trouble writing to file after while loop

[Larz60+]

I rewrote as a class, there were just too many things wronf with the other code
Here's a complete class to get you started.

import json
import os


class NewPassword:
    def __init__(self, filename=None):
        self.usrdatalist = None
        self.filename = filename
        if self.filename is not None:
            if not os.path.exists(filename):
                self.init_file()
            else:
                with open(self.filename, 'r') as f:
                    j = f.read()
                    self.usrdatalist = json.loads(j)

    def init_file(self):
        data = [('user', 'password')]
        with open(self.filename, "w") as f:
            j = json.dumps(data)
            f.write(j)

    def rewrite_file(self):
        with open(self.filename, "w") as f:
            j = json.dumps(self.usrdatalist)
            f.write(j)

    def new_user(self):
        while True:
            username = input("Enter a username: ")
            if username in self.usrdatalist:
                username = input("Name taken, enter new name: ")
                continue
            break
        password = None
        while not password:
            password = input('Enter password: ')
            if len(password) < 12:
                print('Password must be at least 12 characters long')
                password = None
                continue
        self.usrdatalist.append((username, password))
        self.rewrite_file()

def main():
    cp = NewPassword(filename="usrdata.json")
    cp.new_user()

if __name__ == '__main__':
    main()

The password needs to be encoded, double checked, and blanked out while entering
without that, this code is of little use.
If using a gui, you can use getpass (builtin)