More concise formatting for dummy implementations (#3796)

CHANGES.md

@@ -14,6 +14,8 @@
 
 <!-- Changes that affect Black's preview style -->
 
+- More concise formatting for dummy implementations (#3796)
+
 ### Configuration
 
 <!-- Changes to how Black can be configured -->

src/black/linegen.py

@@ -281,7 +281,9 @@ def visit_match_case(self, node: Node) -> Iterator[Line]:
 
     def visit_suite(self, node: Node) -> Iterator[Line]:
         """Visit a suite."""
-        if self.mode.is_pyi and is_stub_suite(node):
+        if (
+            self.mode.is_pyi or Preview.dummy_implementations in self.mode
+        ) and is_stub_suite(node):
             yield from self.visit(node.children[2])
         else:
             yield from self.visit_default(node)
@@ -296,7 +298,9 @@ def visit_simple_stmt(self, node: Node) -> Iterator[Line]:
 
         is_suite_like = node.parent and node.parent.type in STATEMENT
         if is_suite_like:
-            if self.mode.is_pyi and is_stub_body(node):
+            if (
+                self.mode.is_pyi or Preview.dummy_implementations in self.mode
+            ) and is_stub_body(node):
                 yield from self.visit_default(node)
             else:
                 yield from self.line(+1)
@@ -305,7 +309,7 @@ def visit_simple_stmt(self, node: Node) -> Iterator[Line]:
 
         else:
             if (
-                not self.mode.is_pyi
+                not (self.mode.is_pyi or Preview.dummy_implementations in self.mode)
                 or not node.parent
                 or not is_stub_suite(node.parent)
             ):

src/black/lines.py

@@ -165,6 +165,13 @@ def is_def(self) -> bool:
             and second_leaf.value == "def"
         )
 
+    @property
+    def is_stub_def(self) -> bool:
+        """Is this line a function definition with a body consisting only of "..."?"""
+        return self.is_def and self.leaves[-4:] == [Leaf(token.COLON, ":")] + [
+            Leaf(token.DOT, ".") for _ in range(3)
+        ]
+
     @property
     def is_class_paren_empty(self) -> bool:
         """Is this a class with no base classes but using parentheses?
@@ -578,6 +585,8 @@ def _maybe_empty_lines(self, current_line: Line) -> Tuple[int, int]:
             first_leaf.prefix = ""
         else:
             before = 0
+
+        user_had_newline = bool(before)
         depth = current_line.depth
 
         previous_def = None
@@ -589,7 +598,7 @@ def _maybe_empty_lines(self, current_line: Line) -> Tuple[int, int]:
             if self.mode.is_pyi:
                 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)
+                    before = 1 if user_had_newline else 0
                 elif (
                     Preview.blank_line_after_nested_stub_class in self.mode
                     and previous_def.is_class
@@ -624,7 +633,9 @@ def _maybe_empty_lines(self, current_line: Line) -> Tuple[int, int]:
                     before = 2
 
         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)
+            return self._maybe_empty_lines_for_class_or_def(
+                current_line, before, user_had_newline
+            )
 
         if (
             self.previous_line
@@ -648,8 +659,8 @@ def _maybe_empty_lines(self, current_line: Line) -> Tuple[int, int]:
             return 0, 0
         return before, 0
 
-    def _maybe_empty_lines_for_class_or_def(
+    def _maybe_empty_lines_for_class_or_def(  # noqa: C901
-        self, current_line: Line, before: int
+        self, current_line: Line, before: int, user_had_newline: bool
     ) -> Tuple[int, int]:
         if not current_line.is_decorator:
             self.previous_defs.append(current_line)
@@ -715,6 +726,14 @@ def _maybe_empty_lines_for_class_or_def(
                 newlines = 0
         else:
             newlines = 1 if current_line.depth else 2
+            # If a user has left no space after a dummy implementation, don't insert
+            # new lines. This is useful for instance for @overload or Protocols.
+            if (
+                Preview.dummy_implementations in self.mode
+                and self.previous_line.is_stub_def
+                and not user_had_newline
+            ):
+                newlines = 0
         if comment_to_add_newlines is not None:
             previous_block = comment_to_add_newlines.previous_block
             if previous_block is not None:

