def to_yamlf(self, fpath: str, encoding: str='utf8', ignore_none: bool=True, ignore_empty: bool=False) -> str:
        """From instance to yaml file

        :param ignore_none: Properties which is None are excluded if True
        :param fpath: Yaml file path
        :param encoding: Yaml file encoding
        :return: Yaml file path
        """
        return util.save_yamlf(traverse(self, ignore_none, force_value=True, ignore_empty=ignore_empty), fpath, encoding)