By defining other special methods, you can specify the behavior of operators on user-defined types. For example, if you define add method for the Time class, you can use the + operator on Time objects.
def __add__(self,time):
seconds = self.time_to_int() + time.time_to_int()
return Time.int_to_time(seconds)
When you apply the + operator to Time objects, Python invokes __add__. When you print the result, Python invokes __str__. So there is quite a lot happening behind the scenes. Changing the behavior of an operator so that it works with user-defined types is called operator overloading. For every operator in Python there is a corresponding special method, like __add__.
from Thinking in Python
原文：http://www.cnblogs.com/ryansunyu/p/4004431.html