Python while Loops in Depth: Conditions, Sentinels and Input Loops

A for loop repeats once per item. A while loop repeats as long as a condition is true, which makes it the right tool when you do not know in advance how many repetitions you need: until the user types "quit", until savings reach a goal, until a guess is correct.

The basic pattern#

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

1
2
3
4
5
Done

Every working while loop has three parts:

1. Set up a variable before the loop (count = 1).
2. Test it in the condition, which is checked before every repetition.
3. Change it inside the loop, so the condition eventually becomes false.

Looping until a goal is reached#

How many years until ₹1,000 doubles at 8% interest per year?

balance = 1000
year = 0
while balance < 2000:
    balance *= 1.08
    year += 1
print(year, round(balance, 2))

10 2158.92

You could not easily write this with for, because the number of years is the answer, not the input.

Counting digits#

n = 1234
digits = 0
while n > 0:
    n //= 10
    digits += 1
print(digits)

4

Each pass removes the last digit with floor division until nothing is left.

Sentinel values#

A sentinel is a special value that means "stop". Here, -1 ends the input:

total = 0
while True:
    value = int(input("Number (-1 to stop): "))
    if value == -1:
        break
    total += value
print("Total:", total)

Number (-1 to stop): 5
Number (-1 to stop): 12
Number (-1 to stop): -1
Total: 17

while True runs forever on its own; break is the only way out. This shape, "loop forever, break when done", is very common for reading input.

A guessing game#

secret = 6
guess = None
attempts = 0

while guess != secret:
    guess = int(input("Guess (1-10): "))
    attempts += 1
    if guess < secret:
        print("Too low")
    elif guess > secret:
        print("Too high")

print(f"Correct! {attempts} attempts.")

Guess (1-10): 5
Too low
Guess (1-10): 8
Too high
Guess (1-10): 6
Correct! 3 attempts.

guess starts as None so the condition is true on the first check. For a real game, pick the secret with random.randint(1, 10) (see Modules).

Looping while a collection has items#

Because non-empty lists are truthy, while items: keeps going until the list is empty:

stack = [1, 2, 3]
while stack:
    print("popped", stack.pop())

popped 3
popped 2
popped 1

Infinite loops#

If nothing inside the loop ever makes the condition false, it never ends. This loop forgets to update count:

count = 1
while count <= 5:
    print(count)
    # missing: count += 1

It prints 1 forever. Press Ctrl + C in the terminal to stop it. When a loop seems stuck, check that the variable in the condition really changes.

for or while?#

Common mistakes#

Practice#

n = 6
steps = 0
sequence = [n]

while n != 1:
    n = n // 2 if n % 2 == 0 else 3 * n + 1
    steps += 1
    sequence.append(n)

print(sequence, steps)

[6, 3, 10, 5, 16, 8, 4, 2, 1] 8

Key takeaway#