Section 13.3. Classes

13.3. Classes

Recall that a class is a data structure that we can use to define objects that hold together data values and behavioral characteristics. Classes are entities that are the programmatic form of an abstraction for a real-world problem, and instances are realizations of such objects. One analogy is to liken classes to blueprints or molds with which to make real objects (instances). So why the term "class"? The term most likely originates from using classes to identify and categorize biological families of species to which specific creatures belong and can be derived into similar yet distinct subclasses. Many of these features apply to the concept of classes in programming.

In Python, class declarations are very similar to function declarations, a header line with the appropriate keyword followed by a suite as its definition, as indicated below:

def functionName(args):     'function documentation string'     function_suite  class ClassName(object):     'class documentation string'     class_suite

Both allow you to create functions within their declaration, closures or inner functions for functions within functions, and methods for functions defined in classes. The biggest difference is that you run functions but create objects with classes. A class is like a Python container type on steroids. In this section, we will take a close look at classes and what types of attributes they have. Just remember to keep in mind that even though classes are objects (everything in Python is an object), they are not realizations of the objects they are defining. We will look at instances in the next section, so stay tuned for that. For now, the limelight is beamed strictly on class objects.

When you create a class, you are practically creating your own kind of data type. All instances of that class are similar, but classes differ from one another (and so will instances of different classes by nature). Rather than playing with toys that came from the manufacturer and were bestowed upon you as gifts, why not design and build your own toys to play with?

Classes also allow for derivation. You can create subclasses that are classes but inherit all of the features and attributes of the "parent" class. Starting in Python 2.2, you can subclass built-in types instead of just other classes.

13.3.1. Creating Classes

Python classes are created using the class keyword. In the simple form of class declarations, the name of the class immediately follows the keyword:

class ClassName(bases):     'class documentation string'     class_suite

As outlined briefly earlier in this chapter, bases is the set of one or more parent classes from which to derive; and class_suite consists of all the component statements, defining class members, data attributes, and functions. Classes are generally defined at the top-level of a module so that instances of a class can be created anywhere in a piece of source code where the class is defined.

13.3.2. Declaration versus Definition

As with Python functions, there is no distinction between declaring and defining classes because they occur simultaneously, i.e., the definition (the class suite) immediately follows the declaration (header line with the class keyword) and the always recommended, but optional, documentation string. Likewise, all methods must also be defined at this time. If you are familiar with the OOP terms, Python does not support pure virtual functions (à la C++) or abstract methods (as in Java), which coerce the programmer to define a method in a subclass. As a proxy, you can simply raise the NotImplementedError exception in the base class method to get the same effect.