泛型协变和逆变

from typing import TypeVar, Generic

class Beverage:  ❶
    """任何饮料"""

class Juice(Beverage):
    """任何果汁"""

class OrangeJuice(Juice):
    """使用巴西橙子制作的美味果汁"""

T = TypeVar('T')  ❷

class BeverageDispenser(Generic[T]):  ❸
    """一个参数化饮料类型的自动售货机"""
    def __init__(self, beverage: T) -> None:
        self.beverage = beverage

    def dispense(self) -> T:
        return self.beverage

def install(dispenser: BeverageDispenser[Juice]) -> None:  ❹
    """安装一个果汁自动售货机"""


juice_dispenser = BeverageDispenser(Juice())
install(juice_dispenser)

orange_juice_dispenser = BeverageDispenser(OrangeJuice())  #类型检查会报错，不支持子类
install(orange_juice_dispenser)

orange_juice_dispenser = BeverageDispenser(Beverage())  #类型检查会报错，不支持父类
install(orange_juice_dispenser)


# 修改为
T_co = TypeVar('T_co', covariant=True)  ❶#支持协变，支持子类，不支持父类
# T_co = TypeVar('T_co', contravariant=True) # 支持逆变，支持父类，不支持子类


class BeverageDispenser(Generic[T_co]):  ❷
    def __init__(self, beverage: T_co) -> None:
        self.beverage = beverage

    def dispense(self) -> T_co:
        return self.beverage

def install(dispenser: BeverageDispenser[Juice]) -> None:  ❸
    """安装一个果汁自动售货机"""

__EOF__

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