unittest.util.sorted_list_difference fails to deduplicate tail elements

[stefanzetzsche]

Bug report

Issue

sorted_list_difference claims in its docstring that "Duplicate elements in either input list are ignored," but when one list is exhausted before the other, the remaining elements (including duplicates) are appended without deduplication.

Reproducer

from unittest.util import sorted_list_difference

print(sorted_list_difference([], [0, 0]))
# Output: ([], [0, 0])  — expected ([], [0])

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-145546: unittest.util: fix sorted_list_difference tail deduplication #145547

[picnixz]

I'm not sure whether the problem is the docstring or the function. For me it just means that we remove elements that are present in both lists, not that we deduplicate elements in each list. This can be thought about it with unorderable_list_difference and its docstring.