3.2 Itse1359-1420-understanding self

Table of contents

• Preface
  • What you have learned
  • What you will learn
  • Viewing tip
    • Figures
    • Listings
• Preview
• Discussion and sample code
  • Multiple objects from the same class
  • The class named Radio
  • Class methods versus instance methods
• Complete program listing
• Visualizing the program
• Run the program
• Miscellaneous

Preface

This module is one in a collection of modules on Python designed for teaching ITSE 1359 Introduction to Scripting Languages: Python at Austin Community College in Austin, TX.

What you have learned

In an earlier module, you learned how to define a Python class, how to instantiate objects of that class, and how to call methods on those objects.

What you will learn

The class-definition code in the earlier module included the word self in several different locations. I promised that I would revisit the class definition and explain the use of the word self . That is the purpose of this module.

Viewing tip

I recommend that you open another copy of this module in a separate browser window and use the following links to easily find and view the Figuresand the Listings while you are reading about them.

(Note to blind and visually impaired students: most of the Figures and all of the Listings inthis module are presented in plain text format and should be accessible using an audio screen reader or a braille display. Note however that the requiredindentation may not be properly represented by an audio screen reader.)

Figures

• Figure 1 . Visualizing the program.
• Figure 2 . Output from the code in Listing 5.

Listings

• Listing 1 . Multiple objects from the same class.
• Listing 2 . Call the playStation method on an object.
• Listing 3 . Abbreviated version of the class named Radio.
• Listing 4 . Using self to access an instance variable.
• Listing 5 . Complete program listing.

Preview

Listing 5 shows a complete listing of a scaled down version of the radio program from an earlier module. (The program was scaled down simply to reducethe size and to make the presentation more manageable.) Figure 2 shows the output produced by the code in Listing 5 .

The program in Listing 5 will be used to explain the use of the word self in a Python class definition.

Discussion and sample code

With the exception of the following items, you should already understand the code in Listing 5 .

• The use of the word self
• The difference between class variables and instance variables .
• The word object in the parentheses in the first line of the class definition.

I will use fragments of code from the program in Listing 5 to explain the use of the word self in this module.

I will explain the difference between class variables and instance variables as well as theuse of the word object in future modules.

Multiple objects from the same class

You learned in the earlier module that you can instantiate one or more independent objects from the same class and call methods on those objects. Thisis illustrated by the abbreviated code in Listing 1 . (Note that comments and code were omitted from Listing 1 to make it easier to discuss the code of interest.)