Actually, happened only once, several years ago, Python got complete overhaul, they fixed many things from the past and in the process they broke backward version compatibility. Some companies are still running Python 2.7 (will be getting security updates up to 2020) because code they have is huge and it takes time to update it.

Example I gave you is, python 2.7 used print as statement and Python 3+ uses print as function, that's why it changed to print().

So, don't worry, there will not be this kind of radical changes going forward. But someone that is just starting with Python should definitely start learning Python +3. (latest current version is 3.6 with 3.7 just about getting ready) But any 3+ will do. Most Linux distros have preinstalled 3.5 while on Windows you just manually install latest one.