def get_next_id(self):
        try:
            query = {'_id': 'last_id'}
            update = {'$inc': {'id': 1}}
            fn = self.db.internal.find_and_modify
            row = fn(query, update, upsert=True, new=True)
        except Exception as e:
            raise DatabaseException(e)
        return row['id']
def peirce_skill_score(self):
        """
        Multiclass Peirce Skill Score (also Hanssen and Kuipers score, True Skill Score)
        """
        n = float(self.table.sum())
        nf = self.table.sum(axis=1)
        no = self.table.sum(axis=0)
        correct = float(self.table.trace())
        return (correct / n - (nf * no).sum() / n ** 2) / (1 - (no * no).sum() / n ** 2)