課堂練習：Python-三級菜單

#!/usr/bin/env python
# _authon bruce
# @date: 2017/09/09 15:47
menu = {
    "江西":{
        "贛州":{
            "興國":{},
            "於都":{}
        },
        "南昌":{},
        "吉安":{}
    },
    "上海":{
        "閔行":{
            "莘莊":{},
            "七寶":{}
        },
        "松江":{},
    },
}
current_layer = menu #實現動態迴圈
parent_layers = []  #保存所有父級，最後一個永遠都是父級
while True:
    for key in current_layer:
        print (key)
    choice = input(">:").strip()
    if len(choice) == 0:continue
    if choice in current_layer:
        parent_layers.append(current_layer)   #在進入下一層之前，把當前層追加到父級變數中
        current_layer = current_layer[choice]
    elif choice == "b":
        if parent_layers:
            current_layer = parent_layers.pop() #取出父級變數中最後一個值，當做當前層的父級
    else:
        print("無此項")