206 – typing.Self

When a method returns the argument self, use typing.Self to annotate the return type of the method:

from typing import Self

class SomeClass:
    def some_method(self) -> Self:
        ...
        return self

This is especially useful if AnotherClass inherits from SomeClass:

class AnotherClass(SomeClass):
    pass

Because of Self, the return type of AnotherClass.some_method is inferred to be the subclass itself:

from typing import reveal_type

reveal_type(AnotherClass().some_method())  # AnotherClass

If you didn’t use Self, the return type would be the parent class:

from typing import reveal_type

class SomeClass:
    def some_method(self) -> SomeClass:
        ...
        return self

class AnotherClass(SomeClass):
    pass

reveal_type(AnotherClass().some_method())  # SomeClass ?!