python - Python 中 getattribute 背后的确切原理是什么？

Question

我尝试运行下面来自 python reference 的示例代码.

>>> class Meta(type):
...    def __getattribute__(*args):
...       print "Metaclass getattribute invoked"
...       return type.__getattribute__(*args)
...
>>> class C(object):
...     __metaclass__ = Meta
...     def __len__(self):
...         return 10
...     def __getattribute__(*args):
...         print "Class getattribute invoked"
...         return object.__getattribute__(*args)

然后我测试了下面的代码:

In [16]: c = C()
class getattribute invoked
class getattribute invoked

In [17]: c
Class getattribute invoked
Class getattribute invoked
Out[17]: Class getattribute invoked
Metaclass getattribute invoked
Metaclass getattribute invoked
Metaclass getattribute invoked
Metaclass getattribute invoked
Metaclass getattribute invoked
Class getattribute invoked
Metaclass getattribute invoked
Metaclass getattribute invoked
Metaclass getattribute invoked
Metaclass getattribute invoked
<__main__.C at 0x29448d0>

任何人都可以对输出有一个明确的解释吗？我看到显示了很多“Metaclass getattribute invoked”，这意味着 __getattribute__() 被调用了很多次。

Answer 1

我也是这样做的。除了打印参数:

In [2]: class Meta(type):
   ...:     def __getattribute__(*args):
   ...:         print args
   ...:         return type.__getattribute__(*args)
   ...:     

In [3]: class C(object):
   ...:     __metaclass__ = Meta
   ...:     def __len__(self):
   ...:         return 10
   ...:     def __getattribute__(*args):
   ...:         print args
   ...:         return object.__getattribute__(*args)

得到这个结果:

In [8]: c
(<__main__.C object at 0x104aeff10>, '__class__')
(<__main__.C object at 0x104aeff10>, '__class__')
Out[8]: (<__main__.C object at 0x104aeff10>, '__class__')
(<class '__main__.C'>, '__mro__')
(<class '__main__.C'>, '__mro__')
(<class '__main__.C'>, '__module__')
(<class '__main__.C'>, '__name__')
(<class '__main__.C'>, '__dict__')
(<__main__.C object at 0x104aeff10>, '__class__')
(<class '__main__.C'>, '__repr__')
(<class '__main__.C'>, '__class__')
(<class '__main__.C'>, '__module__')
(<class '__main__.C'>, '__name__')
<__main__.C at 0x104aeff10>

非常不言自明。第一个参数是对象，第二个是调用的属性。