我正在使用mysql.connector与mysqldb交互。我在myscript的开头创建了一个数据库连接，并对所有数据库活动（如select/insert/update等）重用同一个连接。我随机得到以下异常，
MySQL连接不可用。

Traceback (most recent call last):
  File "database.py", line 46, in query
    cursor = self.connection.cursor(buffered=True, dictionary=True)
  File "lib/python3.4/site-packages/mysql/connector/connection.py", line 807, in cursor
    raise errors.OperationalError("MySQL Connection not available.")
mysql.connector.errors.OperationalError: MySQL Connection not available.
2017-12-08 13:16:03,845 ERROR util.py 1247 MySQL Connection not available.

上述错误的原因是什么？

import mysql.connector

class Database:
    """
        Simple Database class
    """

    def __init__(self, db_conf, logger):
        self.logger = logger
        try:
            self.connection = mysql.connector.connect(host=db_conf['host'], user=db_conf['user'], password=db_conf['password'],
                                                      database=db_conf['db'], port=db_conf['port'])
        except Exception as e:
            self.logger.exception(e)
            raise

    def insert_or_update(self, query, type):
        """
            Transactional query
        """
        try:
            cursor = self.connection.cursor(buffered=True)
            cursor.execute(query)
            self.connection.commit()

            if type == 'insert':
                last_row_id = cursor.lastrowid
                cursor.close()
                return last_row_id
            elif type == 'update':
                row_count = cursor.rowcount
                cursor.close()
                return row_count
        except Exception as e:
            self.connection.rollback()
            self.logger.info(query)
            self.logger.exception(e)
            raise

    def query(self, query):
        """
            Non transactional query
        """
        try:
            cursor = self.connection.cursor(buffered=True, dictionary=True)
            cursor.execute(query)
            self.connection.commit()
            result = cursor.fetchall()
            cursor.close()
            return result
        except Exception as e:
            self.logger.info(query)
            self.logger.exception(e)
            raise