How to check if input is same as item in list after the first input is wrong?

Question

I was trying to create a login system that will check if the username and password is in the list or not. The problem that I have with the code below is that when the first array of credentials does not match, it will checks the first user-password pair only for the next input. May I know what's the reason for this error and how can I avoid it?

userlist = [["k",'l'],['m','z'],['l','o']] #dummy data
def signin():
     username = input("Please enter username: ")
     for user in userlist:
          if username == user[0]:
               password = input("Please enter password: ")
               if password == user[1]:
                    print("correct password")
                    signin()
               else:
                    print("Incorrect password")
                    signin()
          else:
               print("Unregister username")
               signin()
signin()

Output:

Please enter username: k
Please enter password: l
correct password       
Please enter username: m
Unregister username    
Please enter username: l
Unregister username    
Please enter username: 

Expecting output:

Please enter username: k
Please enter password: l
correct password       
Please enter username: m
Please enter password: z
correct password       
Please enter username: l
Please enter password: o
correct password       
Please enter username: 

Answer 1

Back space your last else so that it is inline with your for loop. Why? Well, right now if you enter l at the first input, it will check this list ["k",'l'] . It is not there so it will go to the else. Putting the else outside the for loop makes sure we iterate through the entire list before making a decision.

userlist = [["k",'l'],['m','z'],['l','o']] #dummy data
def signin():
    username = input("Please enter username: ")
    for user in userlist:
        if username == user[0]:
            password = input("Please enter password: ")
            if password == user[1]:
                print("correct password")
                signin()
            else:
                print("Incorrect password")
                signin()
    else:
        print("Unregister username")
        signin()
signing()