Variables

Big idea

A variable is a name that refers to a value. = attaches the name on the left to the value on the right.

The idea

age = 20
name = "Ada"

Read = as "is now": "age is now 20". It isn't the maths equals sign — it's an instruction, and it runs right to left: work out the value on the right, then attach the name on the left.

Once a name exists, use it anywhere you'd use the value:

name = "Ada"
age = 20
print(name, "is", age)
print(age + 1)

output
Ada is 20
21

There's no "type" to declare, unlike C's int age = 20;. Python works out what kind of value it is by looking at it.

Changing a variable

A variable can be pointed at a new value any time. The old value is forgotten:

score = 10
score = score + 5
score = score * 2
print(score)

30

score = score + 5 looks impossible in maths, but in Python it means "work out score + 5 (that's 15), then make score refer to that".

Names are labels, not boxes

In Python, a variable is a label stuck on a value. Two names can be stuck on the same value:

a = 7
b = a      # b now refers to the same 7
a = 100    # a is moved to a new value; b still refers to 7

For numbers and text this never surprises you. It matters later, with lists — keep this picture in mind.

Watch it

Naming rules

• Letters, digits, and _ only — no spaces or dashes.
• Can't start with a digit: 2nd_place is an error, place_2 is fine.
• Case matters: Age and age are different variables.
• Can't be a keyword Python reserves, like if, for, class, True.

The Python habit is snake_case: lowercase words joined by underscores — total_price, first_name.

2nd_place = "Grace"
print(2nd_place)

output
Grace

Several at once

x, y, z = 1, 2, 3       # x is 1, y is 2, z is 3
a = b = 0               # both start at 0

Think of it like

A name tag. Writing age = 20 sticks the tag "age" on the number 20. Later age = 21 peels it off and sticks it on 21.

Key terms

• Variable — A name that refers to a value.
• Assignment — Attaching a name to a value with =.
• snake_case — The Python naming style: lower_case_with_underscores.