Learning Python

Operator Overloading in Classes

We introduced operator overloading at the start of this chapter; let’s fill in a few blanks here and look at a handful of commonly used overloading methods. Here’s a review of the key ideas behind overloading:

• Operator overloading lets classes intercept normal Python operations.

• Classes can overload all Python expression operators.

• Classes can also overload object operations: printing, calls, qualification, etc.

• Overloading makes class instances act more like built-in types.

• Overloading is implemented by providing specially named class methods.

Here’s a simple example of overloading at work. When we provide specially named methods in a class, Python automatically calls them when instances of the class appear in the associated operation. For instance, the Number class below provides a method to intercept instance construction ( __init __ ), as well as one for catching subtraction expressions ( __ sub__ ). Special methods are the hook that lets you tie into built-in operations:

class Number:
    def __init__(self, start):              # on Number(start)
        self.data = start
    def __sub__(self, other):               # on instance - other
        return Number(self.data - other)    # result is a new instance

>>> from number import Number               # fetch class from module
>>> X = Number(5)                           # calls Number.__init__(X, 5)
>>> Y = X - 2                               # calls Number.__sub__(X, 2)
>>> Y.data
3

Common Operator Overloading Methods

Just about everything you can do to built-in objects such as integers and lists has a corresponding specially ...