title: how to set different PYTHONPATH variables for python3 and python2 respectively
body: <p>I want to add a specific library path only to python2. After adding <code>export PYTHONPATH="/path/to/lib/"</code> to my <code>.bashrc</code>, however, executing python3 gets the error: Your PYTHONPATH points to a site-packages dir for Python 2.x but you are running Python 3.x!</p>
  
  <p>I think it is due to that python2 and python3 share the common <code>PYTHONPATH</code> variable.</p>
  
  <p>So, can I set different <code>PYTHONPATH</code> variables respectively for python2 and python3. If not, how can I add a library path exclusively to a particular version of python?</p>