Python Tip: for...else

Run code only when a loop reaches the end

Learn how to use for…else to tell whether a loop finished or stopped early with break.

Python Tip: for...else

Do you need to run code after a for loop, but only if the loop reaches the end?

Python has a special pattern for this: for...else.

Let’s see how it works and when it helps!

The Flag Pattern

Imagine we want to write a script that checks whether a list contains at least one odd number.

We can use a for loop with a True/False variable, or flag, called found_odd to remember whether the list contains an odd number. We can use break to stop checking as soon as we find one, since checking the rest would waste time.

numbers = [2, 4, 7, 8]
found_odd = False

for number in numbers:
    if number % 2 != 0:
        found_odd = True
        break

if found_odd:
    print("Found an odd number")
else:
    print("All numbers are even")

found_odd starts as False. The loop changes it to True if it finds an odd number. The final if uses that value to decide what to print.

A Shorter Way

The flag version works, but it needs an extra variable and another if statement after the loop.

for...else handles the same two outcomes with less code.

for item in items:
    if stop_condition:
        break
else:
    print("The loop reached the end")

The else lines up with for, so it belongs to the loop. If break stops the loop, Python skips the else. If the loop reaches the end without break, Python runs the else.

Case 1: The Loop Stops Early

Now let’s use this pattern for our odd-number check with a list that contains an odd number.

numbers = [2, 4, 7, 8]

for number in numbers:
    if number % 2 != 0:
        print("Found an odd number")
        break
else:
    print("All numbers are even")

Found an odd number

The loop finds 7, so the first print() runs. Then break stops the loop, so Python skips the else. The second print() never runs.

Case 2: The Loop Finishes

Now let’s look at a list with no odd numbers. Since there is no odd number, the loop never reaches break.

numbers = [2, 4, 6, 8]

for number in numbers:
    if number % 2 != 0:
        print("Found an odd number")
        break
else:
    print("All numbers are even")

All numbers are even

The loop checks every number and reaches the end. Python then runs the else, so the second print() says that all the numbers are even.

One Last Note

for...else makes this pattern shorter, but many Python developers have not seen it before.

If you are writing code for a broad community or for newer developers, the longer found_odd version may be easier to understand.

Use the version that will be clearest to the people reading your code.

Wrap-Up

Use for...else when you want to run code only if a loop reaches the end.

If break stops the loop, Python skips the else.

If the loop reaches the end without break, Python runs the else.

It can make this pattern shorter, but the clearest code is not always the shortest code.

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- Shep4