def prev_period(self):
        """
        The period before this range.

            >>> span = PeriodRange.from_date(date(2000, 1, 1), period="month")
            >>> span.prev_period()
            PeriodRange([datetime.date(1999, 12, 1),datetime.date(2000, 1, 1)))

        :return: A new :class:`~spans.types.PeriodRange` for the period
                 before this period
        """

        return self.from_date(self.prev(self.lower), period=self.period)