fix(3.12_generics_syntax): Fixed formatting of 3.12 generics syntax

CHANGES.md

@@ -16,6 +16,7 @@
 
 - Fix bug where module docstrings would be treated as normal strings if preceeded by
   comments (#4764)
+- Fix bug where python 3.12 generics syntax split line happens weirdly (#4777)
 
 ### Configuration
 

docs/the_black_code_style/future_style.md

@@ -37,6 +37,7 @@ Currently, the following features are included in the preview style:
   normalize file newlines both from and to.
 - `fix_module_docstring_detection`: Fix module docstrings being treated as normal
   strings if preceeded by comments.
+- `fix_type_expansion_split`: Fix type expansions split in generic functions.
 
 (labels/unstable-features)=
 

src/black/linegen.py

@@ -834,7 +834,10 @@ def left_hand_split(
                 current_leaves = tail_leaves if body_leaves else head_leaves
             current_leaves.append(leaf)
             if current_leaves is head_leaves:
-                if leaf.type == leaf_type:
+                if leaf.type == leaf_type and (
+                    Preview.fix_type_expansion_split not in mode
+                    or not (leaf_type == token.LPAR and depth > 0)
+                ):
                     matching_bracket = leaf
                     current_leaves = body_leaves
         if matching_bracket and tail_leaves:

src/black/mode.py

@@ -237,6 +237,7 @@ class Preview(Enum):
     remove_parens_around_except_types = auto()
     normalize_cr_newlines = auto()
     fix_module_docstring_detection = auto()
+    fix_type_expansion_split = auto()
 
 
 UNSTABLE_FEATURES: set[Preview] = {

src/black/resources/black.schema.json

@@ -89,7 +89,8 @@
           "wrap_comprehension_in",
           "remove_parens_around_except_types",
           "normalize_cr_newlines",
-          "fix_module_docstring_detection"
+          "fix_module_docstring_detection",
+          "fix_type_expansion_split"
         ]
       },
       "description": "Enable specific features included in the `--unstable` style. Requires `--preview`. No compatibility guarantees are provided on the behavior or existence of any unstable features."

tests/data/cases/type_expansion.py

@@ -0,0 +1,60 @@
+# flags: --preview --minimum-version=3.12
+
+def f1[T: (int, str)](a,): pass
+
+def f2[T: (int, str)](a: int, b,): pass
+
+def g1[T: (int,)](a,): pass
+
+def g2[T: (int, str, bytes)](a,): pass
+
+def g3[T: ((int, str), (bytes,))](a,): pass
+
+def g4[T: (int, (str, bytes))](a,): pass
+
+def g5[T: ((int,),)](a: int, b,): pass
+
+# output
+
+def f1[T: (int, str)](
+    a,
+):
+    pass
+
+
+def f2[T: (int, str)](
+    a: int,
+    b,
+):
+    pass
+
+
+def g1[T: (int,)](
+    a,
+):
+    pass
+
+
+def g2[T: (int, str, bytes)](
+    a,
+):
+    pass
+
+
+def g3[T: ((int, str), (bytes,))](
+    a,
+):
+    pass
+
+
+def g4[T: (int, (str, bytes))](
+    a,
+):
+    pass
+
+
+def g5[T: ((int,),)](
+    a: int,
+    b,
+):
+    pass