Constants and Variables

A variable is a named value that can change while a program runs. A constant is a named value that is set to a value once and keeps that value for the whole run. The program reads a constant by its name, and no line in the program gives it a different value.

Use a constant when a value is fixed for the whole run, and a variable when the value changes during the run or is different each time the program runs.

Deciding which to use

Ask whether the value is fixed for the whole run or changes. If it is fixed, a constant fits. If it changes, or is different each time the program runs, a variable fits. A value that happens not to change during one run is not automatically a constant: if it can be different the next time the program runs, it is a variable. The choice is about the role the value plays in the program, not its data type.

For a coach trip, the number of seats on the coach is fixed for the run, so it can be a constant. The number of seats booked goes up as people book, so it is a variable.

COACHSEATS = 48  # fixed for the whole run
seats_booked = 33  # this will change

spaces_left = COACHSEATS - seats_booked
print(spaces_left)

seats_booked = 41
spaces_left = COACHSEATS - seats_booked
print(spaces_left)

Output:

15
7

In the program, COACHSEATS is set once and used in both calculations. seats_booked is given a new value before the second calculation, so it is a variable. The constant is never reassigned; the variable is.