Python Variables Explained: Basics, Naming Rules & Practical Examples

📖 Table of Contents

1. What Are Variables?
   • Variable Basics
   • Assignment Operator (=)
2. Variable Naming Rules
   • Valid vs. Invalid Names
   • Reserved Keywords
3. Practical Tasks
   • Task 1: Rectangle Calculator
   • Task 2: Exploring type() and id()

1. What Are Variables?

• A variable is a named storage location in a computer's memory that is used to hold data or values. It allows programmers to store and manipulate data within a program.

Purpose: Variables provide a way to store and manage data that can be used and manipulated throughout a program. They make programs more flexible and allow for dynamic data storage.

Assignment statement: in Python is used to assign a value to a variable. Its primary purpose is to store and manage data within a program.

Imagine variables as labeled boxes:

• You have boxes for storing different things (numbers, words, etc.).
• Each box has a name (label) to identify what's inside.
• You can put things in, take them out, and change what's inside.

Example:

name = "Alice"      # Stores text  
age = 25            # Stores a number  
is_student = True   # Stores True/False  

Assignment Operator (=)

Use = to assign values. No need to declare types!

Key Rules:

• Reassignment: Change a variable’s value anytime.

  x = 10  
  x = "hello"  # Now x is a string!  

• Multiple Assignment:

  a, b = 1, 2          # a=1, b=2  
  x = y = z = 0        # All set to 0  

• Swapping:

  x, y = 10, 20  
  x, y = y, x  # Swap! (Now x=20, y=10)  

Common Mistakes:

• Using spaces: user name = "Bob" ❌
• Starting with numbers: 2nd_place = "Silver" ❌

2. Variable Naming Rules

In Python, valid variable names must adhere to the following rules:

• Begin with a letter or an underscore: The first character of a variable name must be a letter (a-z, A-Z) or an underscore (_).
• Followed by letters, digits, or underscores: After the first character, the variable name can contain letters, digits (0-9), or underscores.
• Case-sensitive: Variable names are case-sensitive. For example, myVariable and myvariable would be considered different variables.
• No reserved keywords: Variable names cannot be Python reserved keywords (e.g., if, for, while, class, etc.).

Valid vs. Invalid Variable Names in Python

✅ Valid Variable Names

• Can contain:
  • Letters (a-z, A-Z)
  • Digits (0-9) but not as the first character
  • Underscores (_)
• Are case-sensitive (age, Age, and AGE are different).
• Should use snake_case (recommended Python style).

Examples:

name = "Alice"  
user_age = 25  
_total = 100  
item2 = "book"  

❌ Invalid Variable Names

• Start with a digit.
• Contain spaces or special symbols (!, @, #, etc.).
• Match Python's reserved keywords (see below).

Examples:

2name = "Bob"      # Error: Starts with digit  
first-name = "John" # Error: Hyphen not allowed  
user age = 30      # Error: Contains space  

Reserved Keywords

Python has 35 reserved keywords (e.g., if, for, while).

Check All Keywords:

import keyword  
print(keyword.kwlist)  

⚠️ Never use these as variable names!

Valid vs. Invalid Names

💡 Tip:
If you accidentally use a keyword, Python will raise a SyntaxError:

class = "Biology"  # Error: 'class' is a keyword  

📝 Interactive Task

Which of these are valid?

1. user_name  
2. 3rd_place  
3. return  
4. _price  
5. my-var  

🌟 Best Practices

✔ Use descriptive names (e.g., student_count vs. n).
✔ Stick to snake_case (e.g., total_score).
✔ Avoid single letters (except in loops like for i in range(5)).

3. Practical Tasks

Task 1: Rectangle Calculator

Given:

length = 10  
width = 5  

Your Task: Calculate and print:

1. Area (length * width)
2. Perimeter (2 * (length + width))

area = length * width  
perimeter = 2 * (length + width)  
print("Area:", area)  
print("Perimeter:", perimeter)  

Task 2: Exploring type() and id()

Code:

x = 10  
print(type(x))  # Output: <class 'int'>  
print(id(x))    # Output: Memory address  

Questions:

1. What happens if you set x = "hello" and check type(x)?
2. Assign y = 10. Does id(y) match id(x)? Why?

💡 Hint: Python reuses memory for small integers!

🔗 Additional Resources:

• Python Variables Video Tutorial
• Common Mistakes to Avoid