Python3操作数据库读取sql语句报错：TypeError:

1. 代码部分

#coding:utf-8
import mysql.connector
import json
from MyEncoder import MyEncoder

class OperationMysql:
    def __init__(self):     
        self.conn = mysql.connector.connect(
                host='xxx.xx.xxx.xxx',  
                port=3306,
                user='xxxxxxxx',
                passwd='xxxxxxxxxx',
                db='xxxxxxx',
                charset='utf8'
                )
        self.cur = self.conn.cursor(dictionary=True)    #self代表全局,#传递dictionary，返回dict类型
        self.cur = self.conn.cursor()

    def search_one(self,sql):
        self.cur.execute(sql)
        result = self.cur.fetchone()    #fetchonone     
        return result[0]
        #result_json = json.dumps(result,cls=MyEncoder) #
        #return result_json

if __name__ == '__main__':
    op_mysql = OperationMysql()
    result1 = op_mysql.search_one("select sessionId from xt_login_log where usercode = (select ygxxid from xt_yhzh where zhmc = 'huangxiaoming') and operationType = 1 ORDER BY operationTime DESC LIMIT 1")
    #result1 = op_mysql.search_one("select * from xt_ygxxb where ygxm = '葛文娟' and jgid = '3dbe692399ba449294a44d27d0e085e1'")
    #print(type(result1))
    print(result1)

2. 报错信息

3. 原因

'NoneType' 是指返回的内容是None空对象，说明sql执行的语句返回结果为空，原因在于这条sql语句查询的内容不存在，而我代码连接的测试环境数据库中不存在“葛文娟”这个用户，所以找不到。