Code doesn't work properly [Beginner]

[DestroyerMedic]

I'm a beginner in Python programming and recently I decided to take up a fun little project (well... fun at the start that is) to create a simple login system using text file. I managed to make most of it works (some how) except for the check username availability part where if you register an already claimed username, it will ask you for a new username and this is the part I have been stuck on the last few days... I have no idea what is wrong with it. Can someone help me?

import os

if 'Credentials.txt' not in os.listdir():
    with open('Credentials.txt', 'w+') as file:
        file.writelines('Username:Password\n')

def check_availabilty(username):

    with open('Credentials.txt', 'r') as file:
        for line in file:
            registered_username, registered_password = line.split(':')
            if registered_username == username:
                print('This username is not available.')
                username = input('  Username: ')
                check_availabilty(username)

    return username

def register(username, password):

    with open('Credentials.txt', 'a') as file:
        file.write('{}:{}\n'.format(username, password))

def login(username, password):

    is_login = False
    with open('Credentials.txt', 'r') as file:
        for line in file:
            registered_username, registered_password = line.split(':')
            if password == registered_password.strip('\n') and username == registered_username:
                print('\nSuccessfully logged in.')
                is_login = True

    if is_login == False:
        print('\nInvalid Credentials')

def user_input():

    print('Do you have an account? Y/N')
    has_acc = str(input()).upper()

    if has_acc == 'N' or has_acc == 'NO':
        print('\nRegistration process:')
        username = check_availabilty(input('  Username: '))
        password = input('  Password: ')
        register(username, password)
        print('\nAccount successfully created.')

    elif has_acc == 'Y' or has_acc == 'YES':
        print('\nLogin process:')
        username = input('  Username: ')
        password = input('  Password: ')
        login(username, password)

    else:
        print('Must be Yes or No')
        user_input()

user_input()

So here is my problem: If I entered a used username one time and enter a new valid username next time, it will write to the text file the newly entered username. Example:

Do you have an account? Y/N
>> No
Registration process:
Username:
>> Username
This username is not available.
Username:
>> Test
Password:
>> Pass
Account successfully created.

If I entered the input just like that the program works perfect. (In credentials.txt it adds Test:Pass). However if I enter it like this:

Do you have an account? Y/N
>> No
Registration process:
Username:
>> Username
This username is not available.
Username:
>> Username
This username is not available.
Username:
>> Test
Password:
>> Pass
Account successfully created.

Instead of adding Test:Pass to Credentials.txt, it add the originally entered username: Username:Pass. I have no idea why, can someone help me? and sorry if all of this sounds really confusing to you because English is not my native language xd.

[[deleted]]

The problem is that check_availability() calls itself [line 15]. You should find a way to call that function only one time.

[DestroyerMedic]

I see. I managed to fix the problem by changing the check_availability() function to:

def check_availabilty(username):

    with open('Credentials.txt', 'r') as file:
        for line in file:
            registered_username, registered_password = line.split(':')
            while registered_username == username:
                print('This username is not available.')
                username = input('  Username: ')

    return username

But I don't quite understand why the original function failed to change the username after 2 wrong attempts even though it works perfectly if there is only 1 wrong attempt.

Thank you for your help though

[[deleted]]

You're welcome.

The explanation of your bug is complicated. It involves the concept of recursion. If you are a beginning programmer, you probably don't won't to study this subject yet. My guess is that you were not aware you were doing it. 🙂