title: How to convince python tox to run tests only for the available python interpreters?
body: <p>I am using python <a href="http://pypi.python.org/pypi/tox" rel="noreferrer">tox</a> to run python unittest for several versions of python, but these python interpreters are not all available on all machines or platforms where I'm running tox.</p>
  
  <p>How can I configure tox so it will run tests only when python interpretors are available.</p>
  
  <p>Example of <code>tox.ini</code>:</p>
  
  <pre><code>[tox]
  envlist=py25,py27
  
  [testenv]
  ...
  [testenv:py25]
  ...
  </code></pre>
  
  <p>The big problem is that I do want to have a list of python environments which is auto-detected.</p>