How can I check with python if a hash is stored in a txt file

Question

I was trying to write a simple log-in-programm, that stores the usernames and password as hashes in a .txt file (I know that that's not safe but I'm just trying to learn). I also tried to implement a system that checks, if a username is already taken. And that's where my problem is. ("#check if it's already taken" in the code)

When the username is already taken, it tells me to choose a new one. But when I'm inputting the same name again, after it told me to take a new one, it just continious with the code, even though the name exists already in the file. So is there a method to get a loop that checks over and over again, if the username, I'm trying to input, is already in the file?

import hashlib, time, os

def register():

#create new username
user = input('Create your username: ')

hashuser = hashlib.sha256(user.encode())
hexhashuser = hashuser.hexdigest()


#check if its already taken
with open('py\Hashed Login\Sign in data.txt') as temporary:
    while hexhashuser in temporary.read():
        user = input('Sorry, that username is already taken. Please chose a new one: ')
        hashuser = hashlib.sha256(user.encode())
        hexhashuser = hashuser.hexdigest()
        
    data = open('py\Hashed Login\Sign in data.txt', 'a' ) 
    data.write(hexhashuser)

    if hexhashuser not in temporary.read():
        data = open('py\Hashed Login\Sign in data.txt', 'a' ) 
        data.write(hexhashuser)

#write to file         
data.write('+')
time.sleep(1.5)


#create new password
password = input('Please choose your new password: ')
check = input('Please enter your password again: ')
while check != password:
     check = input('Wrong password. Please enter it again: ')

hashpassword = hashlib.sha256(password.encode())
hexhashpassword = hashpassword.hexdigest()

data.write(hexhashpassword)


data.write('''
''')
data.close


def login():
print('placeholder')


register()