Learning Python, 2nd Edition

Chapter 23. Advanced Class Topics

Part VI concludes our look at OOP in Python by presenting a few more advanced class-related topics, along with the gotchas and exercises for this part of the book. We encourage you to do the exercises, to help cement the ideas we’ve studied. We also suggest working on or studying larger OOP Python projects as a supplement to this book. Like much in computing, the benefits of OOP tend to become more apparent with practice.

Extending Built-in Types

Besides implementing new kinds of objects, classes are sometimes used to extend the functionality of Python’s built-in types in order to support more exotic data structures. For instance, to add queue insert and delete methods to lists, you can code classes that wrap (embed) a list object, and export insert and delete methods that process the list specially, like the delegation technique studied in Chapter 22. As of Python 2.2, you can also use inheritance to specialize built-in types. The next two sections show both techniques in action.

Extending Types by Embedding

Remember those set functions we wrote in Part IV? Here’s what they look like brought back to life as a Python class. The following example, file setwrapper.py, implements a new set object type, by moving some of the set functions to methods, and adding some basic operator overloading. For the most part, this class just wraps a Python list with extra set operations. Because it’s a class, it also supports multiple instances and customization ...