Class definitions are introduced by the class keyword followed by the class name. By convention, new class names in Python use camel case – sometimes known as Pascal case – with an initial capital letter for each and every component word, without separating underscores. Since classes are a bit awkward to define at the REPL, we'll be using a Python module file to hold the class definitions we use in this chapter.

Let's start with the very simplest class, to which we'll progressively add features. In our example we'll model a passenger aircraft flight between two airports by putting this code into airtravel.py:

"""Model for aircraft flights."""


class Flight:
    pass

The class statement introduces a new block, so we indent on the next line. Empty blocks aren't allowed, so the simplest possible class needs at least a do-nothing pass statement to be syntactically admissible.

Just as with def for defining functions, class is a statement that can occur anywhere in a program and...