Variables

What you'll learn

How to create a variable, update it, and use it inside text with f-strings.

Why it matters

Variables let a program remember a score, a username or the result of a calculation and use it again later.

Explanation

A variable is a name that points to a value. You create one with =:

age = 30

Python works out the type for you; there is nothing else to declare. You can re-assign a variable any time.

Good names are lower-case with underscores: total_price, not TP. Names cannot start with a number or contain spaces.

To put variables inside text, use an f-string: put an f before the opening quote and wrap variables in curly braces.

name = "Asha"
print(f"Hello, {name}!")

Example

name = "Asha"
price = 120
quantity = 3

total = price * quantity
print(f"{name} owes {total} rupees")

quantity = quantity + 1
total = price * quantity
print(f"Now {name} owes {total} rupees")

Step-by-step walkthrough

1. Three variables are created.
2. total = price * quantity calculates 120 * 3 = 360 and stores it.
3. The f-string replaces {name} and {total} with their values: Asha owes 360 rupees.
4. quantity = quantity + 1 makes it 4. The old total is not updated automatically, so we recalculate to get 480.
5. The last line prints Now Asha owes 480 rupees.

Try it yourself

Add a discount = 20 variable and subtract it from the total before printing.

Common mistakes

• Using a name before assigning it, which gives a NameError.
• Forgetting the f in front of an f-string; the braces are then printed literally.
• Expecting a variable to update by itself. After quantity changes, total is still the old number until you recalculate.
• Names with spaces or dashes: total price and total-price are invalid.

Quick quiz

Summary

• name = value creates or updates a variable.
• Use lower_case_with_underscores for names.
• f-strings put values into text.

Next: the different kinds of values.