What are the methods for rewriting parent classes in Python?

1. Basic application

class Animal(object):
    def eat(self):
        print("动物吃东西")


class Cat(Animal):
    def eat(self):
        print("猫吃鱼")
        # 格式一：父类名.方法名(对象)
        Animal.eat(self)
        # 格式二:super(本类名,对象).方法名()
        super(Cat, self).eat()
        # 格式三:super()方法名()
        super().eat()


cat1 = Cat()
cat1.eat()
print(cat1)

2. Practical application

#用元类实现单例模式
class SingletonType(type):
    instance = {}

    def __call__(cls, *args, **kwargs):
        if cls not in cls.instance:
            # 方式一:
            # cls.instance[cls] = type.__call__(cls, *args, **kwargs)
            # 方式二
            # cls.instance[cls] = super(SingletonType, cls).__call__(*args, **kwargs)
            # 方式三
            cls.instance[cls] = super().__call__(*args, **kwargs)
        return cls.instance[cls]


class Singleton(metaclass=SingletonType):
    def __init__(self, name):
        self.name = name


s1 = Singleton('1')
s2 = Singleton('2')
print(id(s1) == id(s2))

3. Note

1. When a class has multiple inheritance, it inherits from multiple parents The classes have the same parent class A. When rewriting its parent class, you need to pay attention to

Method 1: Parent class name.Method name (object)

• Parent class A Will be called multiple times (according to the number of inheritance)

• Pass the required parameters as needed when overriding the parent class

Method 2 :super(class name, object).Method name()

• The parent class A will only be called once

• Rewrite the parent Class methods must pass all parameters

2. When a class has inheritance and the corresponding variables have been overridden in the subclass, changing the variables of the parent class will not affect the subclass.

class Parent(object):
    x = 1

class Child1(Parent):
    pass

class Child2(Parent):
    pass

print(Parent.x, Child1.x, Child2.x)
Child1.x = 2
print(Parent.x, Child1.x, Child2.x)
Parent.x = 3
print(Parent.x, Child1.x, Child2.x)

Output result

1 1 1
1 2 1
3 2 3

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