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ChapterCHAPTER2525Designing with Classes 25So far in this part of the book, we’ve concentrated on using Python’s OOP tool, theclass. But OOP is also about design issues—i.e., how to use classes to model usefulobjects. This chapter will touch on a few core OOP ideas, and present some additionalexamples that are more realistic than those shown so far. Many of the designterms mentioned here (delegation, composition, factories, and more) require moreexplanation than I can provide in this book; if this material sparks your curiosity, Isuggest exploring a text on OOP design, or design patterns as a next step.Python and OOPPython’s implementation of OOP can be summarized by three ideas:InheritanceInheritance is based on attribute lookup in Python (in X.name expressions).PolymorphismIn X.method, the meaning of method depends on the type (class) of X.EncapsulationMethods and operators implement behavior; data hiding is a convention bydefault.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 fromPython’s lack of type declarations. Because attributes are always resolved at runtime,objects that implement the same interfaces are interchangeable; clients don’t need toknow what sorts of objects are implementing the methods they call.Encapsulation means packaging in Python—that is, hiding implementation detailsbehind an object’s interface. It does not mean enforced privacy, as you’ll see inChapter 26. Encapsulation allows the implementation of an object’s interface to bechanged without impacting the users of that object.518