Learning Python

Using the Class Statement

Did all of the above 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 more detail. We met the class statement in our first examples, but let’s formalize some of the ideas we introduced. As in C++, the class statement is Python’s main OOP tool. Unlike in C++, class isn’t really 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.

General Form

As we saw on our quick tour, class is a compound statement with a body of indented statements 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 later in this chapter):

class <name>(superclass,...):       # assign to name
    data = value                    # shared class data
    def method(self,...):           # methods
        self.member = value         # per-instance data

Within the class statement, specially-named methods overload operators; for instance, a function called _ _ init __ is called at instance object construction time, if defined.

Example

At the start of this chapter, we mentioned that classes are mostly just namespaces— a tool for defining names (called attributes) that export data and logic to clients. So how do you get from the statement to a namespace?

Here’s how. Just as with modules, the statements ...