Classes and Instances

Although they are technically two separate object types in the Python model, the classes and instances we put in these trees are almost identical—each type’s main purpose is to serve as another kind of namespace—a package of variables, and a place where we can attach attributes. If classes and instances therefore sound like modules, they should; however, the objects in class trees also have automatically searched links to other namespace objects, and classes correspond to statements, not entire files.

The primary difference between classes and instances is that classes are a kind of factory for generating instances. For example, in a realistic application, we might have an Employee class that defines what it means to be an employee; from that class, we generate actual Employee instances. This is another difference between classes and modules: we only ever have one instance of a given module in memory (that’s why we have to reload a module to get its new code), but with classes, we can make as many instances as we need.

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Operationally, classes will usually have functions attached to them (e.g., computeSalary), and the instances will have more basic data items used by the class’ functions (e.g., hoursWorked). In fact, the object-oriented model is not that different from the classic data-processing model of programs plus records; in OOP, instances are like records with “data,” and classes are the “programs” for processing those records. In OOP, though, we also have the notion of an inheritance hierarchy, which supports software customization better than earlier models.