Fix empty line handling when formatting typing stubs

Black used to erroneously remove all empty lines between functions
preceded by decorators and non-function code in pyi mode. Now a single
empty line is enforced.

CHANGES.md

@@ -7,6 +7,9 @@
 - `Black` now respects `--skip-string-normalization` when normalizing multiline
   docstring quotes (#1637)
 
+- `Black` no longer removes all empty lines between decorators and non-function code
+  when formatting typing stubs. Now `Black` enforces a single empty line. (#1646)
+
 - fixed a crash when PWD=/ on POSIX (#1631)
 
 ### 20.8b1

docs/change_log.md

@@ -9,6 +9,11 @@
 - `Black` now respects `--skip-string-normalization` when normalizing multiline
   docstring quotes (#1637)
 
+- `Black` no longer removes all empty lines between decorators and non-function code
+  when formatting typing stubs. Now `Black` enforces a single empty line. (#1646)
+
+- fixed a crash when PWD=/ on POSIX (#1631)
+
 ### 20.8b1
 
 #### _Packaging_

src/black/__init__.py

@@ -1847,8 +1847,11 @@ def _maybe_empty_lines_for_class_or_def(
                     newlines = 0
                 else:
                     newlines = 1
-            elif current_line.is_def and not self.previous_line.is_def:
-                # Blank line between a block of functions and a block of non-functions
+            elif (
+                current_line.is_def or current_line.is_decorator
+            ) and not self.previous_line.is_def:
+                # Blank line between a block of functions (maybe with preceding
+                # decorators) and a block of non-functions
                 newlines = 1
             else:
                 newlines = 0

tests/data/force_pyi.py

@@ -1,6 +1,12 @@
+import a
+
+@b
 def f(): ...
 
 def g(): ...
 # output
+import a
+
+@b
 def f(): ...
 def g(): ...

tests/test_black.py

@@ -1527,7 +1527,6 @@ def test_tricky_unicode_symbols(self) -> None:
         black.assert_stable(source, actual, DEFAULT_MODE)
 
     def test_single_file_force_pyi(self) -> None:
-        reg_mode = DEFAULT_MODE
         pyi_mode = replace(DEFAULT_MODE, is_pyi=True)
         contents, expected = read_data("force_pyi")
         with cache_dir() as workspace:
@@ -1540,9 +1539,11 @@ def test_single_file_force_pyi(self) -> None:
             # verify cache with --pyi is separate
             pyi_cache = black.read_cache(pyi_mode)
             self.assertIn(path, pyi_cache)
-            normal_cache = black.read_cache(reg_mode)
+            normal_cache = black.read_cache(DEFAULT_MODE)
             self.assertNotIn(path, normal_cache)
-        self.assertEqual(actual, expected)
+        self.assertFormatEqual(expected, actual)
+        black.assert_equivalent(contents, actual)
+        black.assert_stable(contents, actual, pyi_mode)
 
     @event_loop()
     def test_multi_file_force_pyi(self) -> None: