Insert trailing comma when function breaks with single argument

crates/ruff_python_formatter/src/other/parameters.rs

@@ -252,6 +252,19 @@ impl FormatNodeRule<Parameters> for FormatParameters {
             let mut f = WithNodeLevel::new(NodeLevel::ParenthesizedExpression, f);
             // No parameters, format any dangling comments between `()`
             write!(f, [empty_parenthesized("(", dangling, ")")])
+        } else if num_parameters == 1 {
+            // If we have a single argument, avoid the inner group, to ensure that we insert a
+            // trailing comma if the outer group breaks.
+            let mut f = WithNodeLevel::new(NodeLevel::ParenthesizedExpression, f);
+            write!(
+                f,
+                [
+                    token("("),
+                    dangling_open_parenthesis_comments(parenthesis_dangling),
+                    soft_block_indent(&format_inner),
+                    token(")")
+                ]
+            )
         } else {
             // Intentionally avoid `parenthesized`, which groups the entire formatted contents.
             // We want parameters to be grouped alongside return types, one level up, so we

crates/ruff_python_formatter/tests/snapshots/black_compatibility@cases__funcdef_return_type_trailing_comma.py.snap

@@ -179,13 +179,7 @@ def SimplePyFn(
      p,
      q,
  ]:
-@@ -63,16 +67,18 @@
- # long function definition, return type is longer
- # this should maybe split on rhs?
- def aaaaaaaaaaaaaaaaa(
--    bbbbbbbbbbbbbbbbbb,
-+    bbbbbbbbbbbbbbbbbb
- ) -> list[Ccccccccccccccccccccccccccccccccccccccccccccccccccc, Dddddd]: ...
+@@ -68,11 +72,13 @@
  
  
  # long return type, no param list
@@ -204,25 +198,19 @@ def SimplePyFn(
  
  
  # long function name, no param list, no return value
-@@ -93,12 +99,16 @@
+@@ -93,7 +99,11 @@
  
  
  # unskippable type hint (??)
 -def foo(a) -> list[aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa, aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa]:  # type: ignore
 +def foo(
-+    a
++    a,
 +) -> list[
 +    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa, aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
 +]:  # type: ignore
      pass
  
  
- def foo(
--    a,
-+    a
- ) -> list[
-     aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa, aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
- ]:  # abpedeifnore
 @@ -112,7 +122,13 @@
  
  
@@ -333,7 +321,7 @@ def aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa(
 # long function definition, return type is longer
 # this should maybe split on rhs?
 def aaaaaaaaaaaaaaaaa(
-    bbbbbbbbbbbbbbbbbb
+    bbbbbbbbbbbbbbbbbb,
 ) -> list[Ccccccccccccccccccccccccccccccccccccccccccccccccccc, Dddddd]: ...
 
 
@@ -366,15 +354,15 @@ def thiiiiiiiiiiiiiiiiiis_iiiiiiiiiiiiiiiiiiiiiiiiiiiiiis_veeeeeeeeeeeeeeeeeeeee
 
 # unskippable type hint (??)
 def foo(
-    a
+    a,
 ) -> list[
     aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa, aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
 ]:  # type: ignore
     pass
 
 
 def foo(
-    a
+    a,
 ) -> list[
     aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa, aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
 ]:  # abpedeifnore

crates/ruff_python_formatter/tests/snapshots/black_compatibility@cases__preview_return_annotation_brackets_string.py.snap

@@ -19,7 +19,7 @@ def frobnicate(a) -> "ThisIsTrulyUnreasonablyExtremelyLongClassName | list[ThisI
 ```diff
 --- Black
 +++ Ruff
-@@ -1,13 +1,12 @@
+@@ -1,7 +1,6 @@
  # Long string example
  def frobnicate() -> (
 -    "ThisIsTrulyUnreasonablyExtremelyLongClassName |"
@@ -28,13 +28,6 @@ def frobnicate(a) -> "ThisIsTrulyUnreasonablyExtremelyLongClassName | list[ThisI
  ):
      pass
  
- 
- # splitting the string breaks if there's any parameters
- def frobnicate(
--    a,
-+    a
- ) -> "ThisIsTrulyUnreasonablyExtremelyLongClassName | list[ThisIsTrulyUnreasonablyExtremelyLongClassName]":
-     pass
 ```
 
 ## Ruff Output
@@ -49,7 +42,7 @@ def frobnicate() -> (
 
 # splitting the string breaks if there's any parameters
 def frobnicate(
-    a
+    a,
 ) -> "ThisIsTrulyUnreasonablyExtremelyLongClassName | list[ThisIsTrulyUnreasonablyExtremelyLongClassName]":
     pass
 ```

crates/ruff_python_formatter/tests/snapshots/black_compatibility@cases__return_annotation_brackets.py.snap

@@ -111,7 +111,7 @@ def foo(a,b) -> tuple[int, int, int,]:
  # Don't lose the comments
 -def double(a: int) -> int:  # Hello
 +def double(
-+    a: int
++    a: int,
 +) -> (  # Hello
 +    int
 +):
@@ -120,7 +120,7 @@ def foo(a,b) -> tuple[int, int, int,]:
  
 -def double(a: int) -> int:  # Hello
 +def double(
-+    a: int
++    a: int,
 +) -> (
 +    int  # Hello
 +):
@@ -168,15 +168,15 @@ def double(a: int) -> int:
 
 # Don't lose the comments
 def double(
-    a: int
+    a: int,
 ) -> (  # Hello
     int
 ):
     return 2 * a
 
 
 def double(
-    a: int
+    a: int,
 ) -> (
     int  # Hello
 ):

crates/ruff_python_formatter/tests/snapshots/format@parentheses__opening_parentheses_comment_value.py.snap

@@ -253,7 +253,7 @@ except (  # d 9
 
 
 def e1(  # e 1
-    x
+    x,
 ):
     pass
 

crates/ruff_python_formatter/tests/snapshots/format@statement__function.py.snap

@@ -928,7 +928,7 @@ def f(  # first
 
 
 def f(  # first
-    a
+    a,
 ):  # second
     ...