title: Understanding Non-Default Python Configuration in tox.ini
body: <p>I'm trying to learn more about the details of configuring tox.ini files.  In <a href="https://github.com/Jupyter-contrib/jupyter_contrib_core/blob/master/tox.ini" rel="nofollow noreferrer">one of the files for the Jupyter project</a> and in <a href="https://github.com/ionelmc/python-nameless/blob/master/tox.ini" rel="nofollow noreferrer">this demo project</a>, rather than using the default python environments, they define new environments in the command:</p>
  
  <pre><code>[testenv]
  basepython =
      py27: {env:TOXPYTHON:python2.7}
      py33: {env:TOXPYTHON:python3.3}
  </code></pre>
  
  <p>What is the advantage to this?  Also, what does the command with env:TOXPYTHON command accomplish?  I couldn't find it in the tox documentation.</p>