The OS will be some Linux distribution for sure. Most of these are coming with preinstalled Python. Probably both versions. All needed is to type python in the terminal to run the interpreter. python2, python3 are shortcuts to the respective version. The python "command" will run the default version for the OS:
victor@jerry:~$ python Python 3.6.0 |Continuum Analytics, Inc.| (default, Dec 23 2016, 12:22:00) [GCC 4.4.7 20120313 (Red Hat 4.4.7-1)] on linux Type "help", "copyright", "credits" or "license" for more information. >>>So, on this Ubuntu MATE the default one is Python 3.6.0.