三级菜单

region = {
    '北京':{
        '朝阳区':{
            '海淀男孩'
        }
    },'上海':{
        '浦东区':{
            '花旗银行'
        }
    }
}
region_mirroring = region
parent_layers = []
while True:
    for key in region_mirroring:
        print(key)
    choice = input('>>>:').strip()
    if len(choice) == 0:continue
    if choice in region_mirroring:
        # returned_value = region_mirroring
        parent_layers.append(region_mirroring)
        region_mirroring = region_mirroring[choice]
    elif choice == 'b':
        # region_mirroring = returned_value
        if parent_layers:#三元运算
            region_mirroring = parent_layers.pop()
    else:
        print('无此项')