MongoDB singleton class

class MongodbConnection(metaclass=Singleton):
    def __init__(self):
        config = configparser.RawConfigParser()
        config.readfp(open(r'singleton.txt'))
        self.__MONGO_HOST = config.get('mongo_creds', 'MONGO_HOST')
        self.__MONGO_USER = config.get('mongo_creds', 'MONGO_USER')
        self.__MONGO_PORT = int(config.get('mongo_creds', 'MONGO_PORT'))
        self.__MONGO_PASSWORD = config.get('mongo_creds', 'MONGO_PASSWORD')
        self.__MONGO_DATABASE = config.get('mongo_creds', 'MONGO_DATABASE')

    def connect_to_mongodb(self):
        self.client = MongoClient(host=self.__MONGO_HOST, port=self.__MONGO_PORT, username=self.__MONGO_USER, password=self.__MONGO_PASSWORD, authSource=self.__MONGO_DATABASE)
        return self.client

    def execute_mongo_query_and_return_dataframe(self, input_query):
        database = self.client[database_name]
        # sample_query_mongo_shell = database[collection_name].find({})
        return pd.DataFrame(list(input_query))

    def close_connection(self):
        self.client.close()