python中__get__,__getattr__,__getattribute__的区别

python中__get__,__getattr__,__getattribute__的区别

博客分类：
• python

getattr getattribute python

__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)
如果class定义了它，则这个class就可以称为descriptor。owner是所有者的类，instance是访问descriptor的实例，如果不是通过实例访问，而是通过类访问的话，instance则为None。（descriptor的实例自己访问自己是不会触发__get__，而会触发__call__，只有descriptor作为其它类的属性才有意义。）（所以下文的d是作为C2的一个属性被调用）

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)

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

__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