Learning Python, 4th Edition

Chapter 30. 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—i.e., 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 those 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. Many 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.

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 X.

Encapsulation

Methods and operators implement behavior; 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 declarations. Because attributes are always resolved at runtime, objects that implement the same interfaces are interchangeable; clients ...