Add blank line after constants in stub file (#360)

Fixes #340

black.py

@@ -1390,10 +1390,31 @@ def _maybe_empty_lines(self, current_line: Line) -> Tuple[int, int]:
                 before = 0 if depth else 1
             else:
                 before = 1 if depth else 2
-        is_decorator = current_line.is_decorator
-        if is_decorator or current_line.is_def or current_line.is_class:
-            if not is_decorator:
-                self.previous_defs.append(depth)
+        if current_line.is_decorator or current_line.is_def or current_line.is_class:
+            return self._maybe_empty_lines_for_class_or_def(current_line, before)
+
+        if (
+            self.previous_line
+            and self.previous_line.is_import
+            and not current_line.is_import
+            and depth == self.previous_line.depth
+        ):
+            return (before or 1), 0
+
+        if (
+            self.previous_line
+            and self.previous_line.is_class
+            and current_line.is_triple_quoted_string
+        ):
+            return before, 1
+
+        return before, 0
+
+    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)
         if self.previous_line is None:
             # Don't insert empty lines before the first line in the file.
             return 0, 0
@@ -1418,9 +1439,13 @@ def _maybe_empty_lines(self, current_line: Line) -> Tuple[int, int]:
                 newlines = 1
             elif current_line.is_class or self.previous_line.is_class:
                 if current_line.is_stub_class and self.previous_line.is_stub_class:
+                    # No blank line between classes with an emty body
                     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
+                newlines = 1
             else:
                 newlines = 0
         else:
@@ -1429,23 +1454,6 @@ def _maybe_empty_lines(self, current_line: Line) -> Tuple[int, int]:
             newlines -= 1
         return newlines, 0
 
-        if (
-            self.previous_line
-            and self.previous_line.is_import
-            and not current_line.is_import
-            and depth == self.previous_line.depth
-        ):
-            return (before or 1), 0
-
-        if (
-            self.previous_line
-            and self.previous_line.is_class
-            and current_line.is_triple_quoted_string
-        ):
-            return before, 1
-
-        return before, 0
-
 
 @dataclass
 class LineGenerator(Visitor[Line]):

tests/data/stub.pyi

@@ -1,3 +1,7 @@
+X: int
+
+def f(): ...
+
 class C:
     ...
 
@@ -16,6 +20,10 @@ def g():
 def h(): ...
 
 # output
+X: int
+
+def f(): ...
+
 class C: ...
 class B: ...