Hm maybe there is something worthwhile here. I implemented a Python version
with the same semantics as itertools.product, except only consuming the
iterators when absolutely necessary; still only consuming each iterator once,
but building its pools incrementally. See
https://gist.github.com/sweeneyde/fb6734d7b9f7d17e132c28af9ecb6270
from itertools import count
it = LazyProductObject(count(0), count(0), "ab", repeat=3)
assert next(it) == (0, 0, "a", 0, 0, "a", 0, 0, "a")
assert next(it) == (0, 0, "a", 0, 0, "a", 0, 0, "b")
assert next(it) == (0, 0, "a", 0, 0, "a", 0, 1, "a")
assert next(it) == (0, 0, "a", 0, 0, "a", 0, 1, "b")
assert next(it) == (0, 0, "a", 0, 0, "a", 0, 2, "a")
assert next(it) == (0, 0, "a", 0, 0, "a", 0, 2, "b")
It looks like it could be made to have a similar tight loop as the current
implementation if we decided to re-write it in C.
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