Code that works in 3.6 but not in 3.5 is fine within typical compatibility definitions. That said, Python nowadays deprecates things quite often (especially thi

Code that works in 3.6 but not in 3.5 is fine within typical compatibility definitions. That said, Python nowadays deprecates things quite often (especially things in the standard library), so if Python wanted to adopt a “major = incompatible” scheme, it would probably go up to Python 10 or so.