Python sets: membership and deduplication do not preserve input order

Operation contract

The delivery fixture finds identifiers already processed and identifiers still pending. It sorts the results solely to make the displayed output deterministic; the set itself has no promised input order. Set membership is useful when deciding whether a duplicate has already been seen, but it removes multiplicity information.

Failure and ownership boundary

If two identical receipt IDs indicate two separate rejected events, a set of IDs cannot tell how often the failure happened. Use a counter or retained events when frequency is part of the question. Python collection exercise: reject repeated receipt IDs before committing state and Python heapq top-k: define ties before ranking frequencies select different evidence.

Working program

received_ids = {41, 42, 43}
processed_ids = {41, 44}
print(sorted(received_ids & processed_ids))
print(sorted(received_ids - processed_ids))
print(len({41, 41, 42}))

Output

[41]
[42, 43]
2

Costs and limits

Expected membership has constant hash-table work for ordinary keys; retaining n distinct values takes O(n) storage. Sorting k displayed IDs adds O(k log k) time and output storage.

Common Mistakes

• Do not promise input order from a set.
• Deduplication loses frequency.

Connected lessons

Python dictionaries: insertion order and duplicate-key replacement, Python collection exercise: reject repeated receipt IDs before committing state, Python heapq top-k: define ties before ranking frequencies.