Introduce Y040: Ban explicit inheritance from object (#217)

Author: AlexWaygood

CHANGELOG.md

@@ -8,6 +8,7 @@ Features:
   acceptable as an annotation for an `__(a)exit__` method argument.
 * Teach the Y029 check to emit errors for `__repr__` and `__str__` methods that return
   `builtins.str` (as opposed to the unqualified `str`).
+* Introduce Y040: Never explicitly inherit from `object` in Python 3 stubs.
 
 ## 22.4.1
 

README.md

@@ -71,11 +71,12 @@ currently emitted:
 | Y037 | Use PEP 604 syntax instead of `typing.Union` and `typing.Optional`. E.g. use `str \| int` instead of `Union[str, int]`, and use `str \| None` instead of `Optional[str]`.
 | Y038 | Use `from collections.abc import Set as AbstractSet` instead of `from typing import AbstractSet`. Like Y027, this error code should be switched off in your config file if your stubs support Python 2.
 | Y039 | Use `str` instead of `typing.Text`. This error code is incompatible with stubs supporting Python 2.
+| Y040 | Never explicitly inherit from `object`, as all classes implicitly inherit from `object` in Python 3. This error code is incompatible with stubs supporting Python 2.
 
 Many error codes enforce modern conventions, and some cannot yet be used in
 all cases:
 
-* Y027, Y038 and Y039 are incompatible with Python 2. These should only be used in stubs
+* Y027, Y038, Y039 and Y040 are incompatible with Python 2. These should only be used in stubs
   that are meant only for Python 3.
 * Y037 (enforcing PEP 604 syntax everywhere) is not yet fully compatible with
   the mypy type checker, which has

pyi.py

@@ -1130,6 +1130,9 @@ def visit_ClassDef(self, node: ast.ClassDef) -> None:
             self.generic_visit(node)
         self.current_class_node = old_class_node
 
+        if any(_is_builtins_object(base_node) for base_node in node.bases):
+            self.error(node, Y040)
+
         # empty class body should contain "..." not "pass"
         if len(node.body) == 1:
             statement = node.body[0]
@@ -1598,3 +1601,4 @@ def parse_options(
 Y037 = "Y037 Use PEP 604 union types instead of {old_syntax} (e.g. {example})."
 Y038 = 'Y038 Use "from collections.abc import Set as AbstractSet" instead of "from typing import AbstractSet" (PEP 585 syntax)'
 Y039 = 'Y039 Use "str" instead of "typing.Text"'
+Y040 = 'Y040 Do not inherit from "object" explicitly, as it is redundant in Python 3'

tests/classdefs.pyi

@@ -9,7 +9,7 @@ from typing import Any, overload
 from _typeshed import Self
 from typing_extensions import final
 
-class Bad:
+class Bad(object):  # Y040 Do not inherit from "object" explicitly, as it is redundant in Python 3
     def __new__(cls, *args: Any, **kwargs: Any) -> Bad: ...  # Y034 "__new__" methods usually return "self" at runtime. Consider using "_typeshed.Self" in "Bad.__new__", e.g. "def __new__(cls: type[Self], *args: Any, **kwargs: Any) -> Self: ..."
     def __repr__(self) -> str: ...  # Y029 Defining __repr__ or __str__ in a stub is almost always redundant
     def __str__(self) -> builtins.str: ...  # Y029 Defining __repr__ or __str__ in a stub is almost always redundant
@@ -19,6 +19,8 @@ class Bad:
     async def __aenter__(self) -> Bad: ...  # Y034 "__aenter__" methods in classes like "Bad" usually return "self" at runtime. Consider using "_typeshed.Self" in "Bad.__aenter__", e.g. "async def __aenter__(self: Self) -> Self: ..."
     def __iadd__(self, other: Bad) -> Bad: ...  # Y034 "__iadd__" methods in classes like "Bad" usually return "self" at runtime. Consider using "_typeshed.Self" in "Bad.__iadd__", e.g. "def __iadd__(self: Self, other: Bad) -> Self: ..."
 
+class AlsoBad(int, builtins.object): ...  # Y040 Do not inherit from "object" explicitly, as it is redundant in Python 3
+
 class Good:
     def __new__(cls: type[Self], *args: Any, **kwargs: Any) -> Self: ...
     @abstractmethod