while Loops

Computers are good at doing the same thing many times without getting bored. A loop repeats a block of code. Python has two kinds: the for loop, which goes through a collection of items, and the while loop, which keeps going as long as a condition stays true.

In this lesson you will write while loops that stop at the right moment, exit early with break, skip a round with continue, use the unusual else clause, and learn how to escape from a loop that never ends.

The while loop

A while loop looks like an if statement: a condition, a colon and an indented block. The difference is that after the block runs, Python goes back and checks the condition again. It repeats until the condition is false.

countdown = 5

while countdown > 0:
    print(countdown)
    countdown -= 1

print("Lift off!")

5
4
3
2
1
Lift off!

Each pass through the block is called an iteration. Trace it: countdown starts at 5, the condition is true, Python prints 5 and subtracts 1. This repeats until countdown is 0, the condition 0 > 0 is false, and the program continues after the loop.

Repeat until a goal is reached

while is the right loop when you do not know in advance how many rounds you need. How many years does it take for a fixed deposit to double at 7% interest a year?

amount = 10000
target = 20000
rate = 0.07
years = 0

while amount < target:
    amount = amount * (1 + rate)
    years += 1

print(f"It takes {years} years to reach ₹{amount:,.2f}")

It takes 11 years to reach ₹21,048.52

Nobody told the loop to run 11 times. It simply kept adding interest until the amount crossed the target.

Infinite loops

If the condition never becomes false, the loop never ends. This is an infinite loop, and it is the most common while loop bug. It usually happens when you forget to update the variable in the condition:

count = 1
while count <= 5:
    print("Hello")
    # forgot: count += 1

This prints "Hello" forever. If it happens in a terminal, press Ctrl+C to stop the program. In the browser editor, stop or reload the page. Before you run a while loop, ask yourself: "what changes inside the loop, and will it make the condition false?"

break and continue

Two statements change the normal flow of a loop:

• break leaves the loop immediately, even if the condition is still true.
• continue skips the rest of the current iteration and jumps back to the condition.

A common pattern is while True: with a break inside. The loop is written to run forever, and break decides when it stops:

correct_pin = "4321"
attempts = ["1111", "", "4321", "9999"]
tries = 0

while True:
    pin = attempts[tries]
    tries += 1
    if pin == "":
        print("Empty PIN, please type again")
        continue
    if pin == correct_pin:
        print(f"PIN accepted after {tries} tries")
        break
    print("Wrong PIN")

Wrong PIN
Empty PIN, please type again
PIN accepted after 3 tries

The list attempts stands in for what a user would type. When the PIN is empty, continue skips the checks and goes straight to the next attempt. When it is correct, break ends the loop, so the fourth attempt is never read.

The else clause on a loop

A while loop can have an else block. It runs when the loop ends normally, because the condition became false. It does not run if the loop was stopped by break. That makes it a neat way to say "the search found nothing".

number = 97
divisor = 2

while divisor * divisor <= number:
    if number % divisor == 0:
        print(number, "is not prime; it divides by", divisor)
        break
    divisor += 1
else:
    print(number, "is prime")

97 is prime

No divisor was found, so the loop ended normally and the else block ran. Change number to 91 and the break runs instead (91 is 7 times 13), so else is skipped. Many programmers find loop else surprising; if it confuses your team, a Boolean variable such as found = False does the same job.

Loops that empty a collection

Because an empty list is falsy, while items: means "while there is anything left". This is perfect for working through a queue:

orders = ["masala dosa", "idli", "filter coffee"]

while orders:
    current = orders.pop(0)
    print(f"Serving {current}, {len(orders)} left")

Serving masala dosa, 2 left
Serving idli, 1 left
Serving filter coffee, 0 left

pop(0) removes and returns the first item, which you will see again in Lists. The list shrinks each time, so the loop is sure to end.

You will also meet the walrus operator from Booleans & Operators in while loops. This common pattern keeps asking until the user types "quit":

while (command := input("> ")) != "quit":
    print("You typed", command)

while or for?

Use a for loop when you are going through items you already have: every name in a list, every line in a file, every number from 1 to 10. Use a while loop when you repeat until something happens: a target is reached, a user quits, a correct answer arrives. You will learn for loops in for Loops.

Common mistakes

• Forgetting to update the condition variable. The loop never ends. Make sure something inside the loop moves it towards false.
• Off-by-one errors. while count < 5 with count starting at 1 runs 4 times, not 5. Trace the first and last iteration by hand.
• Using while with an index to walk through a list. It works, but a for loop does it with less code and no risk of an infinite loop.
• Expecting else to run after break. The loop's else runs only when the loop ends without break.

Key points

• while condition: repeats its block as long as the condition is true.
• Something inside the loop must change so that the condition eventually becomes false.
• Press Ctrl+C in a terminal to stop an infinite loop.
• break leaves the loop now; continue jumps to the next iteration.
• A loop's else block runs only if the loop finished without break.
• while items: loops until a list is empty; use for to visit items you already have.

saved = 0
monthly = 500
months = 0

while saved < 10000:
    saved += monthly
    monthly += 100
    months += 1

print(f"{months} months, total saved ₹{saved:,}")

11 months, total saved ₹11,000