如何完善python类型的提示，以提示在继承类时返回类实例的方法？

Question

我有一个父母，其方法可以返回该类的实例。 该课程有一个（或几个）儿童课。

如何提供类型提示以表明任何子类中的方法调用将返回该孩子类的实例，而不是父母类的实例？

例如，使用以下代码：

from random import randint

class Person:
    def __init__(self, name, age):
        self.name = name
        self.age = age

    @classmethod
    def get_random_person(cls) -> "Person":
        return cls("Random Guy", randint(18, 65))


class Player(Person):
    pass

def welcome_player(player: Player):
    print(f"Welcome {player.name}")


player = Player.get_random_person()
welcome_player(player)

linter mypy会引起错误：

error: Argument 1 to "welcome_player" has incompatible type "Person"; expected "Player"
Found 1 error in 1 file (checked 1 source file)

当然，您可以执行以下操作，但是如果父母类多次继承，那就很重：

from random import randint
from typing import cast


class Person:
    def __init__(self, name, age):
        self.name = name
        self.age = age

    @classmethod
    def get_random_person(cls) -> "Person":
        return cls("Random Guy", randint(18, 65))


class Player(Person):
    @classmethod
    def get_random_person(cls) -> "Player":
        return cast(Player, super().get_random_person())


def welcome_player(player: Player):
    print(f"Welcome {player.name}")


player = Player.get_random_person()
welcome_player(player)

任何建议：以明智的方式实现这一目标？

Answer 1

您可以使用 typing.typing.typevar typing 使用参数BOND 模拟：

from random import randint
from typing import TypeVar, Type


Self = TypeVar('Self', bound='Person')


class Person:
    def __init__(self, name, age):
        self.name = name
        self.age = age

    @classmethod
    def get_random_person(cls: Type[Self]) -> Self:
        return cls("Random Guy", randint(18, 65))


class Player(Person):
    pass


def welcome_player(player: Player):
    print(f"Welcome {player.name}")


player: Player = Player.get_random_person()
welcome_player(player)

Python 3.11将添加 self code> self code> 但尚未正式发布。 键入 - 延迟在较旧版本中可以访问，尽管mypy会抱怨错误：变量“ typing_extensions.self”无效作为类型）。

from random import randint
from typing_extensions import Self


class Person:
    def __init__(self, name, age):
        self.name = name
        self.age = age

    @classmethod
    def get_random_person(cls) -> Self:
        return cls("Random Guy", randint(18, 65))


class Player(Person):
    pass

def welcome_player(player: Player):
    print(f"Welcome {player.name}")


player = Player.get_random_person()
welcome_player(player)