def presenceOf(obj, names, kwargs):
    """
    A validator to test whether or not some named properties are set.
    For those named properties that are not set, an error will
    be recorded in C{obj.errors}.

    @param obj: The object whose properties need to be tested.
    @param names: The names of the properties to test.
    @param kwargs: Keyword arguments.  Right now, all but a
    C{message} value are ignored.
    """
    message = kwargs.get('message', 'cannot be blank.')
    for name in names:
        if getattr(obj, name, '') in ('', None):
            obj.errors.add(name, message)
