Python How to append login details to a list properly?

Soz probs stupid question, but I cannot seem to get my login to accept any usernames or passwords when I reach the login function after adding usernames and passwords from the register function, instead if I enter nothing at all as inputs, it will still proceed and be counted as logged in. I also have a minor error where if I select option A, it asks for the input twice before accepting it. Any help would be appreciated thanks.

store = [] <---The list that stores the usernames and passwords

def menu(): 
mode = input("""Choose options:\n
##########################################################################
a) Login with username and password
b) Add username and passwords to list
To select a mode, enter the corresponding letter of the mode below
##########################################################################\n
> """.format(name)).strip()
return mode

def login():
    if len(store) > 0 : #user has to append usernames and passwords before it asks for login details
        print("Welcome to the login console")
        while True:
            username = input ("Enter Username: ") 
            password = input ("Enter Password: ") 
            for i in store: #finds usernames and passwords in the store list
                if i == username and i == password: break
                print("Username matches!")
                print("Password matches!")
                logged() #jumps to logged function
    else:
        print("You have no usernames and passwords stored!")

def register(): #example where the username is appended. Same applies for the password
    print("Please create a username and password to be stored in a list.\n")
    while True:
        validname = True
        while validname:
            username = input("Please enter a username you would like to add to the password vault. NOTE: Your username must be at least 3 characters long: ").strip().lower()
            if not username.isalnum():
                print("Your username cannot be null, contain spaces or contain symbols \n")
            elif len(username) < 3:
                print("Your username must be at least 3 characters long \n")
            elif len(username) > 30:
                print("Your username cannot be over 30 characters \n")
            else:
                validname = False
                store.append([username]) <-----(USERNAME IS APPENDED TO THE STORE LIST)
validpass = True
    while validpass:
        password = input("Please enter a password you would like to add to the password vault. NOTE: Your password must be at least 8 characters long: ").strip().lower()
        if not password.isalnum():
            print("Your password cannot be null, contain spaces or contain symbols \n")
        elif len(password) < 8:
            print("Your password must be at least 8 characters long \n")
        elif len(password) > 20:
            print("Your password cannot be over 20 characters long \n")
        else:
            validpass = False #The validpass has to be True to stay in the function, otherwise if it is false, it will execute another action, in this case the password is appended.
            vault.append([password])

validinput = True
while validinput:
    exit = input("\nEnter 'end' to exit or any key to continue to add more username and passwords:\n> ")
    if exit in ["end", "End", "END"]:
        menu()
    else:
        validinput = False
        register()
    return register

def logged():
   print("-----------------------------------------------------------\n")
   print("Hello, you are logged in. ")

while True:
    chosen_option = menu() #a custom variable is created that puts the menu function into the while loop

    if chosen_option in ["a", "A"]:
        login()

    if chosen_option in ["b", "B"]:
        register()

    else:
        print("""That was not a valid option, please try again:\n """)