Learning Python - Mark Lutz [353]
Specifically, in this chapter we’re going to code two classes:
Person—a class that creates and processes information about people
Manager—a customization of Person that modifies inherited behavior
Along the way, we’ll make instances of both classes and test out their functionality. When we’re done, I’ll show you a nice example use case for classes—we’ll store our instances in a shelve object-oriented database, to make them permanent. That way, you can use this code as a template for fleshing out a full-blown personal database written entirely in Python.
Besides actual utility, though, our aim here is also educational: this chapter provides a tutorial on object-oriented programming in Python. Often, people grasp the last chapter’s class syntax on paper, but have trouble seeing how to get started when confronted with having to code a new class from scratch. Toward this end, we’ll take it one step at a time here, to help you learn the basics; we’ll build up the classes gradually, so you can see how their features come together in complete programs.
In the end, our classes will still be relatively small in terms of code, but they will demonstrate all of the main ideas in Python’s OOP model. Despite its syntax details, Python’s class system really is largely just a matter of searching for an attribute in a tree of objects, along with a special first argument for functions.
Step 1: Making Instances
OK, so much for the design phase—let’s move on to implementation. Our first task is to start coding the main class, Person. In your favorite text editor, open a new file for the code we’ll be writing. It’s a fairly strong convention in Python to begin module names with a lowercase letter and class names with an uppercase letter; like the name of self arguments in methods, this is not required by the language, but it’s so common that deviating might be confusing to people who later read your code. To conform, we’ll call our new module file person.py and our class within it Person, like this:
# File person.py (start)
class Person:
All our work will be done in this file until later in this chapter. We can code any number of functions and classes in a single module file in Python, and this one’s person.py name might not make much sense if we add unrelated components to it later. For now, we’ll assume everything in it will be Person-related. It probably should be anyhow—as we’ve learned, modules tend to work best when they have a single, cohesive purpose.
Coding Constructors
Now, the first thing we want to do with our Person class is record basic information about people—to fill out record fields, if you will. Of course, these are known as instance object attributes in Python-speak, and they generally are created by assignment to self attributes in class method functions. The normal way to give instance attributes their first values is to assign them to self in the __init__ constructor method, which contains code run automatically by Python each time an instance is created. Let’s add one to our class:
# Add record field initialization
class Person:
def __init__(self, name, job, pay): # Constructor takes 3 arguments
self.name = name # Fill out fields when created
self.job = job # self is the new instance object
self.pay = pay
This is a very common coding pattern: we pass in the data to be attached to an instance as arguments to the constructor method and assign them to self to retain them permanently. In OO terms, self is the newly created instance object, and name, job, and pay become state information—descriptive data saved on an object for later