for/else and while/else

Python loops can have an else block. This is unusual, but it can be useful when you want one block to run only if the loop did not exit with break.

What is happening?

In a for or while loop, the else block runs if the loop finishes normally.

items = [1, 3, 5]

for item in items:
    if item % 2 == 0:
        print("Found an even number")
        break
else:
    print("No even numbers found")

Output:

No even numbers found

If the loop hits break, the else block is skipped.

Why it is useful

This pattern can remove an extra flag variable in search-style code:

for user in users:
    if user.id == target_id:
        target = user
        break
else:
    raise LookupError("User not found")

That makes the success path and the "not found" path live together.

Use it carefully

for/else is powerful, but it is also unfamiliar to many readers. It works best when:

• the loop is short
• the break condition is clear
• the else block naturally means "nothing matched"

If the logic is complex, an explicit helper function or a simpler structure may be easier to read.

Rules of thumb

• Read loop else as "no break happened."
• Use it for short search-style loops.
• Avoid it when the control flow is already complex.
• Prefer clarity over cleverness if teammates may misread the pattern.