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Overview

This exercise creates a comprehensive shopping cart system that handles user registration, product selection, cart management, and invoice generation with tax calculations. It demonstrates real-world e-commerce functionality.
Difficulty Level: IntermediateConcepts Covered: Nested lists, user management, cart operations, mathematical calculations, IVA/tax handling, error handling

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What You’ll Learn

1

Multi-dimensional Data Structures

Work with nested lists where each product contains multiple attributes
2

User Session Management

Manage user data and allow updates during the session
3

Shopping Cart Operations

Implement add, update, delete, and list functionality for cart items
4

Financial Calculations

Calculate subtotals, taxes (IVA), and final totals
5

Error Handling

Use try-except blocks to handle invalid user input gracefully

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Complete Code

Here’s the full shopping cart system implementation: 
usuario = []
# Nombre - precio - categoria
productos = [
    ["arroz lb", 4000, "comestibles"],
    ["azucar lb", 4500, "comestibles"],
    ["platano lb", 1000, "vegetales"],
    ["pollo lb", 6000, "carnes"],
    ["res lb", 7500, "carnes"],
    ["banano lb", 2000, "frutas"],
    ["pera lb", 3000, "frutas"],
    ["jabon en polvo lb", 4200, "limpieza"],
    ["tomate lb", 1500, "vegetales"],
    ["papa lb", 4000, "vegetales"]
]

carrito = []


print()
print("==" * 25)
print("||              Carrito de compras              ||")
print("==" * 25)
try:
    while True:
        print("-" * 50)
        print("                      MENU                      ")
        print("-" * 50)
        print()
        print("[1] Registrar usuario o actualizar datos usuario")
        print("[2] Agregar producto al carrito de compras")
        print("[3] Listar el carrito de compras")
        print("[4] Actualizar carrito de compras")
        print("[5] Eliminar producto en carrito de compras")
        print("[6] Finalizar programa")
        opcion = int(input("->"))
        if opcion == 1:
            print("-" * 50)
            if not usuario:
                nombre = input("Ingrese su nombre(s) =>")
                apellido = input("Ingrese su apellido(s) =>")
                documento = input("Ingrese su numero de documento=>")

                usuario.append(nombre)
                usuario.append(apellido)
                usuario.append(documento)

                print("Usuario registrado correctamente")
            else:
                print("El usuario ya esta registrado, si lo deseas puedes editarlo")
                print()
                nombre = input("Ingrese su nombre(s) =>")
                apellido = input("Ingrese su apellido(s) =>")
                documento = input("Ingrese su numero de documento=>")

                usuario[0] = nombre
                usuario[1] = apellido
                usuario[2] = documento

                print("Usuario actualizado correctamente")
        elif opcion == 2:
            print("-" * 50)
            print("Productos disponibles")
            print("-" * 50)
            cont = 0
            for i in productos:
                print(f"[{cont + 1}] {i[0]} - ({i[1]})")
                cont += 1
            producto = int(input("Ingrese el id del producto que quieres agregar =>"))
            producto = (producto - 1)
            print(f"    ¿Cuantas cantidades de '{productos[producto][0]}' quieres?")
            cantidad = int(input("(Ejm:7)-> "))
            producto_carrito = []
            producto_carrito.append(productos[producto][0])
            producto_carrito.append(productos[producto][1])
            producto_carrito.append(cantidad)
            producto_carrito.append((productos[producto][1] * cantidad))
            producto_carrito.append(productos[producto][2])
            carrito.append(producto_carrito)
            print()
            print("PRODUCTO AGREGADO AL CARRITO")
            print()
        elif opcion == 3:
            print("-" * 50)
            if not carrito:
                print("El carrito esta vacio")
            else:
                print("-" * 35)
                print("carrito actual")
                print("-" * 35)
                subtotal = 0
                for i in carrito:
                    print(f"{i[0]}({i[1]}) * {i[2]} = {i[3]}")
                    subtotal += i[3]
                print("-" * 35)
                iva = subtotal * 0.19
                total = (subtotal + iva)

