• 题目：设置一个老用户列表和一个新用户列表，检查老用户列表中是否与新注册的用户名字有重复（不区分大小写），老用户列表不能被破坏

usernames = ['admin','Tom','john','jack','philip']
usernames_copy = usernames[:]                        #复制列表操作
for i in range(0,len(usernames_copy)):               #更改列表操作
    usernames_copy[i] = usernames_copy[i].lower()
newusers = ['A','B','C','D','tom']
for newuser in newusers:
    if newuser.lower() in usernames_copy:
        print('This username has been used')
    else:
        print("This username hasn't been used")