Python 嘉宾列表问题

某书上的练习题，当作复习8

#3-5 修改嘉宾名单
def alter(someone, other):
    if someone in din_list:
        din_list.remove(someone)
        print("嘉宾 ", someone," 由于某些原因无法赴约.")
        din_list.append(other)
        print("新的嘉宾 ", other," 取而代之加入嘉宾名单.")
    else:
        print(someone," , 不在嘉宾名单中请重新确认.")

#3-6 添加嘉宾
def add(someone):
    if someone in din_list:
        print(someone,"已经存在名单中,无法重复添加.")
    elif len(din_list) > 5:
        print("嘉宾已满,添加失败.")
        print("这是您目前的的名单:",din_list)
    else:
        head = int(input("将该嘉宾加入开头/中间/末尾?(1/2/0):"))
        if head == 1:
            din_list.insert(0, someone)
            print("新的嘉宾 ",someone, " 加入名单开头.")
        elif head == 2:
            half = round(len(din_list) / 2)
            din_list.insert(half, someone)
            print("新的嘉宾 ",someone, " 加入名单中间.")
        elif head == 0:
            din_list.append(someone)
            print("新的嘉宾 ",someone, " 加入了名单末尾.")
        else:
            print("请输入正确的信息.")

#3-7 削减名单,只能有两个嘉宾
def cut_list():
    while len(din_list) > 2 :
        din_list.pop()
    print("削减完毕,名单最终为:",din_list)

def main():
    print("这是您目前的的名单:",din_list)

#3-4 晚餐初始嘉宾名单
din_list = ["张三","李四","宁五"]