while Loops

Big idea

A while loop repeats its block as long as a condition stays True. Use it when you don't know in advance how many rounds you'll need.

The idea

while condition:
    # repeat this

Each round, Python checks the condition. True: run the block, then check again. False: skip past the loop and carry on.

count = 1
while count <= 5:
    print(count)
    count += 1
print("Done!")

output
1
2
3
4
5
Done!

Watch it

Something must change

If nothing inside the loop ever makes the condition False, it runs forever — an infinite loop. In a terminal you'd stop it with Ctrl+C; here, the run times out.

n = 3
while n > 0:
    print(n)
print("Liftoff!")

output
3
2
1
Liftoff!

n never changes, so n > 0 stays True forever. Add n -= 1 inside the loop.

Unknown number of rounds

A classic while job: keep halving until something small is left. You couldn't easily say how many rounds that takes:

n = 100
steps = 0
while n > 1:
    n = n // 2
    steps += 1
print(steps)

6

100 → 50 → 25 → 12 → 6 → 3 → 1: six steps.

Reading until a stop word

while is also the natural shape for "keep reading until the user says stop":

total = 0
number = int(input())
while number != 0:
    total += number
    number = int(input())
print("Total:", total)

input
5
10
20
0

output
Total: 35

Think of it like

"Keep stirring while the sauce is lumpy." You don't know how many stirs it'll take — you just check after each one.

Key terms

• while loop — Repeats while its condition is True.
• Iteration — One round of a loop.
• Infinite loop — A loop whose condition never becomes False.