def _prev_index(self, s):
        self.set_index(max(0, self.index - 1))
def is_active(self):
        """
        Returns True when the start date is today or in the past and the
        task has not yet been completed.
        """
        start = self.start_date()
        return not self.is_completed() and (not start or start <= date.today())