                print("SUBTOTAL =", subtotal)
                print("IVA 19% =", iva)
                print("TOTAL =", total)
                print("-" * 35)
        elif opcion == 4:
            print("-" * 50)
            if not carrito:
                print("El carrito esta vacio")
            else:
                print("Carrito")
                cont = 0
                for i in carrito:
                    print(f"[{cont + 1}] {i[0]}({i[1]}) * {i[2]} = {i[3]}")
                    cont += 1
                producto = int(input("Ingrese el id del producto que quieres actualizar =>"))
                producto = (producto - 1)
                cantidad_nueva = int(input(f"    ¿Cuantas cantidades de '{carrito[producto][0]}' quieres? =>"))
                carrito[producto][2] = cantidad_nueva
                carrito[producto][3] = (carrito[producto][3] * cantidad_nueva)
                print()
                print("PRODUCTO ACTUALIZADO CORRECTAMENTE")
                print()
        elif opcion == 5:
            print("-" * 50)
            if not carrito:
                print("El carrito esta vacio")
            else:
                print("Carrito")
                cont = 0
                for i in carrito:
                    print(f"[{cont + 1}] {i[0]}({i[1]}) * {i[2]} = {i[3]}")
                    cont += 1
                producto = int(input("Ingrese el id del producto que quieres eliminar =>"))
                producto = (producto - 1)
                del carrito[producto]
                print()
                print("PRODUCTO ELIMINADO CORRECTAMENTE")
                print()
        elif opcion == 6:
            print("Fue un gusto atenderte")
            exit()
        else:
            print("la opcion no esta dentro del menu")
except ValueError:
    print("Ingresaste un valor invalido")

​
Code Breakdown

​
Section 1: Data Structures

usuario = []
productos = [
    ["arroz lb", 4000, "comestibles"],
    ["azucar lb", 4500, "comestibles"],
    # ... more products
]
carrito = []
Three Key Data Structures:
  • usuario: A list storing user information [nombre, apellido, documento]
  • productos: A list of lists where each inner list is [nombre, precio, categoria]
  • carrito: A list of lists where each item is [nombre, precio, cantidad, total, categoria]

​
Section 2: User Registration (Option 1)

if opcion == 1:
    if not usuario:
        # First-time registration
        nombre = input("Ingrese su nombre(s) =>")
        apellido = input("Ingrese su apellido(s) =>")
        documento = input("Ingrese su numero de documento=>")
        
        usuario.append(nombre)
        usuario.append(apellido)
        usuario.append(documento)
        
        print("Usuario registrado correctamente")
    else:
        # Update existing user
        usuario[0] = nombre
        usuario[1] = apellido
        usuario[2] = documento
An empty list evaluates to False in Python, so if not usuario checks if the list is empty. This pattern determines whether to register a new user or update an existing one.

​
Section 3: Add Product to Cart (Option 2)

elif opcion == 2:
    print("Productos disponibles")
    cont = 0
    for i in productos:
        print(f"[{cont + 1}] {i[0]} - ({i[1]})")
        cont += 1
    
    producto = int(input("Ingrese el id del producto que quieres agregar =>"))
    producto = (producto - 1)  # Convert to 0-based index
    
    cantidad = int(input(f"¿Cuantas cantidades de '{productos[producto][0]}' quieres? "))
    
    producto_carrito = []
    producto_carrito.append(productos[producto][0])  # name
    producto_carrito.append(productos[producto][1])  # price
    producto_carrito.append(cantidad)                 # quantity
    producto_carrito.append((productos[producto][1] * cantidad))  # total
    producto_carrito.append(productos[producto][2])  # category
    
    carrito.append(producto_carrito)
1

Display Products

Show all available products with 1-based numbering for user convenience
2

Get Selection

Convert user’s 1-based selection to 0-based index for list access
3

Calculate Total

Multiply unit price by quantity to get line item total
4

Build Cart Item

Create a new list with all product details including calculated total

​
Section 4: View Cart with Invoice (Option 3)

elif opcion == 3:
    if not carrito:
        print("El carrito esta vacio")
    else:
        print("carrito actual")
        subtotal = 0
        for i in carrito:
            print(f"{i[0]}({i[1]}) * {i[2]} = {i[3]}")
            subtotal += i[3]
        
        iva = subtotal * 0.19
        total = (subtotal + iva)
        
        print("SUBTOTAL =", subtotal)
        print("IVA 19% =", iva)
        print("TOTAL =", total)
IVA Calculation: IVA (Impuesto al Valor Agregado) is the Value Added Tax used in many Spanish-speaking countries. In this example, it’s calculated as 19% of the subtotal.

