Should I use Python 2 or Python 3 for my development activity?
You should use Python 3 going forward. As of January 2020 Python 2 will be in EOL (End Of Life) status and receive no further official support. After that date, there will be no further updates nor bugfixes. Since this end-of-life date has been planned for nearly a decade (the first end-of-life date was slated to happen in 2014, and was pushed back to 2020), and nearly all popular libraries have already ported their code, Python 2.x is well on its way to obsolescence. As such, we can only recommend learning and teaching Python 3.
For historical information that may be useful in porting or maintaining remaining Python 2 systems, please see previous page revisions.