三级菜单作业，优化，保存在文件;

三级菜单作业:

1、菜单内容保存在文件 menu

　　menu内容：

　　{‘jiangsu‘: {‘wuxi‘: {‘宜兴‘:{‘官林‘:{}}, ‘jiangying‘:{}, ‘xishan‘:{}}, ‘nanjing‘: {‘qixia‘, ‘gulou‘, ‘changning‘, ‘zhongshan‘}},
‘zhejiang‘: {‘嘉兴‘: {‘a2‘, ‘a3‘, ‘a4‘},‘宁波‘: {‘a1‘, ‘a2‘, ‘a3‘}},
‘guangzhou‘: {1: {‘a‘}, 2: {‘b‘}, 3:{‘c‘}}
}　　

2、用函数取menu内容，转化内容为字典格式;   函数： eval()

3、菜单优化方式做逻辑结构;

with open(‘menu‘) as poem:
    f=poem.read()
dic_city=eval(f)
# print(type(dic_city))
last_layer=[]
current_layer=dic_city
while True:
    for i in current_layer:
        print(i)
    choice = input(‘pls input your next_destion:‘).strip()
    if len(choice)==0:continue
    if choice in current_layer:
        # print(choice)
        last_layer.append(current_layer)
        current_layer = current_layer[choice]
        # print(last_layer)
        # print(current_layer)
    if choice==‘b‘ and last_layer:
        # if last_layer:
        current_layer = last_layer[-1]
        last_layer.pop()