How to annotate the type of a class attribute to be overridden?

Question

Python

Let's say we have an abstract class:

import abc
from pydantic import BaseModel

class Abstract(abc.ABC):
    context_class: ClassVar[Type['BaseModel']]
    error: ClassVar[Type[Exception]]

    def __init__(self, data: Dict) -> None:
        self.context = self.context_class(**data)

    @abc.abstractmethod
    def process(self) -> None:
        pass

And there is its heir, in which the `context_class` attribute is overridden by the `BaseModel` heir:

class Context(BaseModel):
    email: str

class Concrete(Abstract):
    context_class = Context

    def process(self) -> None:
        print(self.context.email)

When starting Mypy - an error falls: how to correctly specify the type in an abstract class? Clarification: Python 3.8.2 Mypy 0.770 Pydantic 1.6.1
... error: "BaseModel" has no attribute "email"

Answer 1

Solution found:

import abc
from typing import ClassVar, Type, Dict, Generic, TypeVar
from pydantic import BaseModel

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


class Abstract(abc.ABC, Generic[T]):
    context_class: ClassVar[Type[T]]
    error: ClassVar[Type[Exception]]

    def __init__(self, data: Dict) -> None:
        self.context = self.context_class(**data)

    @abc.abstractmethod
    def process(self) -> None:
        pass


class Context(BaseModel):
    email: str


class Concrete(Abstract[Context]):
    context_class = Context

    def process(self) -> None:
        print(self.context.email)