Add --skip-magic-trailing-comma

README.md

@@ -97,6 +97,10 @@ Options:
 
   -S, --skip-string-normalization
                                   Don't normalize string quotes or prefixes.
+  -C, --skip-magic-trailing-comma
+                                  Don't use trailing commas as a reason to
+                                  split lines.
+
   --check                         Don't write the files back, just return the
                                   status.  Return code 0 means nothing would
                                   change.  Return code 1 means some files
@@ -127,18 +131,19 @@ Options:
                                   paths are excluded. Use forward slashes for
                                   directories on all platforms (Windows, too).
                                   Exclusions are calculated first, inclusions
-                                  later.  [default: /(\.eggs|\.git|\.hg|\.mypy
-                                  _cache|\.nox|\.tox|\.venv|\.svn|_build|buck-
-                                  out|build|dist)/]
+                                  later.  [default: /(\.direnv|\.eggs|\.git|\.
+                                  hg|\.mypy_cache|\.nox|\.tox|\.venv|\.svn|_bu
+                                  ild|buck-out|build|dist)/]
 
   --force-exclude TEXT            Like --exclude, but files and directories
                                   matching this regex will be excluded even
-                                  when they are passed explicitly as arguments.
+                                  when they are passed explicitly as
+                                  arguments.
 
   --stdin-filename TEXT           The name of the file when passing it through
-                                  stdin. Useful to make sure Black will respect
-                                  --force-exclude option on some editors that
-                                  rely on using stdin.
+                                  stdin. Useful to make sure Black will
+                                  respect --force-exclude option on some
+                                  editors that rely on using stdin.
 
   -q, --quiet                     Don't emit non-error messages to stderr.
                                   Errors are still emitted; silence those with

docs/blackd.md

@@ -54,6 +54,9 @@ The headers controlling how source code is formatted are:
 - `X-Skip-String-Normalization`: corresponds to the `--skip-string-normalization`
   command line flag. If present and its value is not the empty string, no string
   normalization will be performed.
+- `X-Skip-Magic-Trailing-Comma`: corresponds to the `--skip-magic-trailing-comma`
+  command line flag. If present and its value is not the empty string, trailing commas
+  will not be used as a reason to split lines.
 - `X-Fast-Or-Safe`: if set to `fast`, `blackd` will act as _Black_ does when passed the
   `--fast` command line flag.
 - `X-Python-Variant`: if set to `pyi`, `blackd` will act as _Black_ does when passed the

docs/installation_and_usage.md

@@ -52,6 +52,10 @@ Options:
 
   -S, --skip-string-normalization
                                   Don't normalize string quotes or prefixes.
+  -C, --skip-magic-trailing-comma
+                                  Don't use trailing commas as a reason to
+                                  split lines.
+
   --check                         Don't write the files back, just return the
                                   status.  Return code 0 means nothing would
                                   change.  Return code 1 means some files
@@ -82,13 +86,19 @@ Options:
                                   paths are excluded. Use forward slashes for
                                   directories on all platforms (Windows, too).
                                   Exclusions are calculated first, inclusions
-                                  later.  [default: /(\.eggs|\.git|\.hg|\.mypy
-                                  _cache|\.nox|\.tox|\.venv|\.svn|_build|buck-
-                                  out|build|dist)/]
+                                  later.  [default: /(\.direnv|\.eggs|\.git|\.
+                                  hg|\.mypy_cache|\.nox|\.tox|\.venv|\.svn|_bu
+                                  ild|buck-out|build|dist)/]
 
   --force-exclude TEXT            Like --exclude, but files and directories
                                   matching this regex will be excluded even
-                                  when they are passed explicitly as arguments.
+                                  when they are passed explicitly as
+                                  arguments.
+
+  --stdin-filename TEXT           The name of the file when passing it through
+                                  stdin. Useful to make sure Black will
+                                  respect --force-exclude option on some
+                                  editors that rely on using stdin.
 
   -q, --quiet                     Don't emit non-error messages to stderr.
                                   Errors are still emitted; silence those with

docs/the_black_code_style.md

@@ -438,6 +438,9 @@ into one item per line.
 How do you make it stop? Just delete that trailing comma and _Black_ will collapse your
 collection into one line if it fits.
 
