Variables

Why this matters

Programs work with information: prices, names, scores. A variable is a name you give to a piece of information so you can use it again later.

The idea

You create a variable with =. The name goes on the left, the value on the right:

age = 30
name = "Sam"

This is called assignment. Read it as “let age be 30”, not as the equals sign from maths class.

Once a variable exists you can use its name anywhere you could use the value:

age = 30
print(age)        # shows 30
print(age + 1)    # shows 31

You can do arithmetic with + (add), - (subtract), * (multiply) and / (divide). A variable can also hold the result of a calculation:

apples = 3
pears = 4
fruit = apples + pears
print(fruit)      # shows 7

Lines starting with # are comments: Python ignores them, so you can leave notes for humans.

Try it

Use the editor to create a few variables of your own and print them. Try changing a value and running the program again. Everything that uses that variable changes with it.

Your turn

A coffee shop sells coffee for 3.75 dollars a cup, and you are buying 4 cups. The starter code already has the variables price and quantity.

1. Create a variable called total that holds price multiplied by quantity.
2. Print total.

Use the variables rather than typing the answer yourself. That way the program still works if the price changes.

Recap

• name = value creates (or updates) a variable.
• A variable can hold the result of a calculation.
• Use * to multiply and +, -, / for the other basic operations.

Common mistakes

• Using a variable before creating it: print(total) before total = ... fails with NameError.
• Spelling or capitalisation differences: Total and total are different names.
• Putting the value on the left: 5 = x is a SyntaxError. The name always goes on the left.