为什么for循环在列表末尾返回一个空的列表值？

Question

我编写了一段简单的Python代码，用来比较listcurrent_usernames（尝试登录的用户）和listusernames（已经登录的用户）。如果来自current_usernames的用户已经在usernames中，该用户将被拒绝，否则该用户将受到欢迎。如果current_usernames中没有用户，那么应该只有一个输出，即：There are no users in the system at this point。你知道吗

代码：

usernames = ["User1", "User2", "User3", "User4", "User5", "Admin"]
current_usernames = ["User6", "User7", "User3", "User8", "User5"]

for current_user in current_usernames:
    if current_user not in usernames:
        if current_user == "Admin":
            print("Hello Admin!!! Welcome back!")
        else:
            print("Hello " + current_user)
    else:
        print("Sorry, " + current_user + " is already logged in.")
else:
    print("There are no users in the system at this point!")

输出：

Hello User6 
Hello User7
Sorry, User3 is already logged in.
Hello User8
Sorry, User5 is already logged in.
There are no users in the system at this point!

问题：

为什么for循环在循环的末尾返回There are no users in the system at this point!？你知道吗

我希望循环在Hello User5之后停止，因为在current_usernames左侧没有更多的用户。你知道吗

Answer 1

for循环的else子句在循环正常完成时触发，而不是通过break语句触发。由于循环不包含break，因此else将始终被触发。你知道吗

else子句背后的思想是使编写类似线性搜索的内容更容易：

for x in sequence:
    if x == key:
        print('found it')
        break
else:
    print('Not found')

似乎您混淆了else循环后面的for和通常在条件语句中找到的else以下内容将满足您的要求：

if current_users:
    for current_user in current_usernames:
       ...
else:
    print('No users currently trying to get into the system')