https://medium.com/@yanorestes/what-are-the-differences-between-python-2-and-python-3-b4302fec2c7e

Before starting with Python, there’s one question that always comes to mind — which language version should I learn? Perhaps a more difficult question than this one what comes from who already programs in Python — should I create my programs based on which version of Python?

Dealing with some other programming languages, this question may seem a bit unusual, because what works in one version also works on the next version. The point is that the two major versions of Python (2 and 3, the only ones in use) have crucial differences. Python 3 is not backward compatible, and this ends up bringing some confusion and doubts to us developers.