Stubs: preserve blank line between attributes and methods (#2736)

CHANGES.md

@@ -17,6 +17,8 @@
 - Tuple unpacking on `return` and `yield` constructs now implies 3.8+ (#2700)
 - Unparenthesized tuples on annotated assignments (e.g
   `values: Tuple[int, ...] = 1, 2, 3`) now implies 3.8+ (#2708)
+- For stubs, one blank line between class attributes and methods is now kept if there's
+  at least one pre-existing blank line (#2736)
 
 ### Packaging
 

src/black/lines.py

@@ -448,7 +448,14 @@ def _maybe_empty_lines(self, current_line: Line) -> Tuple[int, int]:
         depth = current_line.depth
         while self.previous_defs and self.previous_defs[-1] >= depth:
             if self.is_pyi:
-                before = 0 if depth else 1
+                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 depth:
+                    before = 0
+                else:
+                    before = 1
             else:
                 if depth:
                     before = 1
@@ -532,9 +539,15 @@ def _maybe_empty_lines_for_class_or_def(
             elif (
                 current_line.is_def or current_line.is_decorator
             ) and not self.previous_line.is_def:
+                if not current_line.depth:
                     # Blank line between a block of functions (maybe with preceding
                     # decorators) and a block of non-functions
                     newlines = 1
+                else:
+                    # In classes empty lines between attributes and methods should
+                    # be preserved. The +1 offset is to negate the -1 done later as
+                    # this function is indented.
+                    newlines = min(2, before + 1)
             else:
                 newlines = 0
         else:

tests/data/stub.pyi

@@ -2,32 +2,55 @@ X: int
 
 def f(): ...
 
+
+class D: 
+    ...
+
+
 class C:
     ...
 
 class B:
-    ...
+    this_lack_of_newline_should_be_kept: int
+    def b(self) -> None: ...
+
+    but_this_newline_should_also_be_kept: int
 
 class A:
+    attr: int
+    attr2: str
+
     def f(self) -> int:
         ...
 
     def g(self) -> str: ...
 
+
+
 def g():
     ...
 
 def h(): ...
 
+
 # output
 X: int
 
 def f(): ...
 
+class D: ...
 class C: ...
-class B: ...
+
+class B:
+    this_lack_of_newline_should_be_kept: int
+    def b(self) -> None: ...
+
+    but_this_newline_should_also_be_kept: int
 
 class A:
+    attr: int
+    attr2: str
+
     def f(self) -> int: ...
     def g(self) -> str: ...