code: def relation_id(relation_name=None, service_or_unit=None):
      
      if not relation_name and not service_or_unit:
          return os.environ.get('JUJU_RELATION_ID', None)
      elif relation_name and service_or_unit:
          service_name = service_or_unit.split('/')[0]
          for relid in relation_ids(relation_name):
              remote_service = remote_service_name(relid)
              if remote_service == service_name:
                  return relid
      else:
          raise ValueError('Must specify neither or both of relation_name and service_or_unit')
docs: The relation ID for the current or a specified relation