Python for Loop: Process Items, Break and Use Loop Else

Start with a small, explicit task

Inspect a sequence of fictional package labels until the first damaged package. Return the labels inspected before the stop. Repeat with no damage and with an empty queue. The task is about control flow, so there are no file reads, services or random results to distract from the trace.

Run the complete example

This original example was checked with Python 3.12.14. Save it as python_for_loop.py and run it with Python 3.12. It uses only the standard library. The assertions check the stated results.

def inspect_packages(labels):
    visited = []
    for label in labels:
        visited.append(label)
        if label == "damaged":
            outcome = "stopped"
            break
    else:
        outcome = "complete"
    return visited, outcome
stopped = inspect_packages(["sealed", "damaged", "sealed"])
complete = inspect_packages(["sealed", "sealed"])
empty = inspect_packages([])
assert stopped == (["sealed", "damaged"], "stopped")
assert complete == (["sealed", "sealed"], "complete")
assert empty == ([], "complete")
print(stopped)
print(complete)
print(empty)

Explain the result

The damaged label is included in visited because it is appended before the condition is tested. The following sealed label is never visited: break transfers control beyond the loop and skips its else block. Moving the append below the check would produce a different trace, so statement order is part of the behavior.

In the complete case, every label is visited and no break occurs. The empty input also reaches else because nothing causes an early break. The name complete is the rule chosen for this exercise; an actual workflow might treat an empty inspection set differently. Define that business rule rather than deriving it from Python syntax.

Avoid these mistakes

Indent else at the level of for when it is a loop else. Be cautious about adding or removing items from the same container being traversed: the resulting visits may no longer match your intended queue. Use a separate output collection when that makes the operation clearer.