENING_BRACKETS = set(BRACKET.keys())
 CLOSING_BRACKETS = set(BRACKET.values())
 BRACKETS = OPENING_BRACKETS | CLOSING_BRACKETS
+ALWAYS_NO_SPACE = CLOSING_BRACKETS | {token.COMMA, token.COLON, STANDALONE_COMMENT}
 
 
-def whitespace(leaf: Leaf) -> str:
+def whitespace(leaf: Leaf) -> str:  # noqa C901
     """Return whitespace prefix if needed for the given `leaf`."""
     NO = ''
     SPACE = ' '
@@ -785,24 +825,12 @@ def whitespace(leaf: Leaf) -> str:
     t = leaf.type
     p = leaf.parent
     v = leaf.value
-    if t == token.COLON:
-        return NO
-
-    if t == token.COMMA:
-        return NO
-
-    if t == token.RPAR:
+    if t in ALWAYS_NO_SPACE:
         return NO
 
     if t == token.COMMENT:
         return DOUBLESPACE
 
-    if t == STANDALONE_COMMENT:
-        return NO
-
-    if t in CLOSING_BRACKETS:
-        return NO
-
     assert p is not None, f"INTERNAL ERROR: hand-made leaf without parent: {leaf!r}"
     prev = leaf.prev_sibling
     if not prev:
@@ -834,7 +862,7 @@ def whitespace(leaf: Leaf) -> str:
             if prevp.parent and prevp.parent.type == syms.subscript:
                 return NO
 
-        elif prevp.parent and prevp.parent.type == syms.factor:
+        elif prevp.parent and prevp.parent.type in {syms.factor, syms.star_expr}:
             return NO
 
     elif prev.type in OPENING_BRACKETS:
@@ -931,7 +959,14 @@ def whitespace(leaf: Leaf) -> str:
 
     elif p.type == syms.subscript:
         # indexing
-        if not prev or prev.type == token.COLON:
+        if not prev:
+            assert p.parent is not None, "subscripts are always parented"
+            if p.parent.type == syms.subscriptlist:
+                return SPACE
+
+            return NO
+
+        elif prev.type == token.COLON:
             return NO
 
     elif p.type == syms.atom:
@@ -940,9 +975,9 @@ def whitespace(leaf: Leaf) -> str:
             return NO
 
     elif (
-        p.type == syms.listmaker or
-        p.type == syms.testlist_gexp or
-        p.type == syms.subscriptlist
+        p.type == syms.listmaker
+        or p.type == syms.testlist_gexp
+        or p.type == syms.subscriptlist
     ):
         # list interior, including unpacking
         if not prev:
@@ -956,7 +991,7 @@ def whitespace(leaf: Leaf) -> str:
         if prev.type == token.DOUBLESTAR:
             return NO
 
-    elif p.type == syms.factor or p.type == syms.star_expr:
+    elif p.type in {syms.factor, syms.star_expr}:
         # unary ops
         if not prev:
             prevp = preceding_leaf(p)
@@ -1020,16 +1055,13 @@ def is_delimiter(leaf: Leaf) -> int:
     if leaf.type == token.COMMA:
         return COMMA_PRIORITY
 
-    if leaf.type == token.NAME and leaf.value in LOGIC_OPERATORS:
-        return LOGIC_PRIORITY
-
     if leaf.type in COMPARATORS:
         return COMPARATOR_PRIORITY
 
     if (
-        leaf.type in MATH_OPERATORS and
-        leaf.parent and
-        leaf.parent.type not in {syms.factor, syms.star_expr}
+        leaf.type in MATH_OPERATORS
+        and leaf.parent
+        and leaf.parent.type not in {syms.factor, syms.star_expr}
     ):
         return MATH_PRIORITY
 
@@ -1081,13 +1113,18 @@ def generate_comments(leaf: Leaf) -> Iterator[Leaf]:
         yield Leaf(STANDALONE_COMMENT, line)
 
 
-def split_line(line: Line, line_length: int, inner: bool = False) -> Iterator[Line]:
+def split_line(
+    line: Line, line_length: int, inner: bool = False, py36: bool = False
+) -> Iterator[Line]:
     """Splits a `line` into potentially many lines.
 
