def x(y):
    while True:
        yield y

p = product(x(1), x(2))

next(p)  # this string will never be reached.

Hi Ramil, itertools.product() expect its argument to be finite iterables, it needs to keep all their elements in memory anyway at it "cycles around" to produce all possible pairs.

Basically, product(x(1), x(2)) is equivalent to product(tuple(x(1)), tuple(x(2))).


I see that the documentation does not mention that the arguments must be finite, could you open a PR to improve it?

New changeset cfc6ce4d40f2f01314b7e283fb972a7bb3ed3faa by Ramil Nugmanov in branch 'master':
bpo-40806: Clarify that itertools.product immediately consumes its inpt (GH-20492)
https://github.com/python/cpython/commit/cfc6ce4d40f2f01314b7e283fb972a7bb3ed3faa

Updated the docs to note that inputs are fully consumed before the iterator is run.

FYI, notes on infinite iterable inputs likely belong the FAQs because they aren't specific to product().  Similar effects would be seen with list(x(1)), sorted(x(1)), set(x(1)), tuple(x(1)), etc.

New changeset e4cc3a7c1f5ba9ea2c3015a5bf09cb5b93db5d47 by Miss Islington (bot) in branch '3.9':
bpo-40806: itertools.product immediately consumes its inputs (GH-20492) (GH-20498)
https://github.com/python/cpython/commit/e4cc3a7c1f5ba9ea2c3015a5bf09cb5b93db5d47