Python else on Loops

3Python Loop else: explained step by step

Loops can have an else block, and it does not behave like the else you know from if. It surprises almost everyone the first time, which is exactly why it deserves its own lesson. The short version: the else block runs when the loop finished on its own, and it is skipped when break ended the loop.

The rule in action

Here are two searches of the same guest list. The first one finds its guest and breaks, so the else is skipped. The second never finds its guest, runs out of items, and the else runs.

guests = ["Ana", "Raj", "Lee"]

for guest in guests:
    if guest == "Raj":
        print("Found Raj")
        break
else:
    print("Raj is not on the list")

for guest in guests:
    if guest == "Max":
        print("Found Max")
        break
else:
    print("Max is not on the list")

Read it as "no break"

The word else is a poor name here, so give it a better one in your head: no break. Read the block as "if the loop ended with no break, do this". For a search, that means "I looked at every item and never found a match", which is exactly the moment you want to report "not found".

Only break skips the else. A continue does not, and neither does the loop simply running out of items. (A return or an error leaves even more than the loop, so the else does not run then either.)

An empty sequence still "finished"

A loop over nothing runs zero rounds, but it did finish normally, and no break happened. So the else block runs:

orders = []
for order in orders:
    print("Packing", order)
else:
    print("Loop finished, even with no orders")

Keep this in mind when an empty list should be treated differently from "searched and found nothing". In that case check if not orders: first.

Classic use: is this number prime?

A prime number has no divisor between 2 and itself. The loop tries each possible divisor. If one fits, the number is not prime and there is no point trying the rest, so break. If none fit, the loop runs out and the else says the number is prime.

number = 29
for divisor in range(2, number):
    if number % divisor == 0:
        print(number, "is divisible by", divisor)
        break
else:
    print(number, "is prime")

This simple version has a hole: for 0 and 1, range(2, number) is empty, the loop "finishes", and the else would claim they are prime. Handle numbers below 2 before the loop.

else works on while loops too

For a while loop, the else block runs when the condition becomes False, and is skipped when a break ends the loop earlier.

queue = ["Ana", "Raj", "Lee"]
vip = "Lee"
position = 0
while position < len(queue):
    if queue[position] == vip:
        print(vip, "is at position", position)
        break
    position += 1
else:
    print(vip, "is not in the queue")

The most common confusion: whose else is it?

The indentation decides who owns an else. An else lined up with the if belongs to that if, and it runs on every round where the condition was False. An else lined up with for or while belongs to the loop.

guests = ["Ana", "Raj"]
for guest in guests:
    if guest == "Max":
        print("Found Max")
    else:
        print(guest, "is not Max")

This prints one line per guest, because the else belongs to the if. For the "not on the list" message, you want it to move left so it lines up with for.

When something else is clearer

for ... else is compact, but many programmers rarely use it, so readers may need a moment to decode it. A short comment such as # no break helps. Two alternatives are worth knowing:

• A flag variable is clearer when you need to know the result later, or when several things happen if nothing is found.
• A function with return (you will meet functions soon) lets you return as soon as you find a match, with the "not found" case after the loop and no else at all.

found = False
for guest in guests:
    if guest == "Max":
        found = True
        break
if not found:
    print("Max is not on the list")