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Python Variables Explained: Basics, Naming Rules & Practical Examples

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Learn Python variables with this beginner-friendly guide. Understand variable basics, naming rules (including valid vs invalid names), reserved keywords, and practice with hands-on coding tasks.

📖 Table of Contents

  1. What Are Variables?
  2. Variable Naming Rules
  3. Practical Tasks

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

| Valid ✅ | Invalid ❌ |
|——————-|——————–|
| user_name | user-name (hyphen)|
| _total | 2nd_place (starts with digit)|
| price2 | class (reserved keyword)|

💡 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
📚 Answer ✅ Valid: 1, 4 ❌ Invalid: 2 (starts with digit), 3 (keyword), 5 (hyphen)

🌟 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))
💡 Solution ```python 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: