python - 具有多个基类的多重继承

Question

请帮我理解Python中多重继承的概念(我来自C#背景，不支持多重继承)。

我使用的是Python 3.7.6。

在下面的代码中，类 Apple 继承了类 ToyFruit、NaturalFruit、FakeFruit 和 >机器人水果。虽然 ToyFruit、NaturalFruit 和 FakeFruit 继承 Fruit 基类，但 RoboticFruit 有一个不同的BaseClass Robot。

我注意到 RoboticFruit 和 Robot 根本没有被调用。

class Fruit:
    def __init__(self, name):
        print("This is the Fruit __init__ function")
        self.test = "BaseClass"
        self.name = name
        print("Fruit object created")


class NaturalFruit(Fruit):
    def __init__(self, name):
        print("This is the NaturalFruit __init__ function")
        super().__init__(name)
        self.type = "Natural"
        print("This is a Natural Fruit")
        self.test = "NaturalClass"
        print("Natural Fruit object created")


class FakeFruit(Fruit):
    def __init__(self, name):
        print("This is the FakeFruit __init__ function")
        super().__init__(name)
        self.type = "Fake"
        print("This is a Fake Fruit")
        self.test = "FakeClass"
        print("Fake Fruit object created")


class ToyFruit(Fruit):
    def __init__(self, name):
        print("This is the ToyFruit __init__ function")
        super().__init__(name)
        self.type = "Toy"
        print("This is the Toy Fruit")
        self.test = "ToyClass"
        print("Toy Fruit object created")


class Robot:
    def __init__(self, name):
        print("This is the ROBOT __init__ function")
        self.test = "RobotClass"
        self.name = name
        print("Robot object created")


class RoboticFruit(Robot):
    def __init__(self, name):
        super().__init__("RoboticFruit")
        print("Robotic Fruit")


class Apple(ToyFruit, NaturalFruit, FakeFruit, RoboticFruit):
    def __init__(self):
        super().__init__("Apple")
        print("Apple object created")


apple = Apple()
# print(apple.name)
print(apple.test)

输出:-

This is the ToyFruit __init__ function
This is the NaturalFruit __init__ function
This is the FakeFruit __init__ function
This is the Fruit __init__ function
Fruit object created
This is a Fake Fruit
Fake Fruit object created
This is a Natural Fruit
Natural Fruit object created
This is the Toy Fruit
Toy Fruit object created
Apple object created
ToyClass

如果我将订单交换为

class Apple(RoboticFruit, ToyFruit, NaturalFruit, FakeFruit):

然后，ToyFruit、NaturalFruit、FakeFruit 和 Fruit __init__ 方法是根本没有被调用。 我不明白为什么 RoboticFruit 类构造函数被跳过。

Answer 1

在多重继承的情况下，super() 委托(delegate)给方法解析顺序 (MRO) 中的下一个对象。我们可以看到Apple类的MRO:

print(Apple.__mro__)

# output:
(
<class '__main__.Apple'>, 
<class '__main__.ToyFruit'>, 
<class '__main__.NaturalFruit'>, 
<class '__main__.FakeFruit'>, 
<class '__main__.Fruit'>, 
<class '__main__.RoboticFruit'>, 
<class '__main__.Robot'>, 
<class 'object'>
)

所以我认为 RoboticFruit 和 Robot 没有被调用，因为类中没有像 super().__init__(name) 这样的调用Fruit，这是该顺序中 RoboticFruit 的前一个 (MRO)。如果您在 Fruit 中添加对 super() 的调用，它应该可以正常工作:

class Fruit:
    def __init__(self, name):
        print("This is the Fruit __init__ function")
        super().__init__(name)
        self.test = "BaseClass"
        self.name = name
        print("Fruit object created")