While Loops

A for loop repeats once for each item in a collection. Sometimes you don't know in advance how many times to repeat. You just want to keep going while something is true: until the user types the right password, until the money runs out, until the game ends. That's what a while loop does.

The while loop

A while loop checks a condition. If it's True, the body runs, and then Python checks the condition again. Once the condition is False, the loop ends.

count = 1

while count <= 5:
    print("Count is", count)
    count += 1

print("Loop finished")

Step by step, Python does this:

Three things make this work:

1. A variable is set before the loop (count = 1).
2. The condition uses it (count <= 5).
3. The body changes it (count += 1).

The condition is only checked at the start of each pass, not in the middle of the body. And if it's False the very first time (say count = 10), the body runs zero times and Python goes straight to the line after the loop.

Infinite loops

If the condition never becomes False, the loop runs forever. That's called an infinite loop, and it's the most common while bug. Forgetting the count += 1 line above would cause one.

Try this one, and press Stop to end it:

n = 1
while n > 0:
    n += 1

Tip: If your program seems frozen, check that something inside the loop changes the condition.

Looping until something happens

while shines when the number of repeats depends on the data. How many years does it take for savings to double at 5% interest?

balance = 1000
years = 0

while balance < 2000:
    balance *= 1.05
    years += 1

print(f"It takes {years} years to reach {balance:.2f}")

balance *= 1.05 multiplies the balance by 1.05 each year, which adds 5%. After year 1 it's 1050.00, after year 2 1102.50, and so on. Nobody had to work out the number of years in advance; the loop simply stops on the first check where balance < 2000 is False, after 15 years.

Asking until the input is valid

Keep asking until the user gives an acceptable answer. Run this and try typing a number outside the range first:

rating = int(input("Rate us from 1 to 5: "))

while rating < 1 or rating > 5:
    print("Please enter a number between 1 and 5.")
    rating = int(input("Rate us from 1 to 5: "))

print(f"Thanks for rating us {rating}!")

Notice that the question appears twice. The first input(), before the loop, gets an answer to check. The second, inside the loop, asks again after each wrong answer. Without it, rating would never change, and the loop would repeat forever. A valid answer the first time skips the loop body entirely.

while True with break

Another common style is an endless while True: loop that you leave with break when you're done. while True: means "repeat forever", because the condition can never be False. The only way out is break, which ends the loop immediately and jumps to the first line after it. It's handy when the exit check belongs in the middle of the loop:

total = 0

while True:
    entry = input("Enter a number (or 'done'): ")
    if entry == "done":
        break
    total += float(entry)

print(f"Total: {total}")

The order inside the loop matters. The if entry == "done" check comes before float(entry), because float("done") would be a ValueError. Each pass: ask, check whether to stop, and only then convert and add.

continue

continue skips the rest of the current pass and jumps back to the condition:

n = 0

while n < 10:
    n += 1
    if n % 2 == 0:
        continue      # skip evens
    print(n, end=" ")

This prints 1 3 5 7 9. Notice where n += 1 is: at the top of the body, before the continue. If it came after the continue, it would be skipped for even numbers, n would get stuck at 2, and the loop would never end. In a while loop, make sure the line that changes the condition can't be skipped.

attempts = 0

while attempts < 3:
    password = input("Password: ")
    if password == "python":
        print("Welcome!")
        break
    attempts += 1
else:
    print("Too many attempts. Locked out.")

A guessing game

Here's everything together. The random module picks a secret number. random.randint(1, 20) gives a whole number from 1 to 20, and unlike range(), both ends are included:

import random

secret = random.randint(1, 20)
guesses = 0

print("I'm thinking of a number between 1 and 20.")

while True:
    guess = int(input("Your guess: "))
    guesses += 1

    if guess < secret:
        print("Too low!")
    elif guess > secret:
        print("Too high!")
    else:
        print(f"Correct! You got it in {guesses} guesses.")
        break

for or while?

If you can write it as a for loop, that's usually simpler and safer, because a for loop can't run forever by accident.

Exercises

Exercise 1: Powers of two

Use a while loop to print every power of 2 that is less than 1000: 1 2 4 8 ... 512.

power = 1

Exercise 2: Digit sum

Add up the digits of number using a while loop, % 10 (last digit) and // 10 (remove the last digit). For 4719 it should print 21.

number = 4719
total = 0

Exercise 3: Password check

Keep asking for the password until the user types python123, then print Access granted. Allow at most 3 attempts. After 3 wrong attempts, print Account locked.

attempts = 0

The else on a loop runs only if the loop finished without hitting break. It's a neat trick for "searched everything, didn't find it" situations.

Exercise 4: Fix the infinite loop

This loop is meant to print 10, 8, 6, 4, 2, but it never stops. Press Run, then Stop, and fix it.

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

Summary

• while condition: repeats the body as long as the condition is true.
• Something inside the loop must eventually make the condition false, or the loop runs forever.
• while is ideal when you don't know the number of repeats in advance, such as input validation or games.
• while True: with break exits from the middle of a loop. continue skips to the next pass.
• Prefer for when you're going through a collection or a fixed range.