ruff/crates/ty_python_semantic/resources/mdtest/stubs/class.md

Class definitions in stubs

Cyclical class definition

[environment]
python-version = "3.12"

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]: ...

class Bar(Foo[Bar]): ...

reveal_type(Bar)  # revealed: Literal[Bar]
reveal_type(Bar.__mro__)  # revealed: tuple[Literal[Bar], Literal[Foo[Bar]], Literal[object]]

Access to attributes declared 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