Extending Classes with Mixins

Contributed by: David Beazley and Brian K. Jones

Difficulty level: Easy

Visit the Python Cookbook: Classes and Objects Lab to test out this Shortcut in a live practice environment.

Problem

You have a collection of generally useful methods that you would like to make available for extending the functionality of other class definitions. However, the classes where the methods might be added aren’t necessarily related to one another via inheritance. Thus, you can’t just attach the methods to a common base class.

Solution

The problem addressed by this Shortcut often arises in code where one is interested in the issue of class customization. For example, maybe a library provides a basic set of classes along with a set of optional customizations that can be applied if desired by the user.

To illustrate: suppose you have an interest in adding various customizations (e.g., logging, set-once, type checking) to mapping objects. Here are a set of mixin classes that do that:

class LoggedMappingMixin(object):
    '''
    Add logging to get/set/delete operations for debugging.
    '''
    __slots__ = ()

    def __getitem__(self, key):
        print('Getting ' + str(key))
        return super().__getitem__(key)

    def __setitem__(self, key, value):
        print('Setting {} = {!r}'.format(key, value))
        return super().__setitem__(key, value)

    def __delitem__(self, key):
        print('Deleting ' + str(key))
        return super().__delitem__(key)

class SetOnceMappingMixin(object):
    '''
    Only allow a key to be set once.