​
Section 5: Update Cart Item (Option 4)

elif opcion == 4:
    if not carrito:
        print("El carrito esta vacio")
    else:
        # Display cart items
        cont = 0
        for i in carrito:
            print(f"[{cont + 1}] {i[0]}({i[1]}) * {i[2]} = {i[3]}")
            cont += 1
        
        producto = int(input("Ingrese el id del producto que quieres actualizar =>"))
        producto = (producto - 1)
        cantidad_nueva = int(input(f"¿Cuantas cantidades de '{carrito[producto][0]}' quieres? =>"))
        
        carrito[producto][2] = cantidad_nueva
        carrito[producto][3] = (carrito[producto][1] * cantidad_nueva)
When updating quantity, the code also recalculates the line total by multiplying the unit price (index 1) by the new quantity.

​
Section 6: Delete Cart Item (Option 5)

elif opcion == 5:
    if not carrito:
        print("El carrito esta vacio")
    else:
        # Display cart
        cont = 0
        for i in carrito:
            print(f"[{cont + 1}] {i[0]}({i[1]}) * {i[2]} = {i[3]}")
            cont += 1
        
        producto = int(input("Ingrese el id del producto que quieres eliminar =>"))
        producto = (producto - 1)
        del carrito[producto]

​
Section 7: Error Handling

try:
    while True:
        # All menu logic here
except ValueError:
    print("Ingresaste un valor invalido")
The try-except block catches ValueError exceptions that occur when users enter non-numeric input for operations expecting integers (like menu choices or quantities). This prevents the program from crashing unexpectedly.

​
How to Run

1

Save the File

Save the code in a file named shopping_cart.py
2

Run the Program

Execute from your terminal:
python shopping_cart.py
3

Follow the Workflow

  1. Register a user (Option 1)
  2. Add products to cart (Option 2)
  3. View cart and invoice (Option 3)
  4. Update quantities if needed (Option 4)
  5. Remove items if needed (Option 5)
  6. Exit when done (Option 6)

​
Example Shopping Session

==================================================
||              Carrito de compras              ||
==================================================
--------------------------------------------------
                      MENU                      
--------------------------------------------------

[1] Registrar usuario o actualizar datos usuario
[2] Agregar producto al carrito de compras
[3] Listar el carrito de compras
[4] Actualizar carrito de compras
[5] Eliminar producto en carrito de compras
[6] Finalizar programa
->1
--------------------------------------------------
Ingrese su nombre(s) =>Juan
Ingrese su apellido(s) =>Perez
Ingrese su numero de documento=>12345678
Usuario registrado correctamente

->2
--------------------------------------------------
Productos disponibles
--------------------------------------------------
[1] arroz lb - (4000)
[2] azucar lb - (4500)
[3] platano lb - (1000)
[4] pollo lb - (6000)
...
Ingrese el id del producto que quieres agregar =>1
    ¿Cuantas cantidades de 'arroz lb' quieres?
(Ejm:7)-> 2

PRODUCTO AGREGADO AL CARRITO

->3
--------------------------------------------------
-----------------------------------
carrito actual
-----------------------------------
arroz lb(4000) * 2 = 8000
-----------------------------------
SUBTOTAL = 8000
IVA 19% = 1520.0
TOTAL = 9520.0
-----------------------------------

​
Enhancement Ideas

  1. Discount System: Add coupon codes or promotional discounts
  2. Stock Management: Track inventory and prevent ordering out-of-stock items
  3. Payment Processing: Simulate payment methods and receipt generation
  4. Order History: Save completed orders for the user
  5. Product Search: Add search functionality by name or category
  6. Category Filtering: Show products filtered by category
  7. Multiple Users: Support multiple user accounts with login
  8. Data Persistence: Save cart and user data to files
  9. Price Formatting: Use proper currency formatting for display
  10. Duplicate Prevention: Check if product already in cart and update quantity instead

​
Common Issues and Solutions

Issue: Line 116 has a bug in the total calculation:
carrito[producto][3] = (carrito[producto][3] * cantidad_nueva)
Problem: This multiplies the old total by new quantity instead of recalculating from unit price.Fix: Should be:
carrito[producto][3] = (carrito[producto][1] * cantidad_nueva)
Problem: User enters an invalid product ID.Solution: Add validation:
if 0 <= producto < len(carrito):
    # perform operation
else:
    print("ID de producto invalido")

​
Key Takeaways

  • Nested lists can represent complex data structures like product catalogs and shopping carts
  • Converting between 1-based user display and 0-based indexing is a common pattern
  • Financial calculations should account for subtotals, taxes, and final totals
  • Empty list checking with if not list is a Pythonic way to test for empty collections
  • Try-except blocks provide graceful error handling for user input
  • Recalculating derived values (like totals) is important when updating source values (like quantities)
  • Menu-driven applications benefit from clear visual separators and consistent formatting

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