running python 2 vs 3

[notbob]

Dumb noob questions: 

I've installed python 3.3 on my Slack box, which by default comes with
python 2.7.  I know how to fire up the different IDLE environments,
but how do I differentiate between the scripts?  IOW, up till now,
I've used .py on all my 2.7 files.  How do I know not to run a .py in
python3 or visa versa?  Or do I?  What's the excepted convention for
differentiating between the two?

nb