Python Loop else Clause

Most languages only let you attach else to an if. Python also lets you attach it to loops. The Python for else (and while else) clause runs a block of code only if the loop finished without hitting break. It sounds odd at first, but once you see the search pattern it solves, it becomes a neat tool.

What is the else clause on a loop?

A loop else is a block written right after a for or while loop, at the same indentation as the loop keyword. Python runs it when the loop ends normally:

• a for loop ends normally when it runs out of items;
• a while loop ends normally when its condition becomes False.

If the loop is ended by break, which you learned in break, continue and pass, the else block is skipped.

for song in ["Intro", "Sunrise", "Outro"]:
    print(f"Playing {song}")
else:
    print("Playlist finished.")

Playing Intro
Playing Sunrise
Playing Outro
Playlist finished.

No break happened, so the else block ran after the last song.

What happens when the loop breaks?

Add a break and the picture changes. Here, playback stops if a song is marked as explicit:

for song in ["Intro", "Explicit Mix", "Outro"]:
    if song == "Explicit Mix":
        print("Parental controls: playback stopped.")
        break
    print(f"Playing {song}")
else:
    print("Playlist finished.")

Playing Intro
Parental controls: playback stopped.

"Playlist finished." never appears, because the loop ended with break.

The for-else search pattern

The classic use of for...else is searching. You break as soon as you find a match, and the else handles the "not found" case. Here's a sign-up form checking whether a username is already taken:

taken_usernames = ["sky_runner", "pixel_cat", "jazzfan99", "neo_coder"]
new_username = "pixel_cat"

for name in taken_usernames:
    if name == new_username:
        print(f"Sorry, {new_username} is already taken.")
        break
else:
    print(f"{new_username} is available!")

Sorry, pixel_cat is already taken.

Change new_username to a name that isn't in the list, and the loop checks every item, never breaks, and runs the else:

taken_usernames = ["sky_runner", "pixel_cat", "jazzfan99", "neo_coder"]
new_username = "moon_baker"

for name in taken_usernames:
    if name == new_username:
        print(f"Sorry, {new_username} is already taken.")
        break
else:
    print(f"{new_username} is available!")

moon_baker is available!

Real-world example: is a number prime?

A prime number can only be divided evenly by 1 and itself. To test a number, try every possible divisor. If one divides evenly, it's not prime, so break. If none do, the else declares it prime.

number = 29

for divisor in range(2, number):
    if number % divisor == 0:
        print(f"{number} is not prime (divisible by {divisor}).")
        break
else:
    print(f"{number} is prime!")

29 is prime!

Put that check inside another loop and you can list every prime up to 20. The else belongs to the inner for, so it runs once for each number that had no divisor:

for number in range(2, 21):
    for divisor in range(2, number):
        if number % divisor == 0:
            break
    else:
        print(number, end=" ")

2 3 5 7 11 13 17 19

For 2, range(2, 2) is empty, so the inner loop runs zero times and the else runs straight away. That's correct, since 2 is prime.

How does while-else work?

A while loop's else runs when the condition turns False, and is skipped if the loop ends with break:

seconds = 3
while seconds > 0:
    print(seconds)
    seconds -= 1
else:
    print("Liftoff!")

3
2
1
Liftoff!

A more useful example is a login with limited attempts. Success breaks out of the loop; running out of attempts lets the loop end normally, which triggers the else:

attempts = 0

while attempts < 3:
    pin = input("Enter your PIN: ")
    if pin == "4071":
        print("Card unlocked.")
        break
    attempts += 1
    print("Wrong PIN.")
else:
    print("Card blocked. Please contact your bank.")

In this run the user typed 1234, 0000 and 9999, all wrong:

Enter your PIN: 1234
Wrong PIN.
Enter your PIN: 0000
Wrong PIN.
Enter your PIN: 9999
Wrong PIN.
Card blocked. Please contact your bank.

Had they typed 4071 at any point, the program would print Card unlocked. and skip the else.

Loop else vs a flag variable: when should you use it?

Without else, you'd track the result with a flag variable, a boolean that records whether something happened:

taken_usernames = ["sky_runner", "pixel_cat", "jazzfan99"]
new_username = "moon_baker"

found = False
for name in taken_usernames:
    if name == new_username:
        found = True
        break

if not found:
    print(f"{new_username} is available!")

moon_baker is available!

Both versions are correct. Here's how to choose:

A good rule: use loop else when the "not found" action sits right after the loop and nothing else needs to know. If you're working with a team that rarely sees it, a short comment like # no break next to the else helps.

Common mistakes with loop else

1. Indenting else under the if instead of the loop

for name in ["sky_runner", "pixel_cat"]:
    if name == "pixel_cat":
        print("Taken!")
        break
    else:
        print("Available!")

Available!
Taken!

This else belongs to the if, so it runs for every name that doesn't match. A loop else must line up with the for keyword.

2. Forgetting the break

Without a break inside the loop, a loop's else always runs, which makes it pointless. If your else runs when you didn't expect it, check that the break is actually reached.

3. Thinking else means "the loop didn't run"

for item in []:
    print(item)
else:
    print("else still runs")

else still runs

An empty loop ends normally with no break, so the else runs. Loop else is about break, not about whether the body ran.

Practice exercise

ingredients = ["flour", "butter", "sugar", "almonds", "eggs"]
nuts = ["almonds", "peanuts", "walnuts"]

for ingredient in ingredients:
    if ingredient in nuts:
        print(f"Warning: contains {ingredient}!")
        break
else:
    print("Nut-free recipe.")

Warning: contains almonds!

What's next?

Congratulations, you've finished the Control Flow chapter! You've been looping over collections of items for a while now, so it's time to learn them properly. Start Chapter 6 with Python lists.