Single cont value
        :param elem:
        :param container_type:
        :param params:
        :return:

async def container_val(self, elem, container_type, params=None):
        """
        Single cont value
        :param elem:
        :param container_type:
        :param params:
        :return:
        """
        if hasattr(container_type, "serialize_archive"):
            raise ValueError("not supported")
        if self.writing:
            return await self._dump_container_val(
                self.iobj, elem, container_type, params
            )
        else:
            raise ValueError("Not supported")