'Make sure `val` is a valid list of tag strings and assign them.
Note: this will erase any previous tags.'

def deserialize_tags(self, item, key, val, user=None, **context):

    new_tags_list = self.validate.basestring_list(key, val)
    _tags_from_strings(item, self.app.tag_handler, new_tags_list, user=user)
    return item.tags
