title: Recommended way to run commands after installing dependencies in the virtualenv
body: <p>I would like to use <em>tox</em> to run <em>py.test</em> on a project which needs additional setup in addition to installing packages into the virtualenv. After creating the virtualenv and installing dependencies, some commands need to be run.</p>
  
  <p>Specifically I'm talking about setting up a <em>node</em> and <em>npm</em> environment using <a href="https://pypi.python.org/pypi/nodeenv" rel="nofollow">nodeenv</a>:</p>
  
  <pre><code>nodeenv --prebuilt -p
  </code></pre>
  
  <p>I see that tox allows me to provide a custom command used for installing dependencies by setting <a href="http://tox.readthedocs.io/en/latest/config.html#confval-install_command=ARGV" rel="nofollow"><code>install_command</code></a> in <code>tox.ini</code>. But I don't think this is what I want because that <em>replaces</em> the command (I assume <code>pip</code>) used to install dependencies.</p>
  
  <p>I thought about using a py.test fixture with <em>session</em> scope to handle setting up nodeenv but that seems hacky to me as I don't want this to happen when py.test is run directly, not via tox.</p>
  
  <p>What is the least insane way of achieving this?</p>