Classes Generate Multiple Instance Objects

To understand how the multiple objects idea works, you have to first understand that there are two kinds of objects in Python’s OOP model: class objects and instance objects. Class objects provide default behavior and serve as factories for instance objects. Instance objects are the real objects your programs process—each is a namespace in its own right, but inherits (i.e., has automatic access to) names in the class from which it was created. Class objects come from statements, and instances come from calls; each time you call a class, you get a new instance of that class.

This object-generation concept is very different from any of the other program constructs we’ve seen so far in this book. In effect, classes are essentially factories for generating multiple instances. By contrast, only one copy of each module is ever imported into a single program (in fact, one reason that we have to call imp.reload is to update the single module object so that changes are reflected once they’ve been made).

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The following is a quick summary of the bare essentials of Python OOP. As you’ll see, Python classes are in some ways similar to both defs and modules, but they may be quite different from what you’re used to in other languages.