I suggest to remove the micro-optimization from Python 3.5 for safety. I'm ok to experiment a new safer implementation on Python 3.6 ;-) We have more time to fix the code in Python 3.6 if new issues are found. Setting the tuple size to zero looks simple and safe, but the overall hack deserves a comment to explain: * why you use a cached tuple * why the reference count can be different than 2: recursive calls * why do you change the tuple size