1.2 KiB
1.2 KiB
Class definitions in stubs
Cyclical class definition
In type stubs, classes can reference themselves in their base class definitions. For example, in
typeshed, we have class str(Sequence[str]): ....
class Foo[T]: ...
# TODO: actually is subscriptable
# error: [non-subscriptable]
class Bar(Foo[Bar]): ...
reveal_type(Bar) # revealed: Literal[Bar]
reveal_type(Bar.__mro__) # revealed: tuple[Literal[Bar], Unknown, Literal[object]]
Access to attributes declarated in stubs
Unlike regular Python modules, stub files often omit the right-hand side in declarations, including
in class scope. However, from the perspective of the type checker, we have to treat them as bindings
too. That is, symbol: type is the same as symbol: type = ....
One implication of this is that we'll always treat symbols in class scope as safe to be accessed from the class object itself. We'll never infer a "pure instance attribute" from a stub.
b.pyi:
from typing import ClassVar
class C:
class_or_instance_var: int
from typing import ClassVar, Literal
from b import C
# No error here, since we treat `class_or_instance_var` as bound on the class.
reveal_type(C.class_or_instance_var) # revealed: int