def serialize_text(self):
        '''Returns a serialized form of the Namepace.

        All the elements in the namespace are sorted by
        URI, joined to the associated prefix with a colon and
        separated with spaces.
        :return: bytes
        '''
        if self._uri_to_prefix is None or len(self._uri_to_prefix) == 0:
            return b''
        od = collections.OrderedDict(sorted(self._uri_to_prefix.items()))
        data = []
        for uri in od:
            data.append(uri + ':' + od[uri])
        return ' '.join(data).encode('utf-8')