Constructs and returns a map from namespace URI to schema location
        URI.  Namespaces without schema locations are excluded.

def get_uri_schemaloc_map(self):
        """Constructs and returns a map from namespace URI to schema location
        URI.  Namespaces without schema locations are excluded."""
        mapping = {}

        for ni in six.itervalues(self.__ns_uri_map):
            if ni.schema_location:
                mapping[ni.uri] = ni.schema_location

        return mapping