How would one install a side by side installation of Python 2.x, and Python 3.x

Python 3 is available from the repositories and can be installed side by side with Python 2:

sudo apt-get install python3 python

You can use the -V switch to find out the minor version numbers:

$ python3 -V
Python 3.1.2

$ python -V
python 2.6.6

comment: please note that python 2 and 3 will use different output channels for their prints whilst the returned code will be zero in case of success.

command=python2 --version
rc=0
stderr=Python 2.7.12

.

command=python3 --version
rc=0
stdout=Python 3.5.2