1.5 Itse1359-1030-variables and identifiers

Table of contents

• Preface
  • Viewing tip
    • Figures
• What is a variable?
  • A pigeonhole in memory
  • The concept of type
    • Strongly typed languages
    • Python is not strongly typed
  • Declaration of variables
    • Dangerous curves ahead!
    • Don't use the same name for two variables
    • A more subtle danger
• Rules for identifiers
  • Give me the rules in plain English
    • Allowable characters
    • Case is significant
    • Numbers
    • Style
• Let's program
  • Start the interactive Python environment
  • Create and use some variables
    • Back to the pigeonholes
    • The assignment operator
    • What is an operand?
    • Addition of variables
    • Assigning the same value to several variables
    • Create three variables
    • Assign different values
  • A more comprehensive explanation
    • A code visualizer tool
    • Creation of variables and objects
    • The important point
    • The sum of two variables
    • Assigning the same value to several variables
    • Breaking a reference to an object
  • Type considerations
    • Advantages and disadvantages
      • The range of values
      • Speed
      • Floating point provides greater range
      • Sometimes range is important, and sometimes not
      • Approximate results
  • Automatic type handling in Python
    • Python takes care of routine type issues automatically
    • Assign some floating point values
    • How is this accomplished?
  • The magic continuation variable
    • How does it work?
    • Sum is saved in the continuation variable
    • The primary purpose of the continuation variable
  • Illegal variable names
  • Variable name spelling errors
    • A serious programming problem
    • Why did this happen?
    • The code visualizer
    • Spelling errors can be dangerous
    • Defending against spelling errors
    • Meaningful variable names
    • Remember, case is significant in variable names
• 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.

This module provides an introduction to the use of variables, and the required syntax of the identifiers used to represent variables.

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 theFigures while you are reading about them.

(Note to blind and visually impaired students: most of the Figures in this module are presented in plaintext format and should be accessible using an audio screen reader or a braille display.)

Figures

• Figure 1 . Create and use some variables.
• Figure 2 . Assigning the same value to several variables.
• Figure 3 . The truth about pigeonholes.
• Figure 4 . Assign a different value to the variable named b.
• Figure 5 . Python takes care of routine type issues automatically.
• Figure 6 . The magic continuation variable.
• Figure 7 . An illegal variable name.
• Figure 8 . A serious programming problem.
• Figure 9 . Visualizer showing result of a spelling error.

What is a variable?

As the name implies, a variable is something whose value changes over time.

A pigeonhole in memory

As a practical matter, a variable is a pigeonhole in memory, which has a nickname, where you can store values.