     They should fit in the allotted `line_length` but might not be able to.
     `inner` signifies that there were a pair of brackets somewhere around the
     current `line`, possibly transitively. This means we can fallback to splitting
     by delimiters if the LHS/RHS don't yield any results.
+
+    If `py36` is True, splitting may generate syntax that is only compatible
+    with Python 3.6 and later.
     """
     line_str = str(line).strip('\n')
     if len(line_str) <= line_length and '\n' not in line_str:
@@ -1110,11 +1147,13 @@ def split_line(line: Line, line_length: int, inner: bool = False) -> Iterator[Li
         # split altogether.
         result: List[Line] = []
         try:
-            for l in split_func(line):
+            for l in split_func(line, py36=py36):
                 if str(l).strip('\n') == line_str:
                     raise CannotSplit("Split function returned an unchanged result")
 
-                result.extend(split_line(l, line_length=line_length, inner=True))
+                result.extend(
+                    split_line(l, line_length=line_length, inner=True, py36=py36)
+                )
         except CannotSplit as cs:
             continue
 
@@ -1126,7 +1165,7 @@ def split_line(line: Line, line_length: int, inner: bool = False) -> Iterator[Li
         yield line
 
 
-def left_hand_split(line: Line) -> 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.
 
     Note: this usually looks weird, only use this for function definitions.
@@ -1142,11 +1181,11 @@ def left_hand_split(line: Line) -> Iterator[Line]:
     matching_bracket = None
     for leaf in line.leaves:
         if (
-            current_leaves is body_leaves and
-            leaf.type in CLOSING_BRACKETS and
-            leaf.opening_bracket is matching_bracket  # type: ignore
+            current_leaves is body_leaves
+            and leaf.type in CLOSING_BRACKETS
+            and leaf.opening_bracket is matching_bracket
         ):
-            current_leaves = tail_leaves
+            current_leaves = tail_leaves if body_leaves else head_leaves
         current_leaves.append(leaf)
         if current_leaves is head_leaves:
             if leaf.type in OPENING_BRACKETS:
@@ -1164,24 +1203,13 @@ def left_hand_split(line: Line) -> Iterator[Line]:
             comment_after = line.comments.get(id(leaf))
             if comment_after:
                 result.append(comment_after, preformatted=True)
-    # Check if the split succeeded.
-    tail_len = len(str(tail))
-    if not body:
-        if tail_len == 0:
-            raise CannotSplit("Splitting brackets produced the same line")
-
-        elif tail_len < 3:
-            raise CannotSplit(
-                f"Splitting brackets on an empty body to save "
-                f"{tail_len} characters is not worth it"
-            )
-
+    split_succeeded_or_raise(head, body, tail)
     for result in (head, body, tail):
         if result:
             yield result
 
 
-def right_hand_split(line: Line) -> Iterator[Line]:
+def right_hand_split(line: Line, py36: bool = False) -> Iterator[Line]:
     """Split 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)
@@ -1194,11 +1222,11 @@ def right_hand_split(line: Line) -> Iterator[Line]:
     for leaf in reversed(line.leaves):
         if current_leaves is body_leaves:
             if leaf is opening_bracket:
-                current_leaves = head_leaves
+                current_leaves = head_leaves if body_leaves else tail_leaves
         current_leaves.append(leaf)
         if current_leaves is tail_leaves:
             if leaf.type in CLOSING_BRACKETS:
-                opening_bracket = leaf.opening_bracket  # type: ignore
+                opening_bracket = leaf.opening_bracket
                 current_leaves = body_leaves
     tail_leaves.reverse()
     body_leaves.reverse()
@@ -1215,8 +1243,14 @@ def right_hand_split(line: Line) -> Iterator[Line]:
             comment_after = line.comments.get(id(leaf))
             if comment_after:
                 result.append(comment_after, preformatted=True)
-    # Check if the split succeeded.
-    tail_len = len(str(tail).strip('\n'))
+    split_succeeded_or_raise(head, body, tail)
+    for result in (head, body, tail):
+        if result:
+            yield result
+
+
+def split_succeeded_or_raise(head: Line, body: Line, tail: Line) -> None:
+    tail_len = len(str(tail).strip())
     if not body:
         if tail_len == 0:
             raise CannotSplit("Splitting brackets produced the same line")
@@ -1227,15 +1261,13 @@ def right_hand_split(line: Line) -> Iterator[Line]:
                 f"{tail_len} characters is not worth it"
             )
 
-    for result in (head, body, tail):
-        if result:
-            yield result
-
 
-def delimiter_split(line: Line) -> Iterator[Line]:
+def delimiter_split(line: Line, py36: bool = False) -> Iterator[Line]:
     """Split according to delimiters of the highest priority.
 
