Variables

What are variables?

A variable is like a labeled box where you can store information. You give it a name and put a value inside. Later, you can use that name to get the value back. In real life:

Your name is a “variable” that stores what people call you
Your age is a “variable” that stores a number
Your favorite color is a “variable” that stores text

In Python, we create variables to store data our programs need to remember.

Creating your first variable

Here’s how to create a variable:

name = "Alice"
age = 25
is_student = True

Let’s break this down:

name is the variable name
= means “store the value on the right in the variable on the left”
"Alice" is the value we’re storing

Naming rules

Python has rules for variable names: Allowed:

user_name = "Dave"      # lowercase with underscores (Python style)
userName = "Dave"       # camelCase (works but not Python style)
age2 = 30              # numbers are OK (not at start)
_private = "secret"    # underscore at start is OK

Not allowed:

2age = 30              # Can't start with number
my-name = "Dave"       # No hyphens (Python thinks it's subtraction)
my name = "Dave"       # No spaces
class = "Python"       # Can't use Python keywords

Python naming convention

In Python, we use lowercase letters with underscores between words. This is called “snake_case” and it’s the standard way to name variables in Python.

# Good Python style
first_name = "Alice"
user_age = 25
is_logged_in = True
shopping_cart_total = 49.99

# Avoid camelCase (this is for other languages)
firstName = "Alice"  # Works, but not Python style
userAge = 25
isLoggedIn = True

Use descriptive names for your variables! Instead of x or temp, use names like user_score or file_path. Your future self (and teammates) will thank you when reading the code later.

Changing variables

Variables can change (that’s why they’re called variables!):

# Start with one value
score = 0
print(score)  # Shows: 0

# Change it
score = 10
print(score)  # Shows: 10

# Change it again
score = score + 5
print(score)  # Shows: 15

Common mistakes

Forgetting quotes around text

# Wrong
name = Alice  # Python looks for a variable called Alice

# Right
name = "Alice"  # This creates text

Using undefined variables

# Wrong
print(score)  # Error: score doesn't exist yet
score = 10

# Right
score = 10
print(score)  # Now it works

Confusing = and ==

# = means "store"
age = 25

# == means "compare" (we'll learn this later)
if age == 25:
    print("Quarter century!")

What’s next?

Before we explore data types, let’s learn about comments - a crucial tool for making your code readable.

Comments

Document your code clearly

Getting started

Data types

Working with data

Control flow

Data structures

What are variables?

Creating your first variable

Naming rules

Python naming convention

Changing variables

Common mistakes

What’s next?

Comments

​What are variables?

​Creating your first variable

​Naming rules

​Python naming convention

​Changing variables

​Common mistakes

​What’s next?

Comments

What are variables?

Creating your first variable

Naming rules

Python naming convention

Changing variables

Common mistakes

What’s next?