+If you must, you can recover the behaviour of early versions of Black with the option
+`--skip-magic-trailing-comma` / `-C`.
+
 ### r"strings" and R"strings"
 
 _Black_ normalizes string quotes as well as string prefixes, making them lowercase. One

src/black/__init__.py

@@ -256,6 +256,7 @@ class Mode:
     target_versions: Set[TargetVersion] = field(default_factory=set)
     line_length: int = DEFAULT_LINE_LENGTH
     string_normalization: bool = True
+    magic_trailing_comma: bool = True
     experimental_string_processing: bool = False
     is_pyi: bool = False
 
@@ -393,6 +394,12 @@ def target_version_option_callback(
     is_flag=True,
     help="Don't normalize string quotes or prefixes.",
 )
+@click.option(
+    "-C",
+    "--skip-magic-trailing-comma",
+    is_flag=True,
+    help="Don't use trailing commas as a reason to split lines.",
+)
 @click.option(
     "--experimental-string-processing",
     is_flag=True,
@@ -520,6 +527,7 @@ def main(
     fast: bool,
     pyi: bool,
     skip_string_normalization: bool,
+    skip_magic_trailing_comma: bool,
     experimental_string_processing: bool,
     quiet: bool,
     verbose: bool,
@@ -542,6 +550,7 @@ def main(
         line_length=line_length,
         is_pyi=pyi,
         string_normalization=not skip_string_normalization,
+        magic_trailing_comma=not skip_magic_trailing_comma,
         experimental_string_processing=experimental_string_processing,
     )
     if config and verbose:
@@ -1018,13 +1027,12 @@ def f(
         versions = detect_target_versions(src_node)
     normalize_fmt_off(src_node)
     lines = LineGenerator(
+        mode=mode,
         remove_u_prefix="unicode_literals" in future_imports
         or supports_feature(versions, Feature.UNICODE_LITERALS),
-        is_pyi=mode.is_pyi,
-        normalize_strings=mode.string_normalization,
     )
     elt = EmptyLineTracker(is_pyi=mode.is_pyi)
-    empty_line = Line()
+    empty_line = Line(mode=mode)
     after = 0
     split_line_features = {
         feature
@@ -1460,6 +1468,7 @@ def get_open_lsqb(self) -> Optional[Leaf]:
 class Line:
     """Holds leaves and comments. Can be printed with `str(line)`."""
 
+    mode: Mode
     depth: int = 0
     leaves: List[Leaf] = field(default_factory=list)
     # keys ordered like `leaves`
@@ -1492,8 +1501,11 @@ def append(self, leaf: Leaf, preformatted: bool = False) -> None:
             )
         if self.inside_brackets or not preformatted:
             self.bracket_tracker.mark(leaf)
-            if self.maybe_should_explode(leaf):
-                self.should_explode = True
+            if self.mode.magic_trailing_comma:
+                if self.has_magic_trailing_comma(leaf):
+                    self.should_explode = True
+            elif self.has_magic_trailing_comma(leaf, ensure_removable=True):
+                self.remove_trailing_comma()
         if not self.append_comment(leaf):
             self.leaves.append(leaf)
 
@@ -1669,10 +1681,14 @@ def contains_unsplittable_type_ignore(self) -> bool:
     def contains_multiline_strings(self) -> bool:
         return any(is_multiline_string(leaf) for leaf in self.leaves)
 
-    def maybe_should_explode(self, closing: Leaf) -> bool:
-        """Return True if this line should explode (always be split), that is when:
-        - there's a trailing comma here; and
-        - it's not a one-tuple.
+    def has_magic_trailing_comma(
+        self, closing: Leaf, ensure_removable: bool = False
+    ) -> bool:
+        """Return True if we have a magic trailing comma, that is when:
+        - there's a trailing comma here
+        - it's not a one-tuple
+        Additionally, if ensure_removable:
+        - it's not from square bracket indexing
         """
         if not (
             closing.type in CLOSING_BRACKETS
@@ -1681,9 +1697,15 @@ def maybe_should_explode(self, closing: Leaf) -> bool:
         ):
             return False
 
-        if closing.type in {token.RBRACE, token.RSQB}:
+        if closing.type == token.RBRACE:
             return True
 
