python 内置函数与魔术函数和覆盖

Possible Duplicate:
Intercept operator lookup on metaclass
How can I intercept calls to python’s “magic” methods in new style classes?

考虑以下代码:

class ClassA(object): 
    def __getattribute__(self, item): 
        print 'custom__getattribute__ - ' + item 
        return '' 
    def __str__(self): 
        print 'custom__str__' 
        return '' 
 
a=ClassA() 
print 'a.__str__: ', 
a.__str__ 
 
print 'str(a): ', 
str(a) 

输出结果令我惊讶:

a.__str__:  custom__getattribute__ - __str__ 
str(a):  custom__str__ 

• str(a) 不应该映射到魔法方法吗 a.__str__()?
• 如果我删除自定义 ClassA.__str__()，那么 ClassA.__getattribute__() 仍然没有接听电话。怎么会？