Python3 魔法方法：属性访问

1、与属性访问有关的魔法方法

__getattr__(self,name)

定义当用户试图获取某一不存在的属性时的行为

__getattribute__(self,name)

定义当该类属性被访问时的行为

__setattr__(self,name,value)

定义一个属性被设置时的行为

__delattr__(self,name)

定义一个属性被删除时的行为

注意：

def __setattr__(self, name, value):
        self.name = value + 1

#此代码试图在对对象的属性进行赋值时让属性的值加1但是
self.name = value + 1这句又会触发 __setattr__从而导致无限递归

正确写法：

def __setattr__(self, name, value):
        super().__setattr__(name,value+1)
#寻找父类的__setattr__方法来对属性name赋值

或者：

def __setattr__(self, name, value):
        self.__dict__[name] = value + 1

以下的代码问题：

class Counter:
        def __init__(self):
                self.counter = 0 # 这里会触发 __setattr__ 调用
        def __setattr__(self, name, value):
                self.counter += 1
“””既然需要 __setattr__ 调用后才能真正设置 self.counter 的值，所以这时候 self.counter 还没有定义，所以没法 += 1，错误的根源。”””
                super().__setattr__(name, value)
        def __delattr__(self, name):
                self.counter -= 1
                super().__delattr__(name)

定义一个count类实现检测属性的增减

class Counter:
    def __init__(self):
        self.counter = 0
    def __setattr__(self, name, value):
        if name== 'counter':
            super().__setattr__(name, value)
        else:
            self.counter += 1
            super().__setattr__(name, value)
    def __delattr__(self, name):
        if name =='counter':
            super().__delattr__(name)
        else:
            self.counter -= 1
            super().__delattr__(name)

class Counter:
        def __init__(self):
                super().__setattr__('counter', 0)
        def __setattr__(self, name, value):
                super().__setattr__('counter', self.counter + 1)
                super().__setattr__(name, value)
        def __delattr__(self, name):
                super().__setattr__('counter', self.counter - 1)
                super().__delattr__(name)