Python 2 or 3

Please use Python 3. That is the modern version of the language.

Many people recommend using Python 2 because some software libraries (particularly in scientific computing) have not been converted yet to the new version of Python. This can be a problem, but really, Python 3 has been around for almost a decade (released in Dec 2008) so get with the game, folks!

For the record, Apple's Mac OS X still is only equipped with Python 2. You can upgrade from the usual sources (e.g. Anaconda). This only makes a difference if you want to run your code locally.

How different are the two versions? Not much. The command 2/3 returns 0 in Python 2 (integer division) while 2/3 returns .66666666666 in Python 3 (floating point division). And printing is a bit different. And there is enough new, good stuff in Python 3 that you really should use it

Summary: Use Python 3, please.