python - 在 Python 中创建基类返回子对象

Question

我有一个包含两个子类的基类，比如 Car 包含子类 HeavyCar 和 LightCar。是否可以让基类的创建返回依赖于变量的子类的对象？像这样:

weightA = 2000
myAcar = Car(weigthA) # myAcar is now of type HeavyCar
weightB = 500
myBcar = Car(weightB) # myBcar is now of type LightCar

我知道执行此操作的正常方法是查看权重变量，然后决定我想要什么类型的对象，然后创建该特定对象。但是，我想由我的类(class)来决定它应该是哪种类型，而不必在类(class)之外为它烦恼，即根本不必查看可变重量。

Answer 1

您可以覆盖 __new__ 以使其返回所需的类型。但是，定义一个执行相同操作的函数会更简单，因为它不太容易出错。

使用 __new__

class Car(object):    
    def __init__(self, weight):
        self.weight = weight

    def __new__(cls, weight):
        if cls is Car:
            if weight > 1000:
                return object.__new__(HeavyCar)
            else:
                return object.__new__(LightCar)
        else:
            return object.__new__(cls)

class LightCar(Car):
    def __init__(self, weight):
        super(LightCar, self).__init__(weight)
        self.speed = "fast"
class HeavyCar(Car):
    pass

assert isinstance(Car(10), LightCar)
assert Car(10).weight == 10 # Car.__init__ invoked
assert hasattr(Car(10), "speed") # LightCar.__init__ invoked as well

assert isinstance(Car(1001), HeavyCar)
assert Car(1001).weight == 1001 # Car.__init__ invoked

使用函数

def create_car(weight):
    if weight > 1000:
        return HeavyCar(weight)
    else:
        return LightCar(weight)

assert isinstance(create_car(10), LightCar)