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 __get_stop_condition(self):
        for covariance in self.__variances:
            if numpy.linalg.norm(covariance) == 0.0:
                return True