Enforce a blank line after a nested class in stubs

CHANGES.md

@@ -18,6 +18,8 @@
   entry (#3393)
 - `with` statements that contain two context managers will be consistently wrapped in
   parentheses (#3589)
+- For stubs, enforce one blank line after a nested class with a body other than just
+  `...` (#3564)
 
 ### Configuration
 

src/black/lines.py

@@ -501,7 +501,7 @@ class EmptyLineTracker:
     mode: Mode
     previous_line: Optional[Line] = None
     previous_block: Optional[LinesBlock] = None
-    previous_defs: List[int] = field(default_factory=list)
+    previous_defs: List[Line] = field(default_factory=list)
     semantic_leading_comment: Optional[LinesBlock] = None
 
     def maybe_empty_lines(self, current_line: Line) -> LinesBlock:
@@ -557,12 +557,18 @@ def _maybe_empty_lines(self, current_line: Line) -> Tuple[int, int]:
         else:
             before = 0
         depth = current_line.depth
-        while self.previous_defs and self.previous_defs[-1] >= depth:
+        while self.previous_defs and self.previous_defs[-1].depth >= depth:
             if self.mode.is_pyi:
                 assert self.previous_line is not None
                 if depth and not current_line.is_def and self.previous_line.is_def:
                     # Empty lines between attributes and methods should be preserved.
                     before = min(1, before)
+                elif (
+                    Preview.blank_line_after_nested_stub_class in self.mode
+                    and self.previous_defs[-1].is_class
+                    and not self.previous_defs[-1].is_stub_class
+                ):
+                    before = 1
                 elif depth:
                     before = 0
                 else:
@@ -616,7 +622,7 @@ def _maybe_empty_lines_for_class_or_def(
         self, current_line: Line, before: int
     ) -> Tuple[int, int]:
         if not current_line.is_decorator:
-            self.previous_defs.append(current_line.depth)
+            self.previous_defs.append(current_line)
         if self.previous_line is None:
             # Don't insert empty lines before the first line in the file.
             return 0, 0

src/black/mode.py

@@ -154,6 +154,7 @@ class Preview(Enum):
     """Individual preview style features."""
 
     add_trailing_comma_consistently = auto()
+    blank_line_after_nested_stub_class = auto()
     hex_codes_in_unicode_sequences = auto()
     multiline_string_handling = auto()
     prefer_splitting_right_hand_side_of_assignments = auto()

tests/data/miscellaneous/nested_class_stub.pyi

@@ -0,0 +1,16 @@
+class Outer:
+    class InnerStub: ...
+    outer_attr_after_inner_stub: int
+    class Inner:
+        inner_attr: int
+    outer_attr: int
+
+# output
+class Outer:
+    class InnerStub: ...
+    outer_attr_after_inner_stub: int
+
+    class Inner:
+        inner_attr: int
+
+    outer_attr: int

tests/test_format.py

@@ -186,6 +186,12 @@ def test_stub() -> None:
     assert_format(source, expected, mode)
 
 
+def test_nested_class_stub() -> None:
+    mode = replace(DEFAULT_MODE, is_pyi=True, preview=True)
+    source, expected = read_data("miscellaneous", "nested_class_stub.pyi")
+    assert_format(source, expected, mode)
+
+
 def test_power_op_newline() -> None:
     # requires line_length=0
     source, expected = read_data("miscellaneous", "power_op_newline")