参考：
https://www.coder.work/article/4516746
https://www.cnblogs.com/leijiangtao/p/11882107.html
https://stackoverflow.com/questions/55116007/pymysql-query-unable-to-rollback

unable to rollback处理：

# 关闭光标对象
cursor.close()
# 关闭数据库连接
sql_connection.close()

注意：connection不要放到全局变量里。

Already closed处理

try:
    self.conn.ping(reconnect=True)
    print "db is connecting"
except:
    traceback.print_exc()
    self.conn = self.to_connect()
    print "db reconnect"