Using classmethods, class objects and generics together with mypy

Question

I use the following code to create subclasses of class G and pass them the type of model they should produce.

from typing import TypeVar, Type


class A:
    @classmethod
    def model_validate(cls):
        print('Done')


T = TypeVar('T', bound=A)


class G[T]:
    def __init__(self, model: Type[T]):
        self.model = model

    def func(self) -> None:
        print(self.model.model_validate())


G[A](A).func()

This works fine, but mypy gives this error:

error: "type[T]" has no attribute "model_validate"  [attr-defined]

What am i doing wrong?

Answer 1

You are mixing up two type variables. T = TypeVar... defines a typevar with bound, then G[T] completely disregards that, creates a new typevar (with no bound) and uses it.

Either use old-style generics (playground):

from typing import Generic, TypeVar

class A:
    @classmethod
    def model_validate(cls) -> None:
        print('Done')

T = TypeVar('T', bound=A)

class G(Generic[T]):
    def __init__(self, model: type[T]) -> None:
        self.model = model

    def func(self) -> None:
        print(self.model.model_validate())


G[A](A).func()

or new-style PEP695 generics (playground):

class A:
    @classmethod
    def model_validate(cls) -> None:
        print('Done')

class G[T: A]:
    def __init__(self, model: type[T]) -> None:
        self.model = model

    def func(self) -> None:
        print(self.model.model_validate())


G[A](A).func()

Both are accepted by mypy. Also note that you don't need typing.Type if you're using a non-EOL python version (built-in type should be used instead since 3.9 as shown above).