By defining other special methods, you can specify the behavior of operators on userdefined types. For example, if you define a method named __add__ for the Time class, you can use the + operator on Time objects.
Here is what the definition might look like:
# inside class Time: def __add__(self, other): seconds = self.time_to_int() + other.time_to_int() return int_to_time(seconds)
And here is how you could use it:
>>> start = Time(9, 45)>>> duration = Time(1, 35)>>> print start + duration11:20:00
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__. For more details, see http://docs.python.org/2/reference/datamodel.html#specialnames.
Exercise 17.4.Write an add method for the Point class.