Creating Classes and Methods

17.7 Operator overloading

By defining other special methods, you can specify the behavior of operators on programmer-defined 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 + duration)
11:20:00

When you apply the + operator to Time objects, Python invokes __add__. When you print the result, Python invokes __str__. So there is a lot happening behind the scenes!

Changing the behavior of an operator so that it works with programmer-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/3/reference/datamodel.html#specialnames

As an exercise, write an add method for the Point class.