Quick note, the more pythonic way of checking par in this case would be just to:

  if par:
    do_stuff()

Python does some really cool, if slightly unintuitive, checks for "truthiness" where 0, [], {}, false, '' and None (could be other cases too, brain's a little foggy today) all evaluate as false. This controls for:

  if par = None:
    do_stuff()

not catching empty lists, empty dicts, etc.