title: how to know which python version I have and how to switch between them and how to know which version has PyObjC
body: <p>I have installed different versions of python during last year. when I open python interpreter, I can see which version I am currently running, But I want to know all the versions I have.</p>
  
  <p>I also want to switch between different python versions.
  I am using OS X 10.7.5 and this is supposed to have PyObjC built-in.
  how do I know if my current version of python(the running version) supports PyObjC.
  are there some specific modules to import and see if PyObjC works in current version.</p>
  
  <p>If the current version (Python 2.7.3) do not access PyObjC but if have PyObjC installed, how
  will change my current version to link it</p>
  
  <p>Thanks in advance for your help</p>