heapq.merge silently drops an input stream when key raises StopIteration

[augusto-rehfeldt]

Bug report

Bug description:

Documented behaviour: heapq.merge docstring: "Similar to sorted(itertools.chain(*iterables)) but returns a generator" and "If key is not None, applies a key function to each element to determine its sort order." sorted propagates an exception from the key function instead of silently discarding input.

Expected: Consumption raises StopIteration or RuntimeError caused by StopIteration.

Actual: Consumption succeeds with [2, 4], silently omitting 1 and 3.

import heapq
from itertools import chain

data = {"lists": [[1, 3], [2, 4]], "raise_StopIteration_on": 1}
lists = data["lists"]

def key(x):
    if x == data["raise_StopIteration_on"]:
        raise StopIteration
    return x

def outcome(f):
    try:
        return ("result", f())
    except Exception as e:
        if isinstance(e, RuntimeError) and isinstance(e.__cause__, StopIteration):
            e = e.__cause__
        return ("exception", type(e).__name__)

if not all(isinstance(xs, list) and all(type(x) is int for x in xs)
           and xs == sorted(xs) for xs in lists):
    print("REFUTATION REJECTED:", "input is not ascending integer lists")
else:
    actual = outcome(lambda: list(heapq.merge(*lists, key=key)))
    expected = outcome(lambda: sorted(chain.from_iterable(lists), key=key))
    if expected == ("exception", "StopIteration") and actual[0] == "result":
        print("REFUTATION CONFIRMED:", data, "actual:", actual, "expected:", expected)
    else:
        print("REFUTATION REJECTED:", "no silent successful omission demonstrated",
              data, "actual:", actual, "expected:", expected)

Output on Python 3.14.6 (Windows-11-10.0.26220-SP0), standard library heapq:

REFUTATION CONFIRMED: {'lists': [[1, 3], [2, 4]], 'raise_StopIteration_on': 1} actual: ('result', [2, 4]) expected: ('exception', 'StopIteration')

This report was found and written by an automated property-testing tool I run (bugforge). The reproducer above was executed and its output is pasted unedited; no person reviewed the report before it was filed. The search script is in https://github.com/augusto-rehfeldt/bugforge-results/tree/main/heapq-20261003-061723-c1

CPython versions tested on:

3.14

Operating systems tested on:

Windows

[ZeroIntensity]

See #158748 (comment).