code: def update_relationship(self, relationship_id=None, relationship_form=None):
          
          if relationship_id is None or relationship_form is None:
              raise NullArgument()
          if not isinstance(relationship_form, objects.RelationshipForm):
              raise InvalidArgument('argument type is not an RelationshipForm')
          if not relationship_form.is_for_update():
              raise InvalidArgument('form is for create only, not update')
          try:
              if self._forms[relationship_form.get_id().get_identifier()] == UPDATED:
                  raise IllegalState('form already used in an update transaction')
          except KeyError:
              raise Unsupported('form did not originate from this session')
          if not relationship_form.is_valid():
              raise InvalidArgument('one or more of the form elements is invalid')
  
          url_path = ('/handcar/services/relationship/families/' +
                      self._catalog_idstr + '/relationships')
          try:
              result = self._put_request(url_path, relationship_form._my_map)
          except Exception:
              raise  # OperationFailed
          self._forms[relationship_form.get_id().get_identifier()] = UPDATED
          return objects.Relationship(result)
docs: Updates an existing relationship.
  
          arg:    relationship_id (osid.id.Id): the ``Id`` of the
                  ``Relationship``
          arg:    relationship_form (osid.relationship.RelationshipForm):
                  the form containing the elements to be updated
          raise:  IllegalState - ``relationship_form`` already used in an
                  update transaction
          raise:  InvalidArgument - the form contains an invalid value
          raise:  NullArgument - ``relationship_form`` is ``null``
          raise:  OperationFailed - unable to complete request
          raise:  PermissionDenied - authorization failure
          raise:  Unsupported - ``relationship_form`` did not originate
                  from ``get_relationship_form_for_update()``
          *compliance: mandatory -- This method must be implemented.*