python - Mypy子类中更具体的参数

Question

我想声明一个带有抽象方法的基类，该方法具有类型化参数，以便实现类可以为该参数指定更具体的类型，例如:

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

但是，mypy 对此的提示是可以理解的:

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"

Python 有什么方法可以促进这样的结构吗？

Answer 1

您可能想要Bounded Parametric Polymorphism 。另请参阅this section about bounded type variables 。

在Worker中，您想说您的方法run是通用的，因为它采用某种未指定类型的值T而T是Job的子类型。从 Worker 派生的类然后用 T 替换具体类型:

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