19.1 Why Use Classes?

Remember when we told you that programs do things with stuff? In simple terms, classes are just a way to define new sorts of stuff, which reflect real objects in your program's domain. For instance, suppose we've decided to implement that hypothetical pizza-making robot we used as an example in Chapter 12. If we implement it using classes, we can model more of its real-world structure and relationships:

Inheritance

Pizza-making robots are a kind of robot, and so possess the usual robot-y properties. In OOP terms, we say they inherit properties from the general category of all robots. These common properties need to be implemented only once for the general case and reused by all types of robots we may build in the future.

Composition

Pizza-making robots are really collections of components that work together as a team. For instance, for our robot to be successful, it might need arms to roll dough, motors to maneuver to the oven, and so on. In OOP parlance, our robot is an example of composition; it contains other objects it activates to do its bidding. Each component might be coded as a class, which defines its own behavior and relationships.

General OOP ideas like inheritance and composition apply to any application that can be decomposed into a set of objects. For example, in typical GUI systems, interfaces are written as collections of widgets—buttons, labels, and so on—which are all drawn when their container is drawn (composition). Moreover, we may be able to write our own custom widgets—buttons with unique fonts, labels with new color schemes, and the like—which are specialized versions of more general interface devices (inheritance).

From a more concrete programming perspective, classes are a Python program unit, just like functions and modules. They are another compartment for packaging logic and data. In fact, classes also define a new namespace much like modules. But compared to other program units we've already seen, classes have three critical distinctions that make them more useful when it comes to building new objects:

Multiple instances

Classes are roughly factories for generating one or more objects. Every time we call a class, we generate a new object, with a distinct namespace. Each object generated from a class has access to the class's attributes and gets a namespace of its own for data that varies per object.

Customization via inheritance

Classes also support the OOP notion of inheritance; they are extended by redefining their attributes outside the class itself. More generally, classes can build up namespace hierarchies, which define names to be used by objects created from classes in the hierarchy.

Operator overloading

By providing special protocol methods, classes can define objects that respond to the sorts of operations we saw work on built-in types. For instance, objects made with classes can be sliced, concatenated, indexed, and so on. Python provides hooks classes can use to intercept and implement any built-in type operation.