有没有办法在python中访问特定的行？

Question

我是python的新手，当我撞墙时，我正在制作一个带有'easygui'的小程序。我想得到一个名为“Back”的按钮带我回到前一行，但我不知道如何。我在网上搜索并发现了while循环，但我无法想出如何把它放进去。感谢任何评论。

from easygui import *

msg = "Enter Login Information";
title = "";
fieldNames = ["Username", "Password"];
fieldValues = [];


fieldValues = multpasswordbox(msg,title,fieldNames);

USERNAME, PASSWORD = fieldValues[0], fieldValues[1];

if USERNAME == "username" and PASSWORD == "password":
#This is where i would like the button to take me
msg = "What would you like to do?"
title = "Options"
choices=['passwords','usernames','other','close']
choice=buttonbox(msg, title,choices)
if choice=='passwords':
        msg = "Here is the info you requested"
        title = "Passwords"
        choices=['Back']
        choice=buttonbox(msg, title,choices)
        if choice=='Back':    #this is the button


elif choice=='usernames':
        msgbox("Here is the info you requested")
elif choice=='other':
        msgbox("Here is the info you requested")


else:
     msgbox("Access Denied");