def add(self, elem, replace=False):
        """Add a requirement.

        :param elem: a string or :class:`Requirement` instance.
        :param replace: if `True`, packages in the set with the same name will
            be removed first.
        """
        if isinstance(elem, text_type):
            elem = Requirement.coerce(elem)
        if replace and elem in self:
            self.remove(elem)
        super(Requirements, self).add(elem)
def _get_snapshot_status(self, snapshot_id):
        resp, body = self.get_snapshot(snapshot_id)
        status = body['status']
        # NOTE(afazekas): snapshot can reach an "error"
        # state in a "normal" lifecycle
        if (status == 'error'):
            raise exceptions.SnapshotBuildErrorException(
                snapshot_id=snapshot_id)

        return status