Variables in Python

Imagine you're building a game. You need to track the player's name, their score, how many lives they have left, and whether they've completed a level. How do you remember all this information while your program runs? That's exactly what variables do—they're your program's memory, storing everything from a single number to complex data. Without variables, your code would be like trying to do math in your head without writing anything down. Let's learn how to give your programs the power to remember!

Definition

A variable is a named reference to a value stored in memory. It's a container that holds data you can use, change, and reference throughout your program.

What Does "Named Reference" Mean?

When you create a variable, you're doing two things:

1. Storing a value in your computer's memory (RAM)
2. Creating a label (the variable name) that points to that memory location

Think of it like this:

• Memory address: 0x7fff5fbff6a0 (where the actual data lives)
• Variable name: age (the human-friendly label you use)
• Value: 20 (the actual data stored)

Instead of remembering cryptic memory addresses, you just use the name age, and Python handles the rest!

What is "Stored in Memory"?

Memory (RAM) is your computer's short-term workspace. When you create a variable:

• Python reserves a small space in RAM
• Stores your value there (number, text, etc.)
• Links your variable name to that location
• When your program ends, this memory is cleared (variables disappear)

Python's Dynamic Typing

Unlike languages like C++ or Java where you must declare types (int age = 20;), Python infers the type automatically:

age = 20          # Python sees 20 and thinks "that's an integer"
name = "Aisha"    # Python sees quotes and thinks "that's a string"
price = 19.99     # Python sees a decimal and thinks "that's a float"

You don't tell Python "this is an integer"—Python figures it out. This is called dynamic typing and makes Python beginner-friendly!

Analogy

A variable is like a labeled jar in your kitchen.

• The label is the variable name (e.g., "sugar")
• The contents are the value (e.g., 2 cups of sugar)
• You can check what's inside anytime (print the variable)
• You can empty and refill it with something new (reassign the variable)
• You can have many jars with different labels

Just like you wouldn't label a jar jar number 3 when you could call it sugar, you give variables meaningful names that describe what they hold.

Visual (Mental Model)

Memory Representation:

Variable Name    Arrow    Value         Type
─────────────    ─────    ─────────    ──────
price           ──────►   19.99        float
name            ──────►   "Aisha"      string (str)
age             ──────►   20           integer (int)
is_active       ──────►   True         boolean (bool)


Assignment Flow:

    Write             Python              Store in Memory
     ↓                  ↓                       ↓
age = 20    →    Interpret: int    →    RAM: age → 20

Examples

Example 1: Basic Variable Creation (Simple)

# Creating variables - Python assigns types automatically
name = "Aisha"          # String (text)
age = 20                # Integer (whole number)
gpa = 3.8               # Float (decimal number)
is_active = True        # Boolean (True/False)

# Display all variables
print(name, age, gpa, is_active)
# Output: Aisha 20 3.8 True

What's happening:

• = is the assignment operator (stores value on right into variable on left)
• Each variable gets its own type based on the value
• print() can display multiple values separated by spaces

Example 2: Variables Can Change (Intermediate)

# Variables are called "variable" because they can vary (change)!

score = 0
print("Starting score:", score)  # Output: Starting score: 0

score = 10  # Reassign - the old value (0) is replaced
print("After level 1:", score)   # Output: After level 1: 10

score = score + 5  # Use the current value to calculate a new one
print("After bonus:", score)     # Output: After bonus: 15

# You can even change the type (though usually you don't want to)
score = "Game Over"
print(score)  # Output: Game Over

Example 3: Checking Variable Types (Intermediate)

# Use type() to see what type Python assigned

name = "Aisha"
age = 20
gpa = 3.8
is_active = True

print(type(name))       # Output: <class 'str'>
print(type(age))        # Output: <class 'int'>
print(type(gpa))        # Output: <class 'float'>
print(type(is_active))  # Output: <class 'bool'>

# Why does this matter? Different types have different capabilities
print(age + 5)      # Output: 25 (math works on numbers)
# print(name + 5)   # ❌ ERROR: can't add string and number
print(name + "!")   # Output: Aisha! (can combine strings)

Example 4: Practical Use Case (Practical)

# E-commerce shopping cart example

# Product information
product_name = "Wireless Mouse"
product_price = 19.99
product_in_stock = True
quantity_available = 47

# Customer order
customer_name = "Alex Chen"
order_quantity = 2

# Calculate total
total_price = product_price * order_quantity

# Display order summary
print("=" * 40)
print("ORDER SUMMARY")
print("=" * 40)
print(f"Customer: {customer_name}")
print(f"Product: {product_name}")
print(f"Price per unit: ${product_price}")
print(f"Quantity: {order_quantity}")
print(f"Total: ${total_price}")
print(f"In stock: {product_in_stock}")
print("=" * 40)

Hands-On: Create Your Personal Info Variables

Let's practice creating variables and using them in sentences.

