Build the storable string corresponding
        to this configuration object.

        :rtype: string

def config_string(self):
        """
        Build the storable string corresponding
        to this configuration object.

        :rtype: string
        """
        return (gc.CONFIG_STRING_SEPARATOR_SYMBOL).join(
            [u"%s%s%s" % (fn, gc.CONFIG_STRING_ASSIGNMENT_SYMBOL, self.data[fn]) for fn in sorted(self.data.keys()) if self.data[fn] is not None]
        )