code: def _parse_unit(unit, placement_str):
      
      if not unit:
          return None
      try:
          return int(unit)
      except (TypeError, ValueError):
          msg = 'unit in placement {} must be digit'.format(placement_str)
          raise ValueError(msg.encode('utf-8'))
docs: Parse a unit as part of the unit placement.
  
      Return the unit as an integer or None.
      Raise a ValueError if the unit is specified but it is not a digit.