python __getattr__, __getattribute__,__get__的用法

class C(object): 
  a = 'abc' 
  def __getattribute__(self, *args, **kwargs): 
    print("__getattribute__() is called") 
    return object.__getattribute__(self, *args, **kwargs) 

  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) 

结果:

__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 

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。