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_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())