Coexistence of Python 2.x and 3.x on same OS

[Edward Diener]

Has there been any official software that allows both the Python 2.x and 
3.x releases to coexist on the same OS so that the end-user can easily 
switch between them when invoking Python scripts after each has been 
installed to their own directories/folders ?

I know of some unoffical solutions, but they require lots of tweaks. 
Given the vagaries of the different OSs on which Python can run I am 
hoping for some offical solution which will work on any of the most 
popular OSs ( Windows, Linux, Mac ).

The situation is so confusing on Windows, where the file associations, 
registry entries, and other internal software which allows a given 
Python release to work properly when invoking Python is so complicated, 
that I have given up on trying to install more than one Python release 
and finding a relaible, foolproof way of switching between them. So 
although I would like to use the latest 3.x series on Windows I have 
decide to stick with the latest 2.x series instead because much software 
using Python does not support 3.x yet.