As for why to not return 1, I can imagine code that checks cpu_count, and only if it returns the "don't know" result would it invoke some more expensive method of determining the CPU count on platforms that cpu_count doesn't support.  Since the os module is the home for "close to the metal" (well, OS) functions, I agree that it does not make sense to throw away the information that cpu_count can't actually determine the CPU count.  Contrawise, I could see the multiprocessing version returning 1, since it is a higher level API and os.cpu_count would be available for those wanting the "don't know" info.