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python中__get__,__getattr__,__getattribute__的区别
__get__,__getattr__和__getattribute都是访问属性的方法,但不太相同。
object.__getattr__(self, name)
当一般位置找不到attribute的时候,会调用getattr,返回一个值或AttributeError异常。
object.__getattribute__(self, name)
无条件被调用,通过实例访问属性。如果class中定义了__getattr__(),则__getattr__()不会被调用(除非显示调用或引发AttributeError异常)
object.__get__(self, instance, owner)
只用在descriptor中。可以通过owner class或者instance来访问属性。
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 | class C( object ): a = ‘abc‘ def __getattribute__( self , * args, * * kwargs): print ( "__getattribute__() is called" ) return object .__getattribute__( self , * args, * * kwargs) # return "haha" def __getattr__( self , name): print ( "__getattr__() is called " ) return name + " from getattr" def __get__( self , instance, owner): print ( "__get__() is called" , instance, owner) return self def foo( self , x): print (x) class C2( object ): d = C() if __name__ = = ‘__main__‘ : c = C() c2 = C2() print (c.a) print (c.zzzzzzzz) c2.d print (c2.d.a) |
输出结果是:
__getattribute__() is called
abc
__getattribute__() is called
__getattr__() is called
zzzzzzzz from getattr
__get__() is called <__main__.C2 object at 0x16d2310> <class ‘__main__.C2‘>
__get__() is called <__main__.C2 object at 0x16d2310> <class ‘__main__.C2‘>
__getattribute__() is called
abc
小结:可以看出,每次通过实例访问属性,都会经过__getattribute__函数。而当属性不存在时,仍然需要访问__getattribute__,不过接着要访问__getattr__。这就好像是一个异常处理函数。
每次访问descriptor(即实现了__get__的类),都会先经过__get__函数。
需要注意的是,当使用类访问不存在的变量是,不会经过__getattr__函数。而descriptor不存在此问题,只是把instance标识为none而已。
python中__get__,__getattr__,__getattribute__的区别