def _register_extension(self, name, ext):
        """register extension

        :param name: extension name
        :param ext: extension object
        """
        ext.init_app(self)
        if name in self._extensions:
            raise exceptions.ConfigException(
                'extension duplicated: {}'.format(name))
        self._extensions[name] = ext
register extension

        :param name: extension name
        :param ext: extension object