Operator Overloading

Besides creating methods of our own, we can change the meaning of the way that many familiar operators work, a technique known as operator overloading. Special methods, whose names begin and end with double underscores, can be defined to ``intercept'' many common operators, allowing you to redefine what such operators as print, + and * or functions like len will do when they're applied to the objects you create. One of the most important operator overloading methods is the __init__ method. This method is called whenever the class name is used as a constructor, and allows you to initialize attributes in your object at the same time as you create it. The __str__ method is called through the print statement; the __repr__ method is called when an object's name is typed in the interpreter. Table 10.1 lists some of the more commonly used methods for overloading.

Table 10.1: Methods for Operator Overloading

Method	Use
__init__(object)	called when class constructor is invoked
__repr__(object)	also called when object name typed in interpreter
__del__(object)	called when an object is destroyed
__str__(object)	called by print(object)
__len__(object)	called by len(object)
__getitem__(object,key)	allows you to intercept subscripting requests
__setitem__(object,key,value)	allows you to set values of subscripted items
__getslice__(object,start,fin)	allows you to intercept slice requests
__setslice__(object,start,fin,value)	allows you to set slices
__add__(object,other)	called by object + other
__radd__(object,other)	called by other + object
__sub__(object,other)	called by object - other
__mul__(object,other)	called by object * other
__mod__(object,other)	called by object % other

In addition, you can define what happens when your object is iterated over by means of the for statement by defining an __iter__ method that simply returns the object itself, and providing a next method which will be called for each iteration. Inside the next method, you need to raise a StopIteration exception when no more items are available. (See Section 10.10 for an example.