Variables

Definition

How does a game remember your score from one moment to the next?

In simple words

Think of a labelled box. score = 0 puts 0 in a box labelled score. score = score + 10 takes what is in the box, adds 10, and puts the result back. The label stays the same; what's inside changes.

Why it matters

Every program stores information: a user's name, a running total, a model's accuracy. Variables are how. Good names, such as total_marks instead of t, make code readable for others and for you next month.

How it works

Naming rules

• Use letters, digits and underscores only, and don't start with a digit: score_2 is fine, 2score is not.
• Names are case-sensitive: Score and score are different.
• Don't use Python keywords such as if, for or class as names.
• Use snake_case: words in lowercase joined by underscores.

= is not 'equals' as in maths. It means 'store'. x = x + 1 makes perfect sense in Python: it adds one to x.

In code

A score that changes as the game goes on.

score = 0
print("Start:", score)

score = score + 10      # a coin
score = score + 25      # a gem
score += 5              # += is a short way to add
print("After collecting:", score)

lives = 3
lives -= 1
print("Lives left:", lives)

player_name = "Aarav"
print(player_name, "scored", score)

Start: 0
After collecting: 40
Lives left: 2
Aarav scored 40

Try it yourself

Common mistakes

• Using a variable before giving it a value: NameError.
• Reading = as 'equals'. It means 'store the right side in the left'.
• Choosing names like x and temp2 that say nothing about the value.

Check yourself

Summary

• A variable is a name for a stored value; = stores it.
• Reassigning changes the value; += and -= are shortcuts.
• Use clear snake_case names that follow the rules.