Improve checking of "__slots__" (#12531)

Don't rely on `object.__slots__` being defined, since it's not
defined at runtime (and currently it's removed in typeshed).

Author: JukkaL

mypy/checker.py

@@ -1439,6 +1439,17 @@ def check_setattr_method(self, typ: Type, context: Context) -> None:
         if not is_subtype(typ, method_type):
             self.msg.invalid_signature_for_special_method(typ, context, '__setattr__')
 
+    def check_slots_definition(self, typ: Type, context: Context) -> None:
+        """Check the type of __slots__."""
+        str_type = self.named_type("builtins.str")
+        expected_type = UnionType([str_type,
+                                   self.named_generic_type("typing.Iterable", [str_type])])
+        self.check_subtype(typ, expected_type, context,
+                           message_registry.INVALID_TYPE_FOR_SLOTS,
+                           'actual type',
+                           'expected type',
+                           code=codes.ASSIGNMENT)
+
     def check_match_args(self, var: Var, typ: Type, context: Context) -> None:
         """Check that __match_args__ contains literal strings"""
         typ = get_proper_type(typ)
@@ -2281,6 +2292,9 @@ def check_assignment(self, lvalue: Lvalue, rvalue: Expression, infer_lvalue_type
                         else:
                             self.check_getattr_method(signature, lvalue, name)
 
+                if name == '__slots__':
+                    typ = lvalue_type or self.expr_checker.accept(rvalue)
+                    self.check_slots_definition(typ, lvalue)
                 if name == '__match_args__' and inferred is not None:
                     typ = self.expr_checker.accept(rvalue)
                     self.check_match_args(inferred, typ, lvalue)

mypy/message_registry.py

@@ -60,6 +60,7 @@ def format(self, *args: object, **kwargs: object) -> "ErrorMessage":
     'Incompatible types in "async with" for "__aexit__"'
 )
 INCOMPATIBLE_TYPES_IN_ASYNC_FOR: Final = 'Incompatible types in "async for"'
+INVALID_TYPE_FOR_SLOTS: Final = 'Invalid type for "__slots__"'
 
 ASYNC_FOR_OUTSIDE_COROUTINE: Final = '"async for" outside async function'
 ASYNC_WITH_OUTSIDE_COROUTINE: Final = '"async with" outside async function'

mypy/typeshed/stdlib/builtins.pyi

@@ -85,7 +85,6 @@ class _SupportsAiter(Protocol[_T_co]):
 class object:
     __doc__: str | None
     __dict__: dict[str, Any]
-    __slots__: str | Iterable[str]
     __module__: str
     __annotations__: dict[str, Any]
     @property

test-data/unit/pythoneval.test

@@ -1312,13 +1312,17 @@ f(E)
 g(E)
 
 [case testInvalidSlots]
+from typing import List
 class A:
     __slots__ = 1
 class B:
     __slots__ = (1, 2)
+class C:
+    __slots__: List[int] = []
 [out]
-_testInvalidSlots.py:2: error: Incompatible types in assignment (expression has type "int", base class "object" defined the type as "Union[str, Iterable[str]]")
-_testInvalidSlots.py:4: error: Incompatible types in assignment (expression has type "Tuple[int, int]", base class "object" defined the type as "Union[str, Iterable[str]]")
+_testInvalidSlots.py:3: error: Invalid type for "__slots__" (actual type "int", expected type "Union[str, Iterable[str]]")
+_testInvalidSlots.py:5: error: Invalid type for "__slots__" (actual type "Tuple[int, int]", expected type "Union[str, Iterable[str]]")
+_testInvalidSlots.py:7: error: Invalid type for "__slots__" (actual type "List[int]", expected type "Union[str, Iterable[str]]")
 
 [case testDictWithStarStarSpecialCase]
 from typing import Dict