def METHOD_NAME(self, ns, label=None):
    """Return :class:`tags instances<~Tag>` for the namespace `ns`, ordered
    by label.
    If `label` is not None the only one instance may be returned, or
    `None` if no tags exists for this label.
    """
    query = Tag.query.filter(Tag.ns == ns)
    if label is not None:
        return query.filter(Tag.label == label).first()
    return query.all()