Example code for SO "Type annotations for abstract classes that are coupled by a shared, arbitrary type"

so64893801.py

from __future__ import annotations

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

R = TypeVar('R', bound='Runner')
T = TypeVar('T')  # result type


class Command(ABC, Generic[T, R]):
    @abstractmethod
    def execute(self, runner: R) -> T:
        raise NotImplementedError()


# Runner is not generic in R
class Runner(ABC, Generic[T]):
    # Runner.run is generic in its owner
    def run(self: R, command: Command[T, R]) -> T:
        return command.execute(self)


class MyRunner(Runner[bool]):
    magic_level: int = 20


class MyCommand(Command[bool, MyRunner]):
    def execute(self, runner: MyRunner) -> bool:
        # Pseudo code to illustrate dependency on runner's attributes
        return runner.magic_level > 10


if __name__ == '__main__':
    command = MyCommand()
    runner = MyRunner()
    reveal_type(runner.run(command))