Case-insensitive comparison of lists

I want to make a code that to see if each new username has already been used in a website.

I have this code:

current_users = ['Rachel', 'Ross', 'Chandler', 'Joey', 'Monica']

new_users = ['Janice', 'Ben', 'Phoebe', 'JOEY']

for new_user in new_users:
    if new_user in current_users:
    print (f'The username {new_user} is alreay in use. You need to enter a new username')

else:
    print (f'The username {new_user} is avaliable')

And this outputs:

The username Janice is avaliable
The username Ben is avaliable
The username Phoebe is avaliable
The username JOEY is avaliable

For "JOEY" the message to be printed was supposed to be this: "The username JOEY is alreay in use. You need to enter a new username".

How can I lowercase (or uppercase) the lists to make this code case-insensitive?

Solution

You should create a set that contains all of the lowercased names. Then, to check whether a name can be used, look up the lowercased version of the name in the set:

names_in_use = {name.lower() for name in current_users}

for new_user in new_users:
    if new_user.lower() in names_in_use:
        print (f'The username {new_user} is already in use. You need to enter a new username')
    else:
        print (f'The username {new_user} is avaliable')

This outputs:

The username Janice is avaliable
The username Ben is avaliable
The username Phoebe is avaliable
The username JOEY is already in use. You need to enter a new username