def get_relationships_by_ids(self, relationship_ids=None):
if relationship_ids is None:<EOL><INDENT>raise NullArgument()<EOL><DEDENT>relationships = []<EOL>for i in relationship_ids:<EOL><INDENT>relationship = None<EOL>url_path = ('<STR_LIT>' +<EOL>self._catalog_idstr +<EOL>'<STR_LIT>' + str(i))<EOL>try:<EOL><INDENT>relationship = self._get_request(url_path)<EOL><DEDENT>except (NotFound, OperationFailed):<EOL><INDENT>if self._relationship_view == PLENARY:<EOL><INDENT>raise<EOL><DEDENT>else:<EOL><INDENT>pass<EOL><DEDENT><DEDENT>if relationship:<EOL><INDENT>if not (self._relationship_view == COMPARATIVE and<EOL>relationship in relationships):<EOL><INDENT>relationships.append(relationship)<EOL><DEDENT><DEDENT><DEDENT>return objects.RelationshipList(relationships)<EOL>
Gets a ``RelationshipList`` corresponding to the given ``IdList``.

        arg:    relationship_ids (osid.id.IdList): the list of ``Ids``
                to retrieve
        return: (osid.relationship.RelationshipList) - the returned
                ``Relationship list``
        raise:  NotFound - an ``Id`` was not found
        raise:  NullArgument - ``relationship_ids`` is ``null``
        raise:  OperationFailed - unable to complete request
        raise:  PermissionDenied - authorization failure
        *compliance: mandatory -- This method must be implemented.*