code: def _parse_filter_arg(self, arg):
          
          statement = arg.rsplit('__', 1)
          if len(statement) == 1:
              return arg, None
          elif len(statement) == 2:
              return statement[0], statement[1]
          else:
              raise QueryException("Can't parse '{}'".format(arg))
docs: Parses a filter arg in the format:
          <colname>__<op>
          :returns: colname, op tuple