users = {"John":"Doe", "GEO":"Rock"}
status = ""

def mainDisplay():
    status = input("Do you have an account?: y/n? Press ! to quit: ")
    if status == "y":
        existUser()
    elif status == "n":
        newUser()
    elif status == "!":
        quit()

def newUser():
    createAccount = input("Please create a username: ")

    if createAccount in users:
        print("\nUsername already taken.\n")
    else:
        createPass = input("Create a password")
        users[createAccount] = createPass
        print("\nUser created\n")

def existUser():
    login = input("Enter your username: ")
    passw = input("Enter your password: ")

    if login in users and users[login] == passw:
        print("\nLogin successful.\n")
    else:
        print("\nError: invalid username or password and/or invalid combination.")

while status != "!":
    mainDisplay()