title: Python iterate list starting from given index in both directions
body: <p>I'm looking for a way to iterate over a list <code>oblist</code> with objects starting at a given index  <code>x</code>. From this index each iteration I would like to get the two neighboring element indices <code>x-1</code> and <code>x+1</code>. The iteration should not stop when one side reaches the end of the list, but the other side is not exhausted yet and still has elements. A <code>None</code> element should be returned in this case for the exhausted side.</p>
  <p>I tried a couple ways with</p>
  <pre><code>for element_indexprev, element_indexnext in zip(range(1,len(oblist)), range(-1,len(oblist), - 1)):
  </code></pre>
  <p>but I'm not getting the desired output. Probably also not the best approach.</p>