Mypy more specific parameter in subclass

Question

I want to declare a base class with an abstract method that has a typed parameter such that implementing classes can specify a more specific type for that parameter, for example:

from abc import ABC, abstractmethod

class Job(ABC):
    pass

class EasyJob(Job):
    pass

class HardJob(Job):
    pass

class Worker(ABC):
    @abstractmethod
    def run(self, job: Job) -> None:
        raise NotImplementedError()

class EasyWorker(Worker):
    def run(self, job: EasyJob) -> None:
        pass

class HardWorker(Worker):
    def run(self, job: HardJob) -> None:
        pass

However, mypy understandably complains about this:

line 14: error: Argument 1 of "run" is incompatible with supertype "Worker"; supertype defines the argument type as "Job"
line 18: error: Argument 1 of "run" is incompatible with supertype "Worker"; supertype defines the argument type as "Job"

Is there any way in Python to facilitate such a structure?

Answer 1

You probably want Bounded Parametric Polymorphism. See also this section about bounded type variables.

In Worker, you want to say that your method run is generic in the sense that it takes a value of some unspecified type T whereas T is a subtype of Job. The classes derived from Worker then substitute T for concrete types:

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


class Job(ABC):
    pass


class EasyJob(Job):
    pass


class HardJob(Job):
    pass

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


class Worker(ABC, Generic[T]):
    @abstractmethod
    def run(self, job: T) -> None:
        raise NotImplementedError()


class EasyWorker(Worker[EasyJob]):
    def run(self, job: EasyJob) -> None:
        pass


class HardWorker(Worker[HardJob]):
    def run(self, job: HardJob) -> None:
        pass