3.4 Itse1359-1430-instance variables

Table of contents

• Preface
  • What you have learned
  • What you will learn
  • Viewing tip
    • Figures
    • Listings
• General background information
• Discussion and sample code
  • Empty objects
    • Instantiate two objects of the empty class
    • Would be almost worthless in Java or C++
    • Add an instance variable to one object
    • Try to display the same instance variable in the other object
    • Visualize the code in Listing 9
  • Object can modify itself at runtime
    • The definition of a class named TestClass
    • Add an instance variable to one object and display it
    • Visualize the code in Listing 10
  • The method named __init__
    • A class with an __init__ method
    • Instantiate and display two objects
    • The incoming parameter to the __init__ method
    • Display first instance variable from both objects
    • The remainder of the program
    • Visualize the code in Listing 11
• Run the program
• Complete program listings
• 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

An earlier module provided an overview of classes and objects in Python. Similarly, an earlier module explained the use of the self word in Python class definitions.

What you will learn

I promised in an earlier module that I would revisit and explain the difference between class variables and instance variables in classes and objects. That is the purpose of this and some future modules.

In this module, you will learn some of the details regarding instance variables. You will also learn that although other OO languages such as Java andC++ use terminology that is common with Python terminology, such as class , object , class variable , and instance variable , those terms havesignificantly different meanings in Python than they do in many other OO languages.

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 . Print the object's references.
• Figure 2 . Output from the code in Listing 2.
• Figure 3 . Output from the code in Listing 3.
• Figure 4 . Output from the code in Listing 8.
• Figure 5 . Output from code in Listing 9.
• Figure 6 . Output from code in Listing 10.
• Figure 7 . Output from code in Listing 11.

Listings

• Listing 1 . Empty objects.
• Listing 2 . Add an instance variable to one object.
• Listing 3 . Try to display the same instance variable in the other object.
• Listing 4 . Object can modify itself at runtime.
• Listing 5 . Add an instance variable to one object and display it.
• Listing 6 . A class with an __init__ method.
• Listing 7 . Instantiate and display two objects.
• Listing 8 . Display first instance variable from both objects.
• Listing 9 . Complete program listing.
• Listing 10 . Complete program listing.
• Listing 11 . Complete program listing.