Python heapq.merge: combine sorted feeds without sorting every value again

Operation contract

Two bounded receipt feeds are sorted by integer timestamp. The merge returns a timestamp-ordered stream of pairs, then the fixture materializes it for display. The inputs stay unchanged. If a feed breaks its sorted contract, merge does not repair it and its output may be wrong; the wrapper rejects such a feed before yielding.

Failure and ownership boundary

The wrapper uses concrete lists so it can validate before a caller consumes any result. A real one-use stream cannot be fully validated without buffering or a staged failure policy. Python exercise: merge two sorted feeds and preserve left-first ties, Python iterators: exhaustion and repeatable collection ownership and Python heapq top-k: define ties before ranking frequencies connect the data-structure choices.

Working program

from heapq import merge

def merged_events(*feeds):
    if len(feeds) > 4 or any(type(feed) is not list or len(feed) > 20 or any(type(row) is not tuple or len(row) != 2 or type(row[0]) is not int or type(row[1]) is not str for row in feed) or any(a[0] > b[0] for a, b in zip(feed, feed[1:])) for feed in feeds):
        raise ValueError("sorted bounded feeds")
    return list(merge(*feeds, key=lambda row: row[0]))

print(merged_events([(1, "R41"), (4, "R43")], [(2, "R42"), (5, "R44")]))
try:
    merged_events([(3, "R43"), (1, "R41")])
except ValueError:
    print("unsorted feed rejected")

Output

[(1, 'R41'), (2, 'R42'), (4, 'R43'), (5, 'R44')]
unsorted feed rejected

Costs and limits

For n total items across k feeds, merging costs O(n log k) comparisons and O(k) heap storage. This wrapper materializes O(n) output and scans inputs for validation first.

Common Mistakes

• heapq.merge assumes every feed is sorted.
• Materializing the iterator removes its streaming-memory benefit.

Connected lessons

Python exercise: merge two sorted feeds and preserve left-first ties, Python iterators: exhaustion and repeatable collection ownership, Python heapq top-k: define ties before ranking frequencies.