🛒 Shopping Cart Program¶
🚀 Open Notebook¶
📺 Video Tutorial¶
Python shopping cart program 🛒 (9:12)
What You’ll Learn¶
In this chapter, you’ll build a practical shopping cart program that demonstrates how to use parallel lists to store related data, collect user input in a loop, and format output professionally. This project brings together list manipulation, user input validation, and receipt generation.
Learning Objectives¶
Build a complete shopping cart system using parallel lists
Implement input loops with break conditions
Store and manipulate related data across multiple lists
Calculate running totals from list data
Format professional-looking receipts with string formatting
Master list indexing and range-based iteration
Concept Explanation¶
What is a Shopping Cart Program?¶
A shopping cart program simulates a simple point-of-sale system where users can add items with prices, and the program generates a receipt showing all items and the total cost. This is a practical application that combines several fundamental programming concepts.
Parallel Lists¶
Parallel lists are two or more lists where elements at the same index are related to each other. In our shopping cart:
foods = ["Apple", "Banana", "Cherry"]
prices = [1.50, 0.75, 2.25]
# foods[0] and prices[0] are related
# foods[1] and prices[1] are related
# foods[2] and prices[2] are related
The key principle: corresponding indices represent related data.
Key Components¶
1. Input Loop with Sentinel Value¶
Uses a while loop that continues until user enters a quit signal (‘q’):
while True:
food = input("Enter a food item (or 'q' to quit): ")
if food.lower() == "q":
break
# Process the input
2. Parallel List Management¶
Maintains two lists that grow together:
foods.append(food)
prices.append(price)
3. Receipt Generation¶
Uses range(len()) pattern to iterate through parallel lists:
for i in range(len(foods)):
print(f"{foods[i]} ${prices[i]}")
4. String Formatting for Alignment¶
Creates professional output with proper spacing:
print(f"{foods[i]:<20} ${prices[i]:>6.2f}")
# :<20 = left-align in 20 characters
# :>6.2f = right-align in 6 chars, 2 decimal places
Why Use Parallel Lists?¶
While dictionaries might seem more appropriate (and they are for more complex programs), parallel lists teach important concepts:
Index synchronization - Understanding how to keep related data aligned
List manipulation - Working with multiple lists simultaneously
Range iteration - Using indices to access multiple lists
Foundation concepts - Prepares you for more advanced data structures
Program Flow¶
Initialize empty lists and total variable
Input Loop - collect items and prices from user
Validation - check for quit condition
Storage - append to both lists maintaining parallel structure
Display - loop through lists to show receipt
Calculate - accumulate total from prices list
Examples¶
Example 1: Basic Parallel Lists¶
# Creating parallel lists for shopping cart
foods = []
prices = []
# Adding items
foods.append("Apple")
prices.append(1.50)
foods.append("Banana")
prices.append(0.75)
# Accessing related data
for i in range(len(foods)):
print(f"{foods[i]}: ${prices[i]}")
# Output:
# Apple: $1.5
# Banana: $0.75
Example 2: Input Loop with Break¶
# Collecting items until user quits
items = []
while True:
item = input("Enter item (or 'q' to quit): ")
if item.lower() == "q":
break
items.append(item)
print(f"You entered {len(items)} items")
Example 3: Calculating Total¶
prices = [10.50, 25.00, 15.75, 8.25]
# Method 1: Using a loop
total = 0
for price in prices:
total += price
print(f"Total: ${total}") # Total: $59.5
# Method 2: Using sum()
total = sum(prices)
print(f"Total: ${total}") # Total: $59.5
Example 4: String Formatting for Receipts¶
# Left-aligned item names, right-aligned prices
items = ["Coffee", "Sandwich", "Cookie"]
prices = [3.50, 8.95, 2.25]
print("=" * 30)
for i in range(len(items)):
print(f"{items[i]:<20} ${prices[i]:>6.2f}")
print("=" * 30)
# Output:
# ==============================
# Coffee $ 3.50
# Sandwich $ 8.95
# Cookie $ 2.25
# ==============================
Example 5: Input Validation¶
# Ensuring valid price input
while True:
try:
price = float(input("Enter price: $"))
if price < 0:
print("Price cannot be negative!")
continue
break
except ValueError:
print("Please enter a valid number!")
Example 6: Enhanced Receipt with Item Numbers¶
foods = ["Pizza", "Burger", "Fries"]
prices = [12.99, 8.50, 3.99]
print(" YOUR ORDER")
print("=" * 35)
for i in range(len(foods)):
# Add 1 to index for human-readable numbering
print(f"{i + 1}. {foods[i]:<20} ${prices[i]:>6.2f}")
print("=" * 35)
# Output:
# YOUR ORDER
# ===================================
# 1. Pizza $ 12.99
# 2. Burger $ 8.50
# 3. Fries $ 3.99
# ===================================
Example 7: Complete Mini Shopping Cart¶
# Complete shopping cart with all features
cart_items = []
cart_prices = []
total = 0
print("Welcome to Mini Mart!")
while True:
item = input("\nItem name (or 'done'): ")
if item.lower() == "done":
break
try:
price = float(input(f"Price for {item}: $"))
cart_items.append(item)
cart_prices.append(price)
except ValueError:
print("Invalid price! Item not added.")
