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The docs and both docstrings say nlargest and nsmallest are equivalent to sorted(iterable, key=key)[:n] and sorted(iterable, key=key, reverse=True)[:n], but that is wrong when n is negative. nsmallest(-1, [5, 3, 8, 1, 9, 2]) returns [] while sorted(...)[:-1] returns [1, 2, 3, 5, 8]. nsmallest builds its first chunk with zip(range(n), it) and nlargest with zip(range(0, -n, -1), it). Both are empty when n is negative, so both return [] at the `if not result` check. n == 0 agrees, so it is only negative n that differs. Changed the slice to [:max(n, 0)] rather than adding a caveat, so the equivalence stays an expression you can actually run. Applies to Doc/library/heapq.rst and to the two docstrings in Lib/heapq.py. test_nsmallest and test_nlargest already check this equivalence, but only with non-negative n, so negative values are added there and the expected expression is corrected to match.
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The docs and the docstrings for
nlargestandnsmallestboth say:That is not true for a negative
n:So
nlargestandnsmallestare not equivalent to that slice when n is a negative number.nsmallestbuilds its first chunk with[(elem, i) for i, elem in zip(range(n), it)].range(-1)is empty, and thats whyresultends up empty and the function returns[]right there at theif not resultcheck.nlargestends up the same way, it just useszip(range(0, -n, -1), it)instead, which is also empty when n is negative.n == 0is fine, both give[], so its only the negative n that differs.Rather than adding a caveat i changed the slice to
[:max(n, 0)], which keeps it an expression you can paste into a REPL. I checked that against both functions over every list up to length 5 with lots of duplicates, and over random inputs with akey=and with generators, and it matches everywhere.test_nsmallestandtest_nlargestinLib/test/test_heapq.pyalready check this equivalence, but the n values they loop over are(0, 1, 2, 10, 100, 400, 999, 1000, 1100), so a negative n never gets tried. i added some negative values there and corrected the expected expression to match.The same wording is in
Doc/library/heapq.rstand in the two docstrings inLib/heapq.py, so i changed all four. but there is no behaviour change.