省市区小程序

#!/usr/bin/env python
#_*_coding:utf-8_*_
memu= {"北京": {"海淀": ["中关村", "四通桥"], "昌平": ["回龙观", "西二旗"]},
        "上海": {"黄浦区": ["南京路", "延安路"], "静安区": ["静安街道", "石门"]}}
flag=True
sheng=''
shi=''
while flag:
    if sheng!='':
        if shi!='':
            for key  in memu[sheng][shi]:
                print key
        else:
            for key in memu[sheng]:
                print key
    else:
        for key in memu.keys():
            print key
    operation= raw_input("请输入菜单,按q退出,按b返回上一级：").strip()
    if operation=="q":
        print"已退出"
        break
    elif operation=='b':
        if sheng=='':
            print "已是顶级菜单"
        else:
            if shi=='':
               sheng=''
            else:
                shi=''
        continue
    else:
        if sheng=='':
            sheng=operation
        else:
            if shi=='':
                shi=operation
            else:
                print"已是区域菜单，请输入B返回或输入Q退出"