import os

cats =["Books","Electronics","Clothing"]

def displaymenu(cats):
    os.system('cls')
    print('Select a category below\n')
    for i in range(len(cats)):
        print('{0:2d} - {1:15s}' .format(i+1, cats[i]))
    print(' d - View current cart')
    print(' c - Checkout')

    sel = input('')

    if sel not in ('1, 2, 3, d, c'):
        print('Invalid selection, please try again')

    return sel to displaycategory()

def displaycategory(catList):
    os.system('cls')
    for i in range(len(catList)):
        print('{0:2d} - {1:20s} {2:8.2f}' .format(i+1, catList[i][0], catList[i][1]))
    print(' d - View current cart')
    print(' x - Return to category menu')

    sel = input('')

    if sel not in ('1', '2', '3', 'd', 'x'):
        print('Invalid selection, please try again')

    return sel

def append(sel, catList, cartList):

    sel = int(sel)

    for i in range(len(catList)):
        if i+1 == sel:
            cartList.append(catList[i])
            return cartList

books = [['Origin',19.95],
         ['Grant' ,24.50],
         ['Prarie Fires',18.95]]

electronics = [['HP laptop' ,429.00],
               ['Eyephone' ,790.00],
               ['Bose 20 speakers',220.00]]

clothing = [['T-shirt' ,9.50],
            ['Shoes' ,45.00],
            ['Pants' ,24.00]]

cart = [] # Empty cart to start

cats =["Books","Electronics","Clothing"] # Tuple for category menu

valid = True

### Defined this sentry to start and keep the program running until valid is defined as 'False' when you might display the summary of all carts.

while valid:

    selection = displaymenu(cats)

    while selection == '1':

        option = displaycategory(books)

        if option == 'd':
            print(cart)
            input('Hit enter to continue')

        elif option == 'x':
            break

        else:
            cart = append(option, books, cart)