title: python says “no module named nest” when it is in the $PATH
body: <p><code>import nest</code> gives the 'no module named nest' error when it is in the <code>$PATH</code>, which means that there is a <code>/opt/nest/lib/python2.7/site-packages:</code> in my system <code>$PATH</code>. At this location there is a directory named <code>nest</code> and the structure inside the <code>nest</code> directory is looks like:</p>
  
  <p><img src="https://i.stack.imgur.com/OFBnZ.png" alt="enter image description here"></p>
  
  <p>, where there is an <code>__init__.py</code> obviously. So why can't python find <code>nest</code>?</p>
  
  <p>more information:</p>
  
  <p>I am sure that I installed nest with python2.7, and run it with the same python2.7.</p>