title: Can a virtualenv inherit from another?
body: <p>I want to create one <code>virtualenv</code> using another as the starting point, is this possible?</p>
  
  <p>I have to use cases in mind:</p>
  
  <ol>
  <li><p>Let's say I have a two <code>virtualenv</code> one for production and one for development. The development environment requires the same packages as the production environment, but it requires others I don't want in the production environment. I don't want to install the common packages twice. </p></li>
  <li><p>I want to experiment with a development version of a package, say <code>matplotlib</code> for example. The development version of the package has the same requirements as the stable version. So I create a <code>virtualenv</code> called <code>matplotib_stable</code> and install the requirements and the stable version. Then I create a second <code>virtualenv</code> called <code>matplotlib_dev</code> and use <code>matplotlib_stable</code> as a starting point (for the <code>matplotlib</code> requirements) but then I install the development version. </p></li>
  </ol>
  
  <p><a href="https://stackoverflow.com/questions/4806448/how-do-i-install-from-a-local-cache-with-pip">How do I install from a local cache with pip?</a> seems to address the issue of downloading packages, but I don't think it deals with modifying <code>sys.path</code>. </p>