'Check that the scale is in the allowed list of scales, or is `None`'

def _check_scale(self, scale):

    if ((scale is not None) and (scale not in TIME_DELTA_SCALES)):
        raise ScaleValueError(u"Scale    value    '{0}'    not    in    allowed    values    {1}".format(scale, TIME_DELTA_SCALES))
    return scale
