def tags_check_convert(cls, tags):
        """Accept one tag as string or multiple tags in list of strings.
        Returns list (with tags in unicode form) or raises ValueError
        """
        # single string check comes first since string is also a Sequence
        if isinstance(tags, string_types):
            return [cls.__tag_check_convert(tags)]
        elif isinstance(tags, Sequence):
            if not tags:
                raise ValueError("Tag list is empty")
            return [cls.__tag_check_convert(tag) for tag in tags]
        else:
            raise ValueError("tags must be a single string or list of sequence of strings")
Accept one tag as string or multiple tags in list of strings.
        Returns list (with tags in unicode form) or raises ValueError