+        if closing.type == token.RSQB:
+            if not ensure_removable:
+                return True
+            comma = self.leaves[-1]
+            return bool(comma.parent and comma.parent.type == syms.listmaker)
+
         if self.is_import:
             return True
 
@@ -1761,6 +1783,7 @@ def is_complex_subscript(self, leaf: Leaf) -> bool:
 
     def clone(self) -> "Line":
         return Line(
+            mode=self.mode,
             depth=self.depth,
             inside_brackets=self.inside_brackets,
             should_explode=self.should_explode,
@@ -1919,10 +1942,9 @@ class LineGenerator(Visitor[Line]):
     in ways that will no longer stringify to valid Python code on the tree.
     """
 
-    is_pyi: bool = False
-    normalize_strings: bool = True
-    current_line: Line = field(default_factory=Line)
+    mode: Mode
     remove_u_prefix: bool = False
+    current_line: Line = field(init=False)
 
     def line(self, indent: int = 0) -> Iterator[Line]:
         """Generate a line.
@@ -1937,7 +1959,7 @@ def line(self, indent: int = 0) -> Iterator[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 = Line(mode=self.mode, depth=complete_line.depth + indent)
         yield complete_line
 
     def visit_default(self, node: LN) -> Iterator[Line]:
@@ -1961,7 +1983,7 @@ def visit_default(self, node: LN) -> Iterator[Line]:
                     yield from self.line()
 
             normalize_prefix(node, inside_brackets=any_open_brackets)
-            if self.normalize_strings and node.type == token.STRING:
+            if self.mode.string_normalization and node.type == token.STRING:
                 normalize_string_prefix(node, remove_u_prefix=self.remove_u_prefix)
                 normalize_string_quotes(node)
             if node.type == token.NUMBER:
@@ -2013,7 +2035,7 @@ def visit_stmt(
 
     def visit_suite(self, node: Node) -> Iterator[Line]:
         """Visit a suite."""
-        if self.is_pyi and is_stub_suite(node):
+        if self.mode.is_pyi and is_stub_suite(node):
             yield from self.visit(node.children[2])
         else:
             yield from self.visit_default(node)
@@ -2022,15 +2044,19 @@ def visit_simple_stmt(self, node: Node) -> Iterator[Line]:
         """Visit a statement without nested statements."""
         is_suite_like = node.parent and node.parent.type in STATEMENT
         if is_suite_like:
-            if self.is_pyi and is_stub_body(node):
+            if self.mode.is_pyi and is_stub_body(node):
                 yield from self.visit_default(node)
             else:
                 yield from self.line(+1)
                 yield from self.visit_default(node)
                 yield from self.line(-1)
 
         else:
-            if not self.is_pyi or not node.parent or not is_stub_suite(node.parent):
+            if (
+                not self.mode.is_pyi
+                or not node.parent
+                or not is_stub_suite(node.parent)
+            ):
                 yield from self.line()
             yield from self.visit_default(node)
 
@@ -2106,6 +2132,8 @@ def visit_STRING(self, leaf: Leaf) -> Iterator[Line]:
 
     def __post_init__(self) -> None:
         """You are in a twisty little maze of passages."""
+        self.current_line = Line(mode=self.mode)
+
         v = self.visit_stmt
         Ø: Set[str] = set()
         self.visit_assert_stmt = partial(v, keywords={"assert"}, parens={"assert", ","})
@@ -4337,6 +4365,7 @@ def do_transform(self, line: Line, string_idx: int) -> Iterator[TResult[Line]]:
         # `StringSplitter` will break it down further if necessary.
         string_value = LL[string_idx].value
         string_line = Line(
+            mode=line.mode,
             depth=line.depth + 1,
             inside_brackets=True,
             should_explode=line.should_explode,
@@ -4930,7 +4959,7 @@ def bracket_split_build_line(
     If `is_body` is True, the result line is one-indented inside brackets and as such
     has its first leaf's prefix normalized and a trailing comma added when expected.
     """
-    result = Line(depth=original.depth)
+    result = Line(mode=original.mode, depth=original.depth)
     if is_body:
         result.inside_brackets = True
         result.depth += 1
@@ -5002,7 +5031,9 @@ def delimiter_split(line: Line, features: Collection[Feature] = ()) -> Iterator[
         if bt.delimiter_count_with_priority(delimiter_priority) == 1:
             raise CannotSplit("Splitting a single attribute from its owner looks wrong")
 
