Verifying Username and Password in a Text File in Python

Once you open a file and iterate through it, the iterator is exhausted. Just because your outer loop goes into another iteration does not restart the file iteration.

When you do for x in [1, 2, 3]:, python calls iter([1, 2, 3] under the hood. For a list, this returns a new iterator object every time. When a similar loop calls iter(accounts_file), a new iterator is not returned: the file object is the iterator.

You have two basic options to restart the iteration:

1. Reopen the file every time. Normally, the idiomatic way to do that is with a with block, as that will also close the file automatically in case of error:

   while not complete:
       usernameInput = input("Username:")
       passwordInput = input("Password:")
       with open("accounts.txt", "r") as accounts_file:
           for line in accounts_file:
               username, password = line.replace("\n","").split(":")
               if usernameInput == username and passwordInput == password:
                   complete = True
                   break
   

   This option is less desirable because it does a lot of disk I/O every time the user enters a wrong option.

2. Load the entire file into a list. That lets you iterate over and over without hitting the disk multiple times. You can pre-load before the loop, or do it lazily after the first input. The first option is shown here:

   with open("accounts.txt", "r") as accounts_file:
       lines = accounts_file.readlines()
   while not complete:
       usernameInput = input("Username:")
       passwordInput = input("Password:")
        for line in lines:
            username, password = line.replace("\n","").split(":")
            if usernameInput == username and passwordInput == password:
                complete = True
                break
   

A third option would be a hybrid of these two, which might work for large files. You could pre-load an index into the file, and then look up small snippets based on user input.

Notice that the else clause is not necessary. complete is False and only changes when you want to break out of the loop.

It's more idiomatic to write while not complete:. The expression in the condition of a while loop is always interpreted as a boolean: there is no need to make the comparison explicit. And even then, bool values True and False are singletons. In the relatively rare situations when you need to compare against them explicitly, use the identity operator is rather than ==. I.e., complete is False is more pythonic than complete == False.

You could actually get rid of the complete variable entirely by using an arcane feature of for loops: the else clause. This clause gets triggered whenever a loop completes without breaking:

while True:
    usernameInput = input("Username:")
    passwordInput = input("Password:")
    with open("accounts.txt", "r") as accounts_file:
        for line in accounts_file:
            username, password = line.replace("\n","").split(":")
            if usernameInput == username and passwordInput == password:
                   break
        else:
            continue
        break

Also, don't use semicolons normally in python. They aren't invalid, but they aren't conventional except in rare situations like command-line interaction.

How about this:-

ACCOUNTS = {}
with open('accounts.txt') as afile:
    for line in afile.readlines():
        tokens = line.split(':')
        ACCOUNTS[tokens[0]] = tokens[1].strip()
print('Please log in')
while True:
    n = input('Please enter your name: ')
    p = input('Please enter your password: ')
    if (password := ACCOUNTS.get(n)) and password == p:
        break
    print('Incorrect username or password')
print('Welcome')

print("Please Login !")
accounts_file = open("accounts.txt", "r")
USERS = []
for line in accounts_file:
    if line.replace("\n","").split(":") != [""]:
        accounts.append(line.replace("\n","").split(":"))
accounts_file.close()


def passCheck(username , password):
    if [username,password] in USERS:
        return   
    else:
        return passCheck(input("Enter your username : ") , input("Enter your password"))
passCheck(input("Enter your username : ") , input("Enter your password"))
print("Welcome!")