["python","version","virtualenv"]
How do I tell a Python script to use a particular version

How do I, in the main.py module (presumably), tell Python which interpreter to use? What I mean is: if I want a particular script to use version 3 of Python to interpret the entire program, how do I do that? Bonus: How would this affect a virtualenv? Am I right in thinking that if I create a virtualenv for my program and then tell it to use a different version of Python, will I encounter some conflicts?
You can add a shebang line the to the top of the script: #!/usr/bin/env python2.7 But that will only work when executing as ./my_program.py. If you execute as python my_program.py, then the whatever Python version that which python returns will be used. In re: to virtualenv use: virtualenv -p /usr/bin/python3.2 or whatever to set it up to use that Python executable.