-    current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)
+    current_line = Line(
+        mode=line.mode, depth=line.depth, inside_brackets=line.inside_brackets
+    )
     lowest_depth = sys.maxsize
     trailing_comma_safe = True
 
@@ -5014,7 +5045,9 @@ def append_to_line(leaf: Leaf) -> Iterator[Line]:
         except ValueError:
             yield current_line
 
-            current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)
+            current_line = Line(
+                mode=line.mode, depth=line.depth, inside_brackets=line.inside_brackets
+            )
             current_line.append(leaf)
 
     for leaf in line.leaves:
@@ -5038,7 +5071,9 @@ def append_to_line(leaf: Leaf) -> Iterator[Line]:
         if leaf_priority == delimiter_priority:
             yield current_line
 
-            current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)
+            current_line = Line(
+                mode=line.mode, depth=line.depth, inside_brackets=line.inside_brackets
+            )
     if current_line:
         if (
             trailing_comma_safe
@@ -5059,7 +5094,9 @@ def standalone_comment_split(
     if not line.contains_standalone_comments(0):
         raise CannotSplit("Line does not have any standalone comments")
 
-    current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)
+    current_line = Line(
+        mode=line.mode, depth=line.depth, inside_brackets=line.inside_brackets
+    )
 
     def append_to_line(leaf: Leaf) -> Iterator[Line]:
         """Append `leaf` to current line or to new line if appending impossible."""
@@ -5069,7 +5106,9 @@ def append_to_line(leaf: Leaf) -> Iterator[Line]:
         except ValueError:
             yield current_line
 
-            current_line = Line(depth=line.depth, inside_brackets=line.inside_brackets)
+            current_line = Line(
+                line.mode, depth=line.depth, inside_brackets=line.inside_brackets
+            )
             current_line.append(leaf)
 
     for leaf in line.leaves:
@@ -5754,7 +5793,7 @@ def should_split_body_explode(line: Line, opening_bracket: Leaf) -> bool:
         return False
 
     return max_priority == COMMA_PRIORITY and (
-        trailing_comma
+        (line.mode.magic_trailing_comma and trailing_comma)
         # always explode imports
         or opening_bracket.parent.type in {syms.atom, syms.import_from}
     )

src/blackd/__init__.py

@@ -32,6 +32,7 @@
 LINE_LENGTH_HEADER = "X-Line-Length"
 PYTHON_VARIANT_HEADER = "X-Python-Variant"
 SKIP_STRING_NORMALIZATION_HEADER = "X-Skip-String-Normalization"
+SKIP_MAGIC_TRAILING_COMMA = "X-Skip-Magic-Trailing-Comma"
 FAST_OR_SAFE_HEADER = "X-Fast-Or-Safe"
 DIFF_HEADER = "X-Diff"
 
@@ -40,6 +41,7 @@
     LINE_LENGTH_HEADER,
     PYTHON_VARIANT_HEADER,
     SKIP_STRING_NORMALIZATION_HEADER,
+    SKIP_MAGIC_TRAILING_COMMA,
     FAST_OR_SAFE_HEADER,
     DIFF_HEADER,
 ]
@@ -114,6 +116,9 @@ async def handle(request: web.Request, executor: Executor) -> web.Response:
         skip_string_normalization = bool(
             request.headers.get(SKIP_STRING_NORMALIZATION_HEADER, False)
         )
+        skip_magic_trailing_comma = bool(
+            request.headers.get(SKIP_MAGIC_TRAILING_COMMA, False)
+        )
         fast = False
         if request.headers.get(FAST_OR_SAFE_HEADER, "safe") == "fast":
             fast = True
@@ -122,6 +127,7 @@ async def handle(request: web.Request, executor: Executor) -> web.Response:
             is_pyi=pyi,
             line_length=line_length,
             string_normalization=not skip_string_normalization,
+            magic_trailing_comma=not skip_magic_trailing_comma,
         )
         req_bytes = await request.content.read()
         charset = request.charset if request.charset is not None else "utf8"