省市区县，实现逐层访问，并逐层返回！

data = {
    '北京': {
        '朝阳': {},
        '海淀': {},
        '昌平': {},
    },
    '上海': {},
    '四川': {
        '广元':{
            '利州区':{},
            '昭化':{},
            '朝天三区':{},
        },
        '成都':{
            '金牛区': {},
            '成华区': {},
            '高新区': {},
        },
        '绵阳':{
            '涪城区': {},
            '游仙区': {},
            '农科区': {},
        },
    },
}

current_layer = data
parent_layers = []
while True:
    for i in current_layer: #循环字典中的key
        print(i)
    choice = input('请输入:')
    if choice in current_layer:
        parent_layers.append(current_layer)
        current_layer = current_layer[choice]
    elif choice == 'b':
        if parent_layers:
            current_layer = parent_layers.pop()
    elif choice == 'q':
        current_layer = data
    elif choice == 'e':
        exit()
    else:
        print('您的输入有误，请重新输入：')