Chapter 25. 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. Many of the design terms mentioned here (delegation, composition, factories, and more) 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

Python’s implementation of OOP can be summarized by three ideas:


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


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


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 don’t need to know what sorts of objects are implementing the methods they call.

Encapsulation means packaging in Python—that is, hiding implementation details behind an object’s interface. It does not mean enforced privacy, as you’ll ...

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