如何在Python中替换SQL连接中的字符串

import pymysql

conn = pymysql.connect(host='数据库主机地址', port=数据库端口号, user='用户名', password='密码', database='数据库名称')

cursor = conn.cursor()

old_string = '旧字符串'
new_string = '新字符串'

sql = "SELECT * FROM 表名 WHERE 字段名 = %s"
sql = sql.replace(old_string, new_string)

cursor.execute(sql)

result = cursor.fetchall()

cursor.close()
conn.close()