Learning Python, 2nd Edition

Chapter 21. Class Coding Details

Did all of Chapter 20 make sense? If not, don’t worry; now that we’ve had a quick tour, we’re going to dig a bit deeper and study the concepts we’ve introduced in further detail. This chapter takes a second pass, to formalize and expand on some of the class coding ideas introduced in Chapter 20.

The Class Statement

Although the Python class statement seems similar to other OOP languages on the surface, on closer inspection it is quite different than what some programmers are used to. For example, as in C++, the class statement is Python’s main OOP tool. Unlike C++, Python’s class is not a declaration. Like def, class is an object builder, and an implicit assignment—when run, it generates a class object, and stores a reference to it in the name used in the header. Also like def, class is true executable code—your class doesn’t exist until Python reaches and runs the class statement (typically, while importing the module it is coded in, but not until).

General Form

class is a compound statement with a body of indented statements usually under it. In the header, superclasses are listed in parentheses after the class name, separated by commas. Listing more than one superclass leads to multiple inheritance (which we’ll say more about in the next chapter). Here is the statement’s general form:

class <name>(superclass,...):       # Assign to name.
    data = value                    # Shared class data
    def method(self,...):           # Methods
        self.member = value         # Per-instance data

Within the