>I once interviewed and by dint of knowing Python’s itertools module, absolutely destroyed the interviewer’s questions that they clearly thought would take some time to do.

If I see such a situation I usually ask the interviewer whether they want a concise solution using libraries or they want to see how I would do this if I had to do it from scratch.

Or I just offer both, I show that I can do it, but that I know it would be easier to do with x.

I think this is a great opportunity to show you're a good communicator as well as a problem solver.