How to check python is installed?

[amurleopard]

https://docs.python.org/3.6/using/unix.html#on-linux

Most linux operating systems come python 2 & 3 installed. Be careful when upgrading python versions, read the operating systems guidelines for python versions

For example:
Debian OS: https://wiki.debian.org/Python
Ubuntu OS: https://wiki.ubuntu.com/Python

Check the versions installed.
version 2

$ python -V

version 3

$ python3 -V

Version information.
Should I use Version 2 or Version 3?

The uppercase -V is the same as using --version