respect magic trailing commas in return types

CHANGES.md

@@ -18,6 +18,7 @@
 
 - Long type hints are now wrapped in parentheses and properly indented when split across
   multiple lines (#3899)
+- Magic trailing commas are now respected in return types. (#3916)
 
 ### Configuration
 

src/black/linegen.py

@@ -573,7 +573,7 @@ def transform_line(
             transformers = [string_merge, string_paren_strip]
         else:
             transformers = []
-    elif line.is_def:
+    elif line.is_def and not should_split_funcdef_with_rhs(line, mode):
         transformers = [left_hand_split]
     else:
 
@@ -652,6 +652,40 @@ def _rhs(
         yield line
 
 
+def should_split_funcdef_with_rhs(line: Line, mode: Mode) -> bool:
+    """If a funcdef has a magic trailing comma in the return type, then we should first
+    split the line with rhs to respect the comma.
+    """
+    if Preview.respect_magic_trailing_comma_in_return_type not in mode:
+        return False
+
+    return_type_leaves: List[Leaf] = []
+    in_return_type = False
+
+    for leaf in line.leaves:
+        if leaf.type == token.COLON:
+            in_return_type = False
+        if in_return_type:
+            return_type_leaves.append(leaf)
+        if leaf.type == token.RARROW:
+            in_return_type = True
+
+    # using `bracket_split_build_line` will mess with whitespace, so we duplicate a
+    # couple lines from it.
+    result = Line(mode=line.mode, depth=line.depth)
+    leaves_to_track = get_leaves_inside_matching_brackets(return_type_leaves)
+    for leaf in return_type_leaves:
+        result.append(
+            leaf,
+            preformatted=True,
+            track_bracket=id(leaf) in leaves_to_track,
+        )
+
+    # we could also return true if the line is too long, and the return type is longer
+    # than the param list. Or if `should_split_rhs` returns True.
+    return result.magic_trailing_comma is not None
+
+
 class _BracketSplitComponent(Enum):
     head = auto()
     body = auto()

src/black/mode.py

@@ -181,6 +181,7 @@ class Preview(Enum):
     string_processing = auto()
     parenthesize_conditional_expressions = auto()
     parenthesize_long_type_hints = auto()
+    respect_magic_trailing_comma_in_return_type = auto()
     skip_magic_trailing_comma_in_subscript = auto()
     wrap_long_dict_values_in_parens = auto()
     wrap_multiple_context_managers_in_parens = auto()

tests/data/preview/return_annotation_brackets_string.py

@@ -2,6 +2,10 @@
 def frobnicate() -> "ThisIsTrulyUnreasonablyExtremelyLongClassName | list[ThisIsTrulyUnreasonablyExtremelyLongClassName]":
     pass
 
+# splitting the string breaks if there's any parameters
+def frobnicate(a) -> "ThisIsTrulyUnreasonablyExtremelyLongClassName | list[ThisIsTrulyUnreasonablyExtremelyLongClassName]":
+    pass
+
 # output
 
 # Long string example
@@ -10,3 +14,10 @@ def frobnicate() -> (
     " list[ThisIsTrulyUnreasonablyExtremelyLongClassName]"
 ):
     pass
+
+
+# splitting the string breaks if there's any parameters
+def frobnicate(
+    a,
+) -> "ThisIsTrulyUnreasonablyExtremelyLongClassName | list[ThisIsTrulyUnreasonablyExtremelyLongClassName]":
+    pass

