fix: Consistently add trailing comma on typed parameters

CHANGES.md

@@ -112,6 +112,7 @@ release:
 - Fix crash when using a walrus in a dictionary (#4155)
 - Fix unnecessary parentheses when wrapping long dicts (#4135)
 - Stop normalizing spaces before `# fmt: skip` comments (#4146)
+- Consistently add trailing comma on typed parameters (#4164)
 
 ### Configuration
 

docs/the_black_code_style/future_style.md

@@ -26,6 +26,8 @@ Currently, the following features are included in the preview style:
   brackets ([see below](labels/hug-parens))
 - `no_normalize_fmt_skip_whitespace`: whitespace before `# fmt: skip` comments is no
   longer normalized
+- `typed_params_trailing_comma`: consistently add trailing commas to typed function
+  parameters
 
 (labels/unstable-features)=
 

src/black/files.py

@@ -131,7 +131,7 @@ def parse_pyproject_toml(path_config: str) -> Dict[str, Any]:
 
 
 def infer_target_version(
-    pyproject_toml: Dict[str, Any]
+    pyproject_toml: Dict[str, Any],
 ) -> Optional[List[TargetVersion]]:
     """Infer Black's target version from the project metadata in pyproject.toml.
 

src/black/linegen.py

@@ -48,6 +48,7 @@
     is_one_sequence_between,
     is_one_tuple,
     is_parent_function_or_class,
+    is_part_of_annotation,
     is_rpar_token,
     is_stub_body,
     is_stub_suite,
@@ -1041,7 +1042,14 @@ def bracket_split_build_line(
             no_commas = (
                 original.is_def
                 and opening_bracket.value == "("
-                and not any(leaf.type == token.COMMA for leaf in leaves)
+                and not any(
+                    leaf.type == token.COMMA
+                    and (
+                        Preview.typed_params_trailing_comma not in original.mode
+                        or not is_part_of_annotation(leaf)
+                    )
+                    for leaf in leaves
+                )
                 # In particular, don't add one within a parenthesized return annotation.
                 # Unfortunately the indicator we're in a return annotation (RARROW) may
                 # be defined directly in the parent node, the parent of the parent ...

src/black/mode.py

@@ -176,6 +176,7 @@ class Preview(Enum):
     no_normalize_fmt_skip_whitespace = auto()
     wrap_long_dict_values_in_parens = auto()
     multiline_string_handling = auto()
+    typed_params_trailing_comma = auto()
 
 
 UNSTABLE_FEATURES: Set[Preview] = {

src/blib2to3/pgen2/parse.py

@@ -50,7 +50,7 @@ def lam_sub(grammar: Grammar, node: RawNode) -> NL:
 
 
 def stack_copy(
-    stack: List[Tuple[DFAS, int, RawNode]]
+    stack: List[Tuple[DFAS, int, RawNode]],
 ) -> List[Tuple[DFAS, int, RawNode]]:
     """Nodeless stack copy."""
     return [(dfa, label, DUMMY_NODE) for dfa, label, _ in stack]

tests/data/cases/typed_params_trailing_comma.py

@@ -0,0 +1,24 @@
+# flags: --preview
+def long_function_name_goes_here(
+    x: Callable[List[int]]
+) -> Union[List[int], float, str, bytes, Tuple[int]]:
+    pass
+
+
+def long_function_name_goes_here(
+    x: Callable[[str, Any], int]
+) -> Union[List[int], float, str, bytes, Tuple[int]]:
+    pass
+
+
+# output
+def long_function_name_goes_here(
+    x: Callable[List[int]],
+) -> Union[List[int], float, str, bytes, Tuple[int]]:
+    pass
+
+
+def long_function_name_goes_here(
+    x: Callable[[str, Any], int],
+) -> Union[List[int], float, str, bytes, Tuple[int]]:
+    pass