itertools.tee(deque) returns (deque, deque) instead of (Iterator, Iterator)
iterator1, iterator2 = tee(deque([b""])) assert isinstance(iterator1, Iterator) # AssertionError
Reproduced with pypy-3.9-7.3.9. Not reproducible with CPython (3.7-3.11).
Workaround: use the illustrative implementation of tee from the Python documentation.
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