Fix empty line handling when formatting typing stubs

Black used to erroneously remove all empty lines between non-function
code and decorators when formatting typing stubs. Now a single empty
line is enforced.

I chose for putting empty lines around decorated classes that have empty
bodies since removing empty lines around such classes would cause a
formatting issue that seems to be impossible to fix.

For example:

```
class A: ...
@some_decorator
class B: ...
class C: ...
class D: ...

@some_other_decorator
def foo(): -> None: ...
```

It is easy to enforce no empty lines between class A, B, and C.
Just return 0, 0 for a line that is a decorator and precedes an stub
class. Fortunately before this commit, empty lines after that class
would be removed already.

Now let's look at the empty line between class D and function foo. In
this case, there should be an empty line there since it's class code next
to function code. The problem is that when deciding to add X empty lines
before a decorator, you can't tell whether it's before a class or a
function. If the decorator is before a function, then an empty line
is needed, while no empty lines are needed when the decorator is
before a class.

So even though I personally prefer no empty lines around decorated
classes, I had to go the other way surrounding decorated classes with
empty lines.

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 non-function code and decorators
+  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 non-function code and decorators
+  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

@@ -1824,7 +1824,11 @@ def _maybe_empty_lines_for_class_or_def(
             return 0, 0
 
         if self.previous_line.is_decorator:
-            return 0, 0
+            if self.is_pyi and current_line.is_stub_class:
+                # Insert an empty line after a decorated stub class
+                return 0, 1
+            else:
+                return 0, 0
 
         if self.previous_line.depth < current_line.depth and (
             self.previous_line.is_class or self.previous_line.is_def
@@ -1847,8 +1851,11 @@ 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 +1,65 @@
-def f(): ...
+from typing import Union
+
+@bird
+def zoo(): ...
+
+class A: ...
+@bar
+class B:
+    def BMethod(self) -> None: ...
+    @overload
+    def BMethod(self, arg : List[str]) -> None: ...
+
+class C: ...
+@hmm
+class D: ...
+class E: ...
+
+@baz
+def foo() -> None:
+    ...
+
+class F (A , C): ...
+def spam() -> None: ...
+
+@overload
+def spam(arg: str) -> str: ...
+
+var = 1
+
+def eggs() -> Union[str, int]: ...
 
-def g(): ...
 # output
-def f(): ...
-def g(): ...
+
+from typing import Union
+
+@bird
+def zoo(): ...
+
+class A: ...
+
+@bar
+class B:
+    def BMethod(self) -> None: ...
+    @overload
+    def BMethod(self, arg: List[str]) -> None: ...
+
+class C: ...
+
+@hmm
+class D: ...
+
+class E: ...
+
+@baz
+def foo() -> None: ...
+
+class F(A, C): ...
+
+def spam() -> None: ...
+@overload
+def spam(arg: str) -> str: ...
+
+var = 1
+
+def eggs() -> Union[str, int]: ...

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: