while loops

A loop repeats a block of code. The fundamentals lesson on repetition explains when to choose a loop that counts and when to choose one that waits for a condition. Python’s while is the second kind: it keeps going while its condition is truthy.

The basic loop

n = 3
while n > 0:
    print(n)
    n -= 1
print("Liftoff")

3
2
1
Liftoff

The loop follows the same shape as if: a condition, a colon, and an indented block. Python checks the condition before every pass, including the first. If it is false from the start, the body never runs. n -= 1 is shorthand for n = n - 1, and the block must change something that the condition depends on, or the loop never ends.

Sentinel loops

Often you do not know how many times to repeat. You keep asking until the user types a special sentinel value. The usual Python shape is while True with a break. break leaves the loop immediately.

This program adds up numbers until the user types nothing and presses Enter:

total = 0
count = 0

while True:
    text = input("Number (blank to stop): ")
    if text == "":
        break
    total += int(text)
    count += 1

print("Total:", total, "from", count, "numbers")

Here the user typed 10, 20 and then just pressed Enter:

Number (blank to stop): 10
Number (blank to stop): 20
Number (blank to stop):
Total: 30 from 2 numbers

while True looks alarming, but it is deliberate: the loop has an exit point in the middle of the body, and it is a break. Writing the input() call once, inside the loop, avoids repeating it before the loop to prime the condition.

continue

continue is the other control word. It skips the rest of the current pass and jumps back to the condition. A guessing game uses it to reject non-numeric input without counting an attempt:

secret = 42
attempts = 0

while True:
    text = input("Guess a number from 1 to 100 (q to quit): ")
    if text == "q":
        print("Bye.")
        break
    if not text.isdigit():
        print("Please type a whole number.")
        continue
    attempts += 1
    guess = int(text)
    if guess < secret:
        print("Too low.")
    elif guess > secret:
        print("Too high.")
    else:
        print(f"Correct, after {attempts} attempts.")
        break

Here the typed answers were 50, hello, 25 and 42:

Guess a number from 1 to 100 (q to quit): 50
Too high.
Guess a number from 1 to 100 (q to quit): hello
Please type a whole number.
Guess a number from 1 to 100 (q to quit): 25
Too low.
Guess a number from 1 to 100 (q to quit): 42
Correct, after 3 attempts.

Notice that "hello" did not count: the attempts total is 3, not 4. The program has two break exits, one for quitting and one for winning.

Debugging a loop with print

When a loop does something you did not expect, the quickest tool is to print the variables that change on every pass. The = form of an f-string, from the f-strings lesson, prints the name next to the value:

n = 6
while n != 1:
    print(f"{n=}")
    if n % 2 == 0:
        n = n // 2
    else:
        n = 3 * n + 1
print("reached", n)

n=6
n=3
n=10
n=5
n=16
n=8
n=4
n=2
reached 1

Each line shows the value of n at the start of a pass, so you can follow the loop step by step and see where it stops behaving as you expect. Delete the print when you are done.

For longer programs, Python can pause on a line and let you inspect variables interactively. Putting breakpoint() on a line starts the built-in debugger there. A later lesson shows how to use it.

Common pitfalls

• Forgetting to update the loop variable. while n > 0: with no n -= 1 runs forever. Stop a runaway program in the terminal with Ctrl+C.
• Off-by-one conditions. while n > 0 counts 3, 2, 1. while n >= 0 would also print 0. Trace through a small case by hand, or with a print.
• Changing the wrong variable. If you update count but the condition tests total, nothing converges.
• continue that skips the update. If the step n += 1 comes after a continue, the loop can stick on the same value forever. Update before you continue.
• Comparing floats for equality. while x != 1.0 with x += 0.1 may never hit exactly 1.0. Use < or <=.

n = 10
while n > 0:
    print(n)
    n -= 3

10
7
4
1

total = 0

while True:
    number = int(input("Number (0 to stop): "))
    if number == 0:
        break
    total += number

print("Total:", total)

Number (0 to stop): 5
Number (0 to stop): 3
Number (0 to stop): 8
Number (0 to stop): 0
Total: 16

secret = 42
max_attempts = 7
attempts = 0
won = False

while attempts < max_attempts:
    text = input("Guess: ")
    if not text.isdigit():
        print("Please type a whole number.")
        continue
    attempts += 1
    guess = int(text)
    if guess == secret:
        print(f"Correct, after {attempts} attempts.")
        won = True
        break
    print("Too low." if guess < secret else "Too high.")

if not won:
    print(f"Out of guesses. The number was {secret}.")

Guess: 10
Too low.
Guess: 99
Too high.
Guess: abc
Please type a whole number.
Guess: 41
Too low.
Guess: 43
Too high.
Guess: 50
Too high.
Guess: 5
Too low.
Guess: 60
Too high.
Out of guesses. The number was 42.

Summary

• while condition: repeats its block as long as the condition is truthy, testing before each pass.
• while True plus break is the standard shape for sentinel loops and loops with several exits.
• break leaves the loop; continue skips to the next pass. Make sure something in the body moves the loop towards its end, and use print(f"{x=}") to trace it.

Next: for loops and range.