def encode_modfun(self, props, msg=None):
        """
        Encodes a dict with 'mod' and 'fun' keys into a protobuf
        modfun pair. Used in bucket properties.

        :param props: the module/function pair
        :type props: dict
        :param msg: the protobuf message to fill
        :type msg: riak.pb.riak_pb2.RpbModFun
        :rtype riak.pb.riak_pb2.RpbModFun
        """
        if msg is None:
            msg = riak.pb.riak_pb2.RpbModFun()
        msg.module = str_to_bytes(props['mod'])
        msg.function = str_to_bytes(props['fun'])
        return msg
Encodes a dict with 'mod' and 'fun' keys into a protobuf
        modfun pair. Used in bucket properties.

        :param props: the module/function pair
        :type props: dict
        :param msg: the protobuf message to fill
        :type msg: riak.pb.riak_pb2.RpbModFun
        :rtype riak.pb.riak_pb2.RpbModFun