justfordemo

def is_yes(prompt: str = "Yes or No? ", default: bool | None = None) -> bool:
    """ prompt user and only accept answer that is in AFFIRMATION or REJECTION sets
    and return True or False accordingly; return alone can be accepted if default
    set to either True or False in which case the default is returned"""
    while True:
        response = input(prompt).strip().casefold()
        if not response and default is not None:
            return default
        if response in AFFIRMATION:
            return True
        if response in REJECTION:
            return False
        print('Sorry, I did not understand that')

def menu_choice(menu_name, menu_options):
    print()
    print(menu_name)
    for key, (details, _) in menu_options.items():
        print(key, details)
    while True:
        option = input('Enter option: ')
        if option in menu_options:
            return option, menu_options[option][1]
        print('Sorry, that option is not available, please select again')

def register():
    print('registering')
    fini = False
    while not fini:
        option, action =  menu_choice('shop', menus['shop'])
        if action is None:
            fini = True
        else:
            action()

def payment():
    print('paying')
    print('Due:', round(order['total']/100, 2))
    # add code  to process payment
    order['paid'] = True  # assume paid for now

def close_out():
    print('closing out')
    if order['basket']:
        payment()
    else:
        print('Nothing in basket')

def add_discount_card():
    if order['discount'] is not None:
        print(f'You have already selected discount {order["discount"]}.')
        if not is_yes('Do you want to replace  it? '):
            return
    order['discount'] = menu_choice('discount', menus['discount'])

def add_new_item():
    items_added  =  []
    while True:
        item  = input('adding new item (just enter to finish): ').strip().lower()
        if not item:
            break
        if item not in items:
            print('Sorry, do not recognise that item')
            continue
        qty = input('How many? ').strip()
        if qty.isdecimal():
            items_added.append([item, int(qty)])
        else:
            print('Sorry, do not understand that')
    if items_added:
        order['basket'].extend(items_added)

def total_and_pay():
    print('total and paying')

    if order['paid']:
        print('Already paid')
        return

    total = 0
    discount = discounts.get(order['discount'], 0)
    for item, qty in order['basket']:
        total += items[item] * qty
    order['total'] = total
    if total >  0:
        total = total - total * discount/100
        payment()

def abandon_basket():
    order['basket'] = None

def basket():
    print('basket')
    for item, qty in order['basket']:
        unit = items[item]
        price = unit * qty
        print(f'{item:10} x {qty} @ {unit:3}p = £{price/100:3.2f}')  # could show total price

AFFIRMATION: frozenset[str] = frozenset(('y', 'yes', 'yup', 'yeh', 'ok', '1'))
REJECTION = frozenset(('n', 'no', 'nah', 'nope', '0'))

items = {'apple': 20, 'pear': 25, 'banana': 20, 'orange': 32}

menus = {
    'main':
        {
            '1': ('start registering your grocery', register),
            '2': ('close the self-check in register', None),
            '3': ('show basked', basket),
            '4': ('Abandon basket', abandon_basket)
        },

    'shop': {
        '1': ('to add a discount card', add_discount_card),
        '2': ('to register a new item', add_new_item),
        '3': ('show basked', basket),
        '4': ('to compute the total and pay', total_and_pay),
        '5': ('quit', None)
        },

    'discount': {
        '1': ('Pensioner Card', 'pension'),
        '2': ('Weekend Card', 'weekend'),
        '3': ('Green Card', 'green')
        }
    }

discounts = {'pension': 10, 'weekend': 5, 'green': 7}
order = {'discount': None, 'basket': [], 'paid': False, 'total': 0}
fini = False
while not fini:
    option, action = menu_choice('main', menus['main'])
    if action is None:
        if order['paid'] or not order['basket']:
            fini = True
        else:
            print('Sorry, you need to pay first (or abandon basket)')
    else:
        action()