title: How do I tell a Python script to use a particular version
body: <p>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?</p>
  
  <p>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, then I may encounter some conflicts?</p>