title: change the priority of pythonpath
body: <p>When I was loading certain modules [namely pygments.lexers Bash Lexer and pygments.formatters LatexFormatter] I was was getting an error that python couldn't find the modules. I then realised that this problem was being caused by my PYTHONPATH, which is set up for using <a href="http://www.paraview.org/" rel="nofollow">paraview</a> with python. It brings its own version of pygments, which doesn't work with nbconvert from the jupyter notebook for some reason [Note it is not totally disfunctional, as PythonLexer, and a few others were called without a problem, it was only the ones that I've mentioned above that couldn't be found].</p>
  
  <p>I have a similar problem with mayavi, which wouldn't work with paraview's version of vtk.</p>
  
  <p>Both of these problems can be resolved simply enough by commenting out the python path in the bashrc, but obviously then paraview won't work.</p>
  
  <p>Is there any way to, for example, reduce the priority of the PYTHONPATH so that the system codes in /etc... are called preferentially, but paraview can still find the ones that it needs in the PYTHONPATH?</p>
  
  <p>I am using python 2.76 on linux mint 17.3, paraview is version 4.4.0, installed from source code as per <a href="http://openfoamwiki.net/index.php/Installation/Linux/OpenFOAM-3.0.x/Ubuntu" rel="nofollow">here</a></p>