'tags as a comma separated list or None'

def tags(self, index, index_is_id=False):

    id = (index if index_is_id else self.id(index))
    matches = self.conn.get('SELECT    concat(name)    FROM    tags    WHERE    tags.id    IN    (SELECT    tag    from    books_tags_link    WHERE    book=?)', (id,))
    if ((not matches) or (not matches[0][0])):
        return None
    matches = [t.lower().strip() for t in matches[0][0].split(',')]
    return ','.join(matches)
