Python3.x aiohttp+sqlalchemy：在回滚无效事务之前无法重新连接

我使用的是aiohttp和sqlalchemy，我创建了一个单例，在需要SQLAlchemy实例时可以帮助我连接(代码如下).

12月11日09:35:29 ip xxx xxx xxx gunicorn[16513]:sqlalchemy.exc.StatementError:(sqlalchemy.exc.InvalidRequestError)在回滚无效事务之前无法重新连接[SQL:'…\n从…\n此处…=%(用户名\u 1)s\n限制%(参数\u 1)s'][参数:[{}]```

有没有办法修复当前的代码？

CONNECTION_DETAILS = {
    'driver': 'pymysql',
    'dialect': 'mysql',
    'host': os.environ.get('HOST'),
    'port': 3306,
    'user': 'master',
    'password': os.environ.get('PASSWORD'),
    'database': 'ourdb',
    'charset': 'utf8'
}

_instance = None

def __new__(cls, *args, **kwargs):
    if not cls._instance:
        con_str = '{dialect}+{driver}://{user}:{password}@' \
                  '{host}:{port}/{database}?charset={charset}'\
            .format(**cls.CONNECTION_DETAILS)
        try:
            engine = sqlalchemy.create_engine(con_str)

            Session = scoped_session(sessionmaker(bind=engine))
            session = Session()  # Create the ORM handle
        except sqlalchemy.exc.OperationalError:
            logger.exception('Establishing database connection error.')

        cls._instance = super().__new__(cls)
        logger.debug("Returning database's session.")
        cls._instance.session = session

        # Initializing tables
        cls._instance.Users = Users
        cls._instance.Services = Services
        cls._instance.APIKeys = APIKeys

    return cls._instance