From 8bf04549ffd276a1bad6eb110e66e6557ee630d9 Mon Sep 17 00:00:00 2001
From: cobalt <61329810+RedGuy12@users.noreply.github.com>
Date: Sat, 27 Jan 2024 15:55:22 -0600
Subject: [PATCH] Consistently add trailing comma on typed parameters (#4164)

Signed-off-by: RedGuy12 <61329810+RedGuy12@users.noreply.github.com>
Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
---
 CHANGES.md                                    |  2 ++
 docs/the_black_code_style/future_style.md     |  2 ++
 src/black/files.py                            |  2 +-
 src/black/linegen.py                          | 10 +++++++-
 src/black/mode.py                             |  1 +
 src/blib2to3/pgen2/parse.py                   |  2 +-
 .../data/cases/typed_params_trailing_comma.py | 24 +++++++++++++++++++
 7 files changed, 40 insertions(+), 3 deletions(-)
 create mode 100644 tests/data/cases/typed_params_trailing_comma.py

diff --git a/CHANGES.md b/CHANGES.md
index e4240eacfca..6278aed77d8 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -14,6 +14,8 @@
 
 <!-- Changes that affect Black's preview style -->
 
+- Consistently add trailing comma on typed parameters (#4164)
+
 ### Configuration
 
 <!-- Changes to how Black can be configured -->
diff --git a/docs/the_black_code_style/future_style.md b/docs/the_black_code_style/future_style.md
index 1cdd25fdb7c..d5faae36911 100644
--- a/docs/the_black_code_style/future_style.md
+++ b/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)=
 
diff --git a/src/black/files.py b/src/black/files.py
index 65951efdbe8..1eb8745572b 100644
--- a/src/black/files.py
+++ b/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.
 
diff --git a/src/black/linegen.py b/src/black/linegen.py
index a276805f2fe..c74ff9c0b4b 100644
--- a/src/black/linegen.py
+++ b/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 ...
diff --git a/src/black/mode.py b/src/black/mode.py
index 128d2b9f108..22352e7c6a8 100644
--- a/src/black/mode.py
+++ b/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] = {
diff --git a/src/blib2to3/pgen2/parse.py b/src/blib2to3/pgen2/parse.py
index ad51a3dad08..ad1d795b51a 100644
--- a/src/blib2to3/pgen2/parse.py
+++ b/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]
diff --git a/tests/data/cases/typed_params_trailing_comma.py b/tests/data/cases/typed_params_trailing_comma.py
new file mode 100644
index 00000000000..a53b908b18b
--- /dev/null
+++ b/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