python 类中内置方法的重写

Posted 2020-10-01

为达成目的，经常会在类中将一些内置方法进行重写，最常见的例如__setattr__，下面就通过内置属性，来查看重写会带来什么变化

先定义一个测试用的类，代码如下

class base:
    def __init__(self):
        pass

inspect.getmembers(base): # 查看内置属性

打印结果如下

(‘__class__‘, <class ‘type‘>)
(‘__delattr__‘, <slot wrapper ‘__delattr__‘ of ‘object‘ objects>)
(‘__dict__‘, mappingproxy({‘__module__‘: ‘__main__‘, ‘__init__‘: <function base.__init__ at 0x0069A0C0>, ‘__getattr__‘: <function base.__getattr__ at 0x0069A108>, ‘__dict__‘: <attribute ‘__dict__‘ of ‘base‘ objects>, ‘__weakref__‘: <attribute ‘__weakref__‘ of ‘base‘ objects>, ‘__doc__‘: None}))
(‘__dir__‘, <method ‘__dir__‘ of ‘object‘ objects>)
(‘__doc__‘, None)
(‘__eq__‘, <slot wrapper ‘__eq__‘ of ‘object‘ objects>)
(‘__format__‘, <method ‘__format__‘ of ‘object‘ objects>)
(‘__ge__‘, <slot wrapper ‘__ge__‘ of ‘object‘ objects>)
(‘__getattr__‘, <function base.__getattr__ at 0x0069A108>)
(‘__getattribute__‘, <slot wrapper ‘__getattribute__‘ of ‘object‘ objects>)
(‘__gt__‘, <slot wrapper ‘__gt__‘ of ‘object‘ objects>)
(‘__hash__‘, <slot wrapper ‘__hash__‘ of ‘object‘ objects>)
(‘__init__‘, <function base.__init__ at 0x0069A0C0>)
(‘__init_subclass__‘, <built-in method __init_subclass__ of type object at 0x00699030>)
(‘__le__‘, <slot wrapper ‘__le__‘ of ‘object‘ objects>)
(‘__lt__‘, <slot wrapper ‘__lt__‘ of ‘object‘ objects>)
(‘__module__‘, ‘__main__‘)
(‘__ne__‘, <slot wrapper ‘__ne__‘ of ‘object‘ objects>)
(‘__new__‘, <built-in method __new__ of type object at 0x1E1FADB8>)
(‘__reduce__‘, <method ‘__reduce__‘ of ‘object‘ objects>)
(‘__reduce_ex__‘, <method ‘__reduce_ex__‘ of ‘object‘ objects>)
(‘__repr__‘, <slot wrapper ‘__repr__‘ of ‘object‘ objects>)
(‘__setattr__‘, <slot wrapper ‘__setattr__‘ of ‘object‘ objects>)
(‘__sizeof__‘, <method ‘__sizeof__‘ of ‘object‘ objects>)
(‘__str__‘, <slot wrapper ‘__str__‘ of ‘object‘ objects>)
(‘__subclasshook__‘, <built-in method __subclasshook__ of type object at 0x00699030>)
(‘__weakref__‘, <attribute ‘__weakref__‘ of ‘base‘ objects>)

base的内置属性

找到__setattr__，发现他是一个 slot wrapper（是否可以理解为插口包装）

‘__setattr__‘, <slot wrapper ‘__setattr__‘ of ‘object‘ objects>

对__setattr__进行重写，在赋值的时候打印key与value

def __setattr__(self , key , value):
    print(key,value)
    self.__dict__[key] = value

再次打印，发现__setattr__变为了普通的方法

(‘__setattr__‘, <function base.__setattr__ at 0x0227F108>)

总结：在重写内置方法时，内置方法会从slot wrapper变为普通的function，slot wrapper-->function

 

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疑问：如果有类继承，父类重写，子类是否也继承？

 1 class base: # 父类
 2     def __init__(self):
 3         pass
 4 
 5     def __setattr__(self, key, value):
 6         print(key, value)
 7         self.__dict__[key] = value
 8 
 9 class Foo(base): # 子类
10     def __init__(self):
11         pass

子类Foo未重写方法

情况1： 子类Foo未重写__setattr__，父类base与子类Foo的内置属性中，__setattr__完全相同

# 父类base
(‘__setattr__‘, <function base.__setattr__ at 0x0243E108>)

# 子类Foo
(‘__setattr__‘, <function base.__setattr__ at 0x0243E108>)

情况2，子类Foo重写__setattr__方法，二者不同

# 父类base
(‘__setattr__‘, <function base.__setattr__ at 0x0243E108>)

# 子类Foo
(‘__setattr__‘, <function Foo.__setattr__ at 0x022BE198>)

 

,总结：在有继承关系时，  1：父类重写内置方法会影响到所有的子类

　　　　　　　　　　　　2：子类将方法重写后与继承特性相同，会覆盖父类的相同的方法

　　　　　　　　　　　　3：在多重继承中，如果父类都重写了该方法，则会继承写在前面的那个父类的方法（具体请参考我的另一片关于多重继承的文章）