我的用户名检查器一直说所有名称都可用，尽管 if/else 测试

Question

I'm attempting to make a simple, small, practice program for checking usernames using if-elif-else statements.

current_users = ['Enrique', 'Jose', 'Pablo', 'John', 'Jake']
new_users = ['Mike', 'Tom', 'Bowser', 'John', 'Howard', 'Ben']

'''for user in current_users:
        user = user.lower()
        print(user)

   for user in new_users:
        user = user.lower()
        print(user)
'''


for user in new_users:
    if user.lower() == current_users:
        print(f"{user}: That username is taken.")
    else:
        print(f"{user}: That username is available.")

I expect for it to print the message "That username is taken" for a username that's on both the new_user list and the current_user list, but instead, they all evaluate the True when I run them in the Sublime text editor.

I know it's a logic error because no actual error message is given, but I can't quite see it yet.

I've tried inverting the loop by starting with checking if a username from the new_users list is NOT from the current_users list.

As you can see, I've tried changing all of the usernames to lowercase to see if it would help, but it doesn't.

Answer 1

When you look for values in a list use "in" operator.

    current_users = ['Enrique', 'Jose', 'Pablo', 'John', 'Jake']
    new_users = ['Mike', 'Tom', 'Bowser', 'John', 'Howard', 'Ben']


    for user in new_users:
        if user in current_users:
            print(f"{user}: That username is taken.")
        else:
            print(f"{user}: That username is available.")

Answer 2

The problem is you were comparing a string to a list of strings user.lower() = current_user .

Moreover, you are comparing user.lower() to user .

example means "Jhon".lower() == "Jhon" will give you False

TRY THIS:

create a temporary list of current_user with all the name in lower case then check if it already has taken or available.

current_users = ['Enrique', 'Jose', 'Pablo', 'John', 'Jake']
new_users = ['Mike', 'Tom', 'Bowser', 'John', 'Howard', 'Ben']

t_current_user = [x.lower() for x in current_users]

for user in new_users:
    if user.lower() in t_current_user:
        print(f"{user}: That username is taken.")
    else:
        print(f"{user}: That username is available.")