def __missing__(self, key):
        if self.default_factory is None:
            raise KeyError(key)
        else:
            ret = self[key] = self.default_factory(name=key)
            return ret
def is_overdue(self):
        """
        Returns True when the due date is in the past and the task has not
        yet been completed.
        """
        return not self.is_completed() and self.days_till_due() < 0