I want to write a typed BaseCollection class for a database layer, which I then plan to subclass for specific collections, e.g., for UserCollection. The goal is to a) have a standardized interface and to implement generic methods such as get() which can be used for all collections.

The problem is that I have two different “base” models for each collection. A model with editable fields (e.g. UserBase) and a DB model with fields that are managed by the database in addition to the editable fields (e.g. UserInDB). So, for the BaseCollection I have two generic types: An input type (IT) and a return type (RT). Here, RT extends IT by additional fields. So, I would like to be able to write something like this:

from abc import ABC
from typing import Generic, Type, TypeVar

from pydantic import BaseModel
from models import UserBase, UserInDB

class ModelInDB:
    id: str
    # ...


IT = TypeVar("IT", bound=BaseModel)

                        
class BaseCollection(ABC, Generic[IT]):
    RT = ??? # Somehow derive gernic RT from IT with bound=(IT, ModelInDB)
    model: Type[RT] = None

    def create(self, data: IT) -> RT:
        data = ... # generic implementation here
        return self.model(**data)

    def get(self, id: str) -> RT
        data = ... # generic implementation here
        return self.model(**data)


class UserCollection(BaseCollection[UserBase]):
    model = User


# now it's time to harvest fruits and use my typed user collection :)