title: which python vs PYTHONPATH
body: <p>If I type in <code>which python</code> I get: <code>/home/USER/anaconda3/bin/python</code>
  If I type in <code>echo $PYTHONPATH</code> I get: <code>/home/USER/terrain_planning/devel/lib/python2.7/dist-packages:/opt/ros/melodic/lib/python2.7/dist-packages</code></p>
  
  <p>Should that not be the same? And is it not better to set it: <code>usr/lib/python/</code>
  How would I do that? Add it to the PYTHONPATH or set the PYTHONPATH to that? But how to set <code>which python</code>?</p>