Step 1: Create a New File

1. Open your python-projects folder (or create it if you haven't)
2. Create a new file called my_info.py
3. Open it in your text editor (VS Code, Notepad, etc.)

Step 2: Create Variables for Personal Information

# my_info.py - Store and display personal information

# Create variables with your information
first_name = "Aisha"           # Replace with your first name
last_name = "Kumar"            # Replace with your last name
year_of_birth = 2004           # Replace with your birth year
current_year = 2025

# Calculate age (approximately)
age = current_year - year_of_birth

# Create a friendly sentence using your variables
print("Hello! My name is", first_name, last_name)
print("I was born in", year_of_birth)
print("I am approximately", age, "years old.")

Step 3: Run Your Program

python my_info.py

Step 4: Expected Output

Hello! My name is Aisha Kumar
I was born in 2004
I am approximately 21 years old.

Common Mistakes

Mistake 1: Using Spaces in Variable Names

Problem: Variable names cannot contain spaces.

# ❌ WRONG:
first name = "Aisha"
# SyntaxError: invalid syntax

# ✅ CORRECT:
first_name = "Aisha"  # Use underscores (snake_case)
firstName = "Aisha"   # Or camelCase (less common in Python)

Mistake 2: Case Sensitivity

Problem: Python treats Name, name, and NAME as three different variables.

name = "Aisha"
Name = "Bob"
NAME = "Charlie"

print(name)  # Output: Aisha
print(Name)  # Output: Bob
print(NAME)  # Output: Charlie

# These are three separate variables!

Mistake 3: Using Reserved Keywords

Problem: You can't name variables after Python's built-in words.

# ❌ WRONG - These are Python keywords:
for = 10      # SyntaxError
if = 20       # SyntaxError
class = 30    # SyntaxError
print = 40    # ⚠️ This "works" but breaks print()!

# ✅ CORRECT - Add extra context:
for_loop_count = 10
if_condition = 20
class_name = 30
print_count = 40

Mini-Project: Interactive Greeting Program

Goal: Create a program that asks for the user's information and provides a personalized greeting with calculations.

Project Requirements:

• ✅ Ask the user for their name
• ✅ Ask for their birth year
• ✅ Calculate their approximate age
• ✅ Ask for their favorite hobby
• ✅ Display a personalized multi-line greeting

Complete Implementation

# greeting.py - Interactive user greeting

# Get user information
user_name = input("What is your name? ")
birth_year = input("What year were you born? ")

# Convert birth_year to integer (input() gives us text)
birth_year = int(birth_year)

# Calculate age
current_year = 2025
age = current_year - birth_year

# Get more info
favorite_hobby = input("What is your favorite hobby? ")

# Display personalized greeting
print("\n" + "=" * 50)
print("✨ PERSONALIZED GREETING ✨")
print("=" * 50)
print(f"Hello, {user_name}! 👋")
print(f"You were born in {birth_year}.")
print(f"That makes you approximately {age} years old!")
print(f"It's awesome that you enjoy {favorite_hobby}! 🎉")
print("=" * 50)

Summary

Key Takeaways:

• Variables store values with names you choose — Instead of remembering complex memory addresses, use meaningful names like user_score or product_price
• Use snake_case for naming — Python convention is lowercase_with_underscores, not camelCase or PascalCase
• Types are inferred automatically — You don't declare types; Python figures them out from the value you assign
• Variables are mutable — You can change a variable's value anytime by reassigning it
• The = operator assigns, not equals — Think "store the value on the right into the variable on the left"

Variable Naming Best Practices:

✅ DO:

• Use descriptive names: user_score not us
• Use snake_case: first_name not firstName
• Start with lowercase letters
• Use plurals for collections: users, scores
• Be consistent throughout your code

❌ DON'T:

• Use single letters (except in loops: i, j)
• Use reserved keywords: for, class, if
• Start with numbers: 1st_place
• Use special characters: user@name, total$
• Be vague: x, temp, data (unless context is clear)