The only legitimate purpose of using six.PY3 is for managing a module that runs in both Python 2 or Python 3; for that purpose it&#x27;s almost essential. When Python 4 eventually comes out you may find vestiges of code that still rely on it, even though it hasn&#x27;t been required for ages. If PY3 is still True, your code will continue to run just fine without changes. If you find it necessary to manage the upgrade from 3 to 4 I&#x27;m sure there will be another package available with the required constants.