src/black/mode.py

@@ -182,6 +182,7 @@ class Preview(Enum):
     skip_magic_trailing_comma_in_subscript = auto()
     wrap_long_dict_values_in_parens = auto()
     wrap_multiple_context_managers_in_parens = auto()
+    dummy_implementations = auto()
 
 
 class Deprecated(UserWarning):

tests/data/miscellaneous/force_py36.py

@@ -1,6 +1,6 @@
 # The input source must not contain any Py36-specific syntax (e.g. argument type
 # annotations, trailing comma after *rest) or this test becomes invalid.
-def long_function_name(argument_one, argument_two, argument_three, argument_four, argument_five, argument_six, *rest): ...
+def long_function_name(argument_one, argument_two, argument_three, argument_four, argument_five, argument_six, *rest): pass
 # output
 # The input source must not contain any Py36-specific syntax (e.g. argument type
 # annotations, trailing comma after *rest) or this test becomes invalid.
@@ -13,4 +13,4 @@ def long_function_name(
     argument_six,
     *rest,
 ):
-    ...
+    pass

tests/data/preview/comments7.py

@@ -278,8 +278,7 @@ class C:
     )
     def test_fails_invalid_post_data(
         self, pyramid_config, db_request, post_data, message
-    ):
+    ): ...
-        ...
 
 
 square = Square(4)  # type: Optional[Square]

tests/data/preview/dummy_implementations.py

@@ -0,0 +1,99 @@
+from typing import NoReturn, Protocol, Union, overload
+
+
+def dummy(a): ...
+def other(b): ...
+
+
+@overload
+def a(arg: int) -> int: ...
+@overload
+def a(arg: str) -> str: ...
+@overload
+def a(arg: object) -> NoReturn: ...
+def a(arg: Union[int, str, object]) -> Union[int, str]:
+    if not isinstance(arg, (int, str)):
+        raise TypeError
+    return arg
+
+class Proto(Protocol):
+    def foo(self, a: int) -> int:
+        ...
+
+    def bar(self, b: str) -> str: ...
+    def baz(self, c: bytes) -> str:
+        ...
+
+
+def dummy_two():
+    ...
+@dummy
+def dummy_three():
+    ...
+
+def dummy_four():
+    ...
+
+@overload
+def b(arg: int) -> int: ...
+
+@overload
+def b(arg: str) -> str: ...
+@overload
+def b(arg: object) -> NoReturn: ...
+
+def b(arg: Union[int, str, object]) -> Union[int, str]:
+    if not isinstance(arg, (int, str)):
+        raise TypeError
+    return arg
+
+# output
+
+from typing import NoReturn, Protocol, Union, overload
+
+
+def dummy(a): ...
+def other(b): ...
+
+
+@overload
+def a(arg: int) -> int: ...
+@overload
+def a(arg: str) -> str: ...
+@overload
+def a(arg: object) -> NoReturn: ...
+def a(arg: Union[int, str, object]) -> Union[int, str]:
+    if not isinstance(arg, (int, str)):
+        raise TypeError
+    return arg
+
+
+class Proto(Protocol):
+    def foo(self, a: int) -> int: ...
+
+    def bar(self, b: str) -> str: ...
+    def baz(self, c: bytes) -> str: ...
+
+
+def dummy_two(): ...
+@dummy
+def dummy_three(): ...
+
+
+def dummy_four(): ...
+
+
+@overload
+def b(arg: int) -> int: ...
+
+
+@overload
+def b(arg: str) -> str: ...
+@overload
+def b(arg: object) -> NoReturn: ...
+
+
+def b(arg: Union[int, str, object]) -> Union[int, str]:
+    if not isinstance(arg, (int, str)):
+        raise TypeError
+    return arg