# Display receipt
print("\n" + "=" * 40)
print(" RECEIPT")
print("=" * 40)
for i in range(len(cart_items)):
print(f"{i+1}. {cart_items[i]:<25} ${cart_prices[i]:>7.2f}")
total += cart_prices[i]
print("=" * 40)
print(f"{'TOTAL':<27} ${total:>7.2f}")
print("=" * 40)
print("Thank you for shopping!\n")
Practice Exercises¶
Beginner Level¶
Simple Cart: Create a program that stores 3 items and their prices in parallel lists, then displays them with total.
Fixed Menu: Create lists of 5 pre-defined items with prices, display them, and let user select by number (1-5).
Item Counter: Modify the shopping cart to display how many items were added before showing the receipt.
Average Price: Calculate and display the average price of all items in the cart.
Price Formatter: Practice string formatting by displaying 10 different prices aligned properly.
Intermediate Level¶
Quantity Tracker: Add a third parallel list to track quantities. Update total calculation to multiply price by quantity.
Remove Item: Add functionality to remove an item from the cart by name before checkout.
Price Range Filter: After checkout, display only items within a specified price range.
Most Expensive Item: Find and display the most expensive item in the cart.
Discount Calculator: Apply a 10% discount to the total if it exceeds $50.
Advanced Level¶
Tax Calculator: Add sales tax calculation (e.g., 8%) to the final total and display both subtotal and total with tax.
Multiple Carts: Allow user to create multiple shopping carts (using lists of lists) and compare totals.
Edit Item: Add functionality to modify an existing item’s name or price before checkout.
Receipt Export: Save the receipt to a text file with timestamp and cart details.
Inventory System: Start with a predefined inventory (items and stock levels) and decrease stock when items are added to cart. Prevent adding out-of-stock items.
Common Mistakes to Avoid¶
Mistake 1: Forgetting to Convert Price to Float¶
Wrong:
price = input("Enter price: $")
prices.append(price) # Stored as string!
total += prices[i] # Error: can't add strings
Correct:
price = float(input("Enter price: $"))
prices.append(price) # Stored as float
total += prices[i] # Works correctly
Why: Input always returns a string. Must convert to float for mathematical operations.
Mistake 2: Misaligned Parallel Lists¶
Wrong:
foods.append(food)
if price > 0: # Conditional append!
prices.append(price)
# Now lists are different lengths!
Correct:
if price > 0:
foods.append(food)
prices.append(price)
# Both lists grow together
Why: Parallel lists must maintain the same length with corresponding indices.
Mistake 3: Incorrect Range Usage¶
Wrong:
for i in range(len(foods) + 1): # Off by one!
print(foods[i]) # IndexError on last iteration
Correct:
for i in range(len(foods)):
print(foods[i]) # Correct indexing
Why: Lists are 0-indexed. If len(foods) is 3, valid indices are 0, 1, 2.
Mistake 4: Not Formatting Decimal Places¶
Wrong:
print(f"Total: ${total}") # Displays: $10.5
Correct:
print(f"Total: ${total:.2f}") # Displays: $10.50
Why: Money should always display 2 decimal places for professionalism and clarity.
Real-World Applications¶
1. E-Commerce Checkout Systems¶
Online shopping carts that track items, quantities, and prices before final purchase. Amazon, eBay, and all e-commerce platforms use sophisticated versions of this concept.
2. Restaurant POS Systems¶
Point-of-sale terminals in restaurants use similar logic to track orders, items, and prices before generating a bill. The parallel list concept scales to include modifiers, special requests, and table numbers.
3. Expense Tracking Apps¶
Personal finance apps track expenses by category and amount, then generate reports and totals. This is essentially a shopping cart for tracking spending.
4. Inventory Management¶
Retail stores use parallel data structures to track product names, SKUs, quantities, prices, and locations. When items are sold, these lists are updated together.
Challenge Projects¶
1. Multi-User Shopping Cart¶
Create a system that supports multiple users, each with their own cart. Use a dictionary where keys are usernames and values are lists of lists (cart data).
Requirements:
User login/registration
Each user has separate cart
Display all users’ totals at the end
Find user who spent the most
2. Smart Grocery List¶
Build a shopping cart that suggests items based on previous purchases and warns about duplicate entries.
Requirements:
Store purchase history
Suggest frequently bought items
Warn if adding duplicate items
Calculate savings compared to regular price
3. Receipt Generator with Barcode¶
Create a detailed receipt system with item codes, taxes, and discounts.
Requirements:
Generate unique item codes
Calculate multiple tax rates
Apply discount codes
Export to formatted text file
4. Budget-Aware Shopping Cart¶
Implement a cart that helps users stay within budget by tracking spending in real-time.
Requirements:
Set initial budget
Show remaining budget after each item
Warn when approaching budget limit
Suggest items to remove if over budget
5. Comparison Shopping Tool¶
Create a program that compares prices across multiple stores for the same items.
Requirements:
Store prices from 3 different stores
Calculate total for each store
Recommend cheapest store
Calculate savings by shopping at cheapest store
🎓 Key Takeaways from Video¶
Lists store multiple items in a single variable
Use loops to repeat actions
Use if-elif-else for conditional logic
💡 These points cover the main concepts from the video tutorial to help reinforce your learning.