def understands_Unset(cls_or_func):
    """
    Decorator for marking validators as supporting the special Unset value.

    This decorator should be applied to every validator that natively supports
    Unset values. Without it, Unset is never validated.

    This decorator works by setting the ``understands_Unset`` attribute on the
    decorated object and returning it intact.
    """
    cls_or_func.understands_Unset = True
    return cls_or_func
