Function to Calculate the Total Price of Items in a Shopping Cart

This section describes a Python function that calculates the total price of items in a shopping cart, applying any applicable discounts.

Code Implementation

from typing import List, Dict

def calculate_total_price(cart_items: List[Dict[str, float]], discount: float = 0.0) -> float:
    """
    Calculate the total price of items in the shopping cart.

    Parameters:
    cart_items (List[Dict[str, float]]): A list of dictionaries where each dictionary contains:
        - 'price' (float): The price of the item.
        - 'quantity' (int): Number of items.
    discount (float): Optional discount rate to apply to the total (as a percentage).

    Returns:
    float: The total price after applying discounts.

    Raises:
    ValueError: If any item has a negative price, quantity, or discount is out of the 0-100 range.
    """
    
    if not (0 <= discount <= 100):
        raise ValueError("Discount must be between 0 and 100.")
    
    total_price = 0.0
    
    # Calculate total price from item prices and quantities
    for item in cart_items:
        price = item.get('price', 0)
        quantity = item.get('quantity', 1)
        
        # Validate price and quantity
        if price < 0 or quantity < 0:
            raise ValueError("Item price and quantity must be non-negative.")
        
        total_price += price * quantity  # Add the item's total price to the total
    
    # Apply discount if applicable
    if discount > 0:
        total_price -= total_price * (discount / 100)
    
    return total_price

Explanation and Commentary

• Imports: The function utilizes List and Dict from the typing module which helps in defining the input types.

• Docstring: A comprehensive docstring is provided to clarify the function’s intent, parameters, and return type.

• Input Validation:

  • It checks if the discount rate is between 0 and 100.
  • It validates that price and quantity values are not negative.
  • Raises ValueError for any invalid input.

• Logic:

  • The function iterates through the cart_items, computes the total price based on individual item price and quantity.
  • If a discount is specified, it deducts the amount from the total before returning.

Example Usage

Here’s how you can use the calculate_total_price function in a real-world scenario:

shopping_cart = [
    {'price': 20.0, 'quantity': 2},  # 2 items at $20 each
    {'price': 5.0, 'quantity': 3},   # 3 items at $5 each
]

# Calculate total price without discount
total_without_discount = calculate_total_price(shopping_cart)
print(f"Total Price without Discount: ${total_without_discount:.2f}")

# Calculate total price with a 10% discount
total_with_discount = calculate_total_price(shopping_cart, discount=10)
print(f"Total Price with 10% Discount: ${total_with_discount:.2f}")

Conclusion

The calculate_total_price function is designed for robustness and clarity. It effectively handles input validation and provides clear output. This function can be further extended to accommodate additional features such as different types of discounts or tax calculations.

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