     This kind of split doesn't increase indentation.
+    If `py36` is True, the split will add trailing commas also in function
+    signatures that contain * and **.
     """
     try:
         last_leaf = line.leaves[-1]
@@ -1249,11 +1281,20 @@ def delimiter_split(line: Line) -> Iterator[Line]:
         raise CannotSplit("No delimiters found")
 
     current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)
+    lowest_depth = sys.maxsize
+    trailing_comma_safe = True
     for leaf in line.leaves:
         current_line.append(leaf, preformatted=True)
         comment_after = line.comments.get(id(leaf))
         if comment_after:
             current_line.append(comment_after, preformatted=True)
+        lowest_depth = min(lowest_depth, leaf.bracket_depth)
+        if (
+            leaf.bracket_depth == lowest_depth
+            and leaf.type == token.STAR
+            or leaf.type == token.DOUBLESTAR
+        ):
+            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])
@@ -1262,8 +1303,9 @@ def delimiter_split(line: Line) -> Iterator[Line]:
             current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)
     if current_line:
         if (
-            delimiter_priority == COMMA_PRIORITY and
-            current_line.leaves[-1].type != token.COMMA
+            delimiter_priority == COMMA_PRIORITY
+            and current_line.leaves[-1].type != token.COMMA
+            and trailing_comma_safe
         ):
             current_line.append(Leaf(token.COMMA, ','))
         normalize_prefix(current_line.leaves[0])
@@ -1276,10 +1318,10 @@ def is_import(leaf: Leaf) -> bool:
     t = leaf.type
     v = leaf.value
     return bool(
-        t == token.NAME and
-        (
-            (v == 'import' and p and p.type == syms.import_name) or
-            (v == 'from' and p and p.type == syms.import_from)
+        t == token.NAME
+        and (
+            (v == 'import' and p and p.type == syms.import_name)
+            or (v == 'from' and p and p.type == syms.import_from)
         )
     )
 
@@ -1298,6 +1340,31 @@ def normalize_prefix(leaf: Leaf) -> None:
     leaf.prefix = ''
 
 
+def is_python36(node: Node) -> bool:
+    """Returns True if the current file is using Python 3.6+ features.
+
+    Currently looking for:
+    - f-strings; and
+    - trailing commas after * or ** in function signatures.
+    """
+    for n in node.pre_order():
+        if n.type == token.STRING:
+            value_head = n.value[:2]  # type: ignore
+            if value_head in {'f"', 'F"', "f'", "F'", 'rf', 'fr', 'RF', 'FR'}:
+                return True
+
+        elif (
+            n.type == syms.typedargslist
+            and n.children
+            and n.children[-1].type == token.COMMA
+        ):
+            for ch in n.children:
+                if ch.type == token.STAR or ch.type == token.DOUBLESTAR:
+                    return True
+
+    return False
+
+
 PYTHON_EXTENSIONS = {'.py'}
 BLACKLISTED_DIRECTORIES = {
     'build', 'buck-out', 'dist', '_build', '.git', '.hg', '.mypy_cache', '.tox', '.venv'
@@ -1319,9 +1386,9 @@ def gen_python_files_in_dir(path: Path) -> Iterator[Path]:
 @dataclass
 class Report:
     """Provides a reformatting counter."""
-    change_count: int = attrib(default=0)
-    same_count: int = attrib(default=0)
-    failure_count: int = attrib(default=0)
+    change_count: int = 0
+    same_count: int = 0
+    failure_count: int = 0
 
     def done(self, src: Path, changed: bool) -> None:
         """Increment the counter for successful reformatting. Write out a message."""
@@ -1420,7 +1487,7 @@ def assert_equivalent(src: str, dst: str) -> None:
         raise AssertionError(
             f"INTERNAL ERROR: Black produced invalid code: {exc}. "
             f"Please report a bug on https://github.com/ambv/black/issues.  "
-            f"This invalid output might be helpful: {log}",
+            f"This invalid output might be helpful: {log}"
         ) from None
 
     src_ast_str = '\n'.join(_v(src_ast))
@@ -1431,7 +1498,7 @@ def assert_equivalent(src: str, dst: str) -> None:
             f"INTERNAL ERROR: Black produced code that is not equivalent to "
             f"the source.  "
             f"Please report a bug on https://github.com/ambv/black/issues.  "
-            f"This diff might be helpful: {log}",
+            f"This diff might be helpful: {log}"
         ) from None
 
 
@@ -1450,7 +1517,7 @@ def assert_stable(src: str, dst: str, line_length: int) -> None:
             f"INTERNAL ERROR: Black produced different code on the second pass "
             f"of the formatter.  "
             f"Please report a bug on https://github.com/ambv/black/issues.  "
-            f"This diff might be helpful: {log}",
+            f"This diff might be helpful: {log}"
         ) from None