tests/data/preview_py_310/funcdef_return_type_trailing_comma.py

@@ -0,0 +1,300 @@
+# normal, short, function definition
+def foo(a, b) -> tuple[int, float]: ...
+
+
+# normal, short, function definition w/o return type
+def foo(a, b): ...
+
+
+# no splitting
+def foo(a: A, b: B) -> list[p, q]:
+    pass
+
+
+# magic trailing comma in param list
+def foo(a, b,): ...
+
+
+# magic trailing comma in nested params in param list
+def foo(a, b: tuple[int, float,]): ...
+
+
+# magic trailing comma in return type, no params
+def a() -> tuple[
+    a,
+    b,
+]: ...
+
+
+# magic trailing comma in return type, params
+def foo(a: A, b: B) -> list[
+    p,
+    q,
+]:
+    pass
+
+
+# magic trailing comma in param list and in return type
+def foo(
+    a: a,
+    b: b,
+) -> list[
+    a,
+    a,
+]:
+    pass
+
+
+# long function definition, param list is longer
+def aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa(
+    bbbbbbbbbbbbbbbbbb,
+) -> cccccccccccccccccccccccccccccc: ...
+
+
+# long function definition, return type is longer
+# this should maybe split on rhs?
+def aaaaaaaaaaaaaaaaa(bbbbbbbbbbbbbbbbbb) -> list[
+    Ccccccccccccccccccccccccccccccccccccccccccccccccccc, Dddddd
+]: ...
+
+
+# long return type, no param list
+def foo() -> list[
+    Loooooooooooooooooooooooooooooooooooong,
+    Loooooooooooooooooooong,
+    Looooooooooooong,
+]: ...
+
+
+# long function name, no param list, no return value
+def thiiiiiiiiiiiiiiiiiis_iiiiiiiiiiiiiiiiiiiiiiiiiiiiiis_veeeeeeeeeeeeeeeeeeeeeeery_looooooong():
+    pass
+
+
+# long function name, no param list
+def thiiiiiiiiiiiiiiiiiis_iiiiiiiiiiiiiiiiiiiiiiiiiiiiiis_veeeeeeeeeeeeeeeeeeeeeeery_looooooong() -> (
+    list[int, float]
+): ...
+
+
+# long function name, no return value
+def thiiiiiiiiiiiiiiiiiis_iiiiiiiiiiiiiiiiiiiiiiiiiiiiiis_veeeeeeeeeeeeeeeeeeeeeeery_looooooong(
+    a, b
+): ...
+
+
+# unskippable type hint (??)
+def foo(a) -> list[aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa, aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa]:  # type: ignore
+    pass
+
+
+def foo(a) -> list[
+    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa, aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+]:  # abpedeifnore
+    pass
+
+def foo(a, b: list[Bad],): ... # type: ignore
+
+# don't lose any comments (no magic)
+def foo( # 1
+    a, # 2
+    b) -> list[ # 3
+               a, # 4
+               b]: # 5
+        ... # 6
+
+
+# don't lose any comments (param list magic)
+def foo( # 1
+    a, # 2
+    b,) -> list[ # 3
+               a, # 4
+               b]: # 5
+        ... # 6
+
+
+# don't lose any comments (return type magic)
+def foo( # 1
+    a, # 2
+    b) -> list[ # 3
+               a, # 4
+               b,]: # 5
+        ... # 6
+
+
+# don't lose any comments (both magic)
+def foo( # 1
+    a, # 2
+    b,) -> list[ # 3
+               a, # 4
+               b,]: # 5
+        ... # 6
+
+# real life example
+def SimplePyFn(
+    context: hl.GeneratorContext,
+    buffer_input: Buffer[UInt8, 2],
+    func_input: Buffer[Int32, 2],
+    float_arg: Scalar[Float32],
+    offset: int = 0,
+) -> tuple[
+    Buffer[UInt8, 2],
+    Buffer[UInt8, 2],
+]: ...
+# output
+# normal, short, function definition
+def foo(a, b) -> tuple[int, float]: ...
+
+
+# normal, short, function definition w/o return type
+def foo(a, b): ...
+
+
+# no splitting
+def foo(a: A, b: B) -> list[p, q]:
+    pass
+
+
+# magic trailing comma in param list
+def foo(
+    a,
+    b,
+): ...
+
+
+# magic trailing comma in nested params in param list
+def foo(
+    a,
+    b: tuple[
+        int,
+        float,
+    ],
+): ...
+
+
+# magic trailing comma in return type, no params
+def a() -> tuple[
+    a,
+    b,
+]: ...
+
+
+# magic trailing comma in return type, params
+def foo(a: A, b: B) -> list[
+    p,
+    q,
+]:
+    pass
+
+
+# magic trailing comma in param list and in return type
+def foo(
+    a: a,
+    b: b,
+) -> list[
+    a,
+    a,
+]:
+    pass
+
+
+# long function definition, param list is longer
+def aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa(
+    bbbbbbbbbbbbbbbbbb,
+) -> cccccccccccccccccccccccccccccc: ...
+
+
+# long function definition, return type is longer
+# this should maybe split on rhs?
+def aaaaaaaaaaaaaaaaa(
+    bbbbbbbbbbbbbbbbbb,
+) -> list[Ccccccccccccccccccccccccccccccccccccccccccccccccccc, Dddddd]: ...
+
+
+# long return type, no param list
+def foo() -> list[
+    Loooooooooooooooooooooooooooooooooooong,
+    Loooooooooooooooooooong,
+    Looooooooooooong,
+]: ...
+
+
+# long function name, no param list, no return value
+def thiiiiiiiiiiiiiiiiiis_iiiiiiiiiiiiiiiiiiiiiiiiiiiiiis_veeeeeeeeeeeeeeeeeeeeeeery_looooooong():
+    pass
+
+
+# long function name, no param list
+def thiiiiiiiiiiiiiiiiiis_iiiiiiiiiiiiiiiiiiiiiiiiiiiiiis_veeeeeeeeeeeeeeeeeeeeeeery_looooooong() -> (
+    list[int, float]
+): ...
+
+
+# long function name, no return value
+def thiiiiiiiiiiiiiiiiiis_iiiiiiiiiiiiiiiiiiiiiiiiiiiiiis_veeeeeeeeeeeeeeeeeeeeeeery_looooooong(
+    a, b
+): ...
+
+
+# unskippable type hint (??)
+def foo(a) -> list[aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa, aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa]:  # type: ignore
+    pass
+
+
+def foo(
+    a,
+) -> list[
+    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa, aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+]:  # abpedeifnore
+    pass
+
+
+def foo(
+    a,
+    b: list[Bad],
+): ...  # type: ignore
+
+
+# don't lose any comments (no magic)
+def foo(a, b) -> list[a, b]:  # 1  # 2  # 3  # 4  # 5
+    ...  # 6
+
+
+# don't lose any comments (param list magic)
+def foo(  # 1
+    a,  # 2
+    b,
+) -> list[a, b]:  # 3  # 4  # 5
+    ...  # 6
+
+
+# don't lose any comments (return type magic)
+def foo(a, b) -> list[  # 1  # 2  # 3
+    a,  # 4
+    b,
+]:  # 5
+    ...  # 6
+
+
+# don't lose any comments (both magic)
+def foo(  # 1
+    a,  # 2
+    b,
+) -> list[  # 3
+    a,  # 4
+    b,
+]:  # 5
+    ...  # 6
+
+
+# real life example
+def SimplePyFn(
+    context: hl.GeneratorContext,
+    buffer_input: Buffer[UInt8, 2],
+    func_input: Buffer[Int32, 2],
+    float_arg: Scalar[Float32],
+    offset: int = 0,
+) -> tuple[
+    Buffer[UInt8, 2],
+    Buffer[UInt8, 2],
+]: ...