Learning Python, 5th Edition

Chapter 31. Designing with Classes

So far in this part of the book, we’ve concentrated on using Python’s OOP tool, the class. But OOP is also about design issues—that is, how to use classes to model useful objects. This chapter will touch on a few core OOP ideas and present some additional examples that are more realistic than many shown so far.

Along the way, we’ll code some common OOP design patterns in Python, such as inheritance, composition, delegation, and factories. We’ll also investigate some design-focused class concepts, such as pseudoprivate attributes, multiple inheritance, and bound methods.

One note up front: some of the design terms mentioned here require more explanation than I can provide in this book. If this material sparks your curiosity, I suggest exploring a text on OOP design or design patterns as a next step. As we’ll see, the good news is that Python makes many traditional design patterns trivial.

Python and OOP

Let’s begin with a review—Python’s implementation of OOP can be summarized by three ideas:

Inheritance

Inheritance is based on attribute lookup in Python (in X.name expressions).

Polymorphism

In X.method, the meaning of method depends on the type (class) of subject object X.

Encapsulation

Methods and operators implement behavior, though data hiding is a convention by default.

By now, you should have a good feel for what inheritance is all about in Python. We’ve also talked about Python’s polymorphism a few times already; it flows from Python’s lack of type ...