repository_pattern.py

from typing import Generic, TypeVar, Optional


class BaseModel:
    pass


M = TypeVar('M', bound=BaseModel)


class BaseRepository(Generic[M]):

    model_class = None  #type: type

    @classmethod
    def get(cls, pk: int) -> M:
        """Find by PK"""
        ...


class MyModel(BaseModel):
    pk = None   #type: int
    name = None #type: str

    def __init__(self, name: str) -> None:
        self.name = name


class MyModelRepository(BaseRepository[MyModel]):

    model_class = MyModel

    @classmethod
    def find_by_name(cls, name: str) -> Optional[MyMydel]:
        ...

def main() -> None:
    ins = MyModelRepository.get(42)
    print(ins.pk)
#    assert isinstance(ins, MyModel)


main()

Funny, you left out one important piece: find_by_name references cls.model_class. Anyway, I got the use case.