from time import sleep

def valrep(x,change): #function goes through a file and replaces specific values
    with open('users.txt', 'r') as file :
        filedata = file.read()
    for aline in open("users.txt","r").readlines():
        word = aline.split()
        if (word[0] == username) and (word[1] == user_password):
            aline2 = aline.replace(word[x], change)
            filedata = filedata.replace(aline,aline2)
    with open('users.txt', 'w') as file:
        file.write(filedata)
        print("SUCCESS")

loop = False
while loop == False:
    username = input("ENTER A VALID USERNAME>>> ")#splits the input into two seperate variables
    user_password = input("ENTER A VALID 4 DIGIT PIN>>> ")
    valnums = '1234567890' #gives a set of valid characters (used later in the program)
    with open("users.txt","r") as user_file:
        for aline in user_file.readlines():
            vals = aline.split()
            if (vals[0] == username) and (vals[1] == user_password):#goes through the text file looking for the valid username and password
                loop2 = False
                while loop2 == False:
                    sleep(3.0) #delays the time for menu to appear
                    print("n"*300,"="*30,"nnnWELCOME TO HAYDON BANK, {} nnn".format(username),"="*30,"n  WHICH SERVICE DO YOU REQUIRE?n","="*30,"nnn    Option 1: View Balancenn    Option 2: Withdrawalnn    Option 3: Change PINnnn","="*30,"n")#this is the menu
                    balance = vals[2] #sets respective part of the file as the balance variable
                    print("n"*6)
                    option = input("INPUT OPTION PRESS 'Q' TO QUIT>>> ")
                    if option == '1':
                        print("£",balance)
                        sleep(2.0)
                    elif option == '2':
                        withdrawal = input("SElECT AMOUNT TO WITHDRAW:n £10n £20n £50n £100n OR INPUT AN AMOUNT THAT IS DIVISIBLE BY 10nnnINPUT AMOUNT>>> £")
                        if withdrawal%10 == 0:
                            print("You selected:£",withdrawal)
                            newbalance = float(balance) - float(withdrawal)#sets the newbalance variable
                            valrep(2,str(newbalance))
                            print("New Balance:£",newbalance)
                            sleep(2.0)
                    elif option == '3':
                        loop3 = True
                        while loop3 == True:
                            old_pass, new_pass = input("input old PIN and new PIN: ").split(" ")
                            if len(new_pass) != 4: #checks if new_pass is 4 characters long
                                print("INCORRECT LENGTH")
                            elif False in [c in valnums for c in new_pass]: #loops through valnums and compares the variable new_pass to have valid characters
                                print("INVALID CHARACTERS")
                            elif old_pass == user_password: #checks if old_pass is equal to user_password
                                valrep(1,new_pass)
                                print("New PIN: ",new_pass)
                                loop3 = False
                                sleep(2.0)
                    elif option.lower() == 'q':
                        loop2 = True
                        print("Bye", username)
                        sleep(5.0)
                    else:
                        print("INVALID INPUT")
            else:
                print("INVALID INPUT")
                break

user0001 1234 123.45
user0002 1234 123.45
user0003 1234 123.45
...
user9999 1234 123.45

"INVALID INPUT"
"INVALID INPUT"
"INVALID INPUT"
...

ENTER A VALID USERNAME>>> user9999
ENTER A VALID 4 DIGIT PIN>>> 1234

 ==============================


WELCOME TO HAYDON BANK, user0001


 ==============================
  WHICH SERVICE DO YOU REQUIRE?
 ==============================


    Option 1: View Balance

    Option 2: Withdrawal

    Option 3: Change PIN


 ==============================





INPUT OPTION PRESS 'Q' TO QUIT>>>