Fix an issue where extra empty lines are added. (psf#3470)

CHANGES.md

@@ -23,6 +23,8 @@
   regular and f-strings start with an empty span (#3463)
 - Fix a crash in preview advanced string processing where a standalone comment is placed
   before a dict's value (#3469)
+- Fix an issue where extra empty lines are added when a decorator has `# fmt: skip`
+  applied or there is a standalone comment between decorators (#3470)
 - Do not put the closing quotes in a docstring on a separate line, even if the line is
   too long (#3430)
 - Long values in dict literals are now wrapped in parentheses; correspondingly

src/black/lines.py

@@ -520,7 +520,8 @@ def maybe_empty_lines(self, current_line: Line) -> LinesBlock:
                 and (self.semantic_leading_comment is None or before)
             ):
                 self.semantic_leading_comment = block
-        elif not current_line.is_decorator:
+        # `or before` means this decorator already has an empty line before
+        elif not current_line.is_decorator or before:
             self.semantic_leading_comment = None
 
         self.previous_line = current_line

tests/data/preview/comments9.py

@@ -114,6 +114,31 @@ def first_method(self):
         pass
 
 
+# Regression test for https://github.com/psf/black/issues/3454.
+def foo():
+    pass
+    # Trailing comment that belongs to this function
+
+
+@decorator1
+@decorator2  # fmt: skip
+def bar():
+    pass
+
+
+# Regression test for https://github.com/psf/black/issues/3454.
+def foo():
+    pass
+    # Trailing comment that belongs to this function.
+    # NOTE this comment only has one empty line below, and the formatter
+    # should enforce two blank lines.
+
+@decorator1
+# A standalone comment
+def bar():
+    pass
+
+
 # output
 
 
@@ -252,3 +277,29 @@ class MyClass:
     # More comments.
     def first_method(self):
         pass
+
+
+# Regression test for https://github.com/psf/black/issues/3454.
+def foo():
+    pass
+    # Trailing comment that belongs to this function
+
+
+@decorator1
+@decorator2  # fmt: skip
+def bar():
+    pass
+
+
+# Regression test for https://github.com/psf/black/issues/3454.
+def foo():
+    pass
+    # Trailing comment that belongs to this function.
+    # NOTE this comment only has one empty line below, and the formatter
+    # should enforce two blank lines.
+
+
+@decorator1
+# A standalone comment
+def bar():
+    pass