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Exercise: Shopping Cart
Calculate the cost of a shopping cart

Exercise: Shopping Cart

Introduction

We've learned a bunch of Python now! Concepts like operators, variables, and being able to construct strings and print them to the console allow us to put Python to work in a variety of situations!

As a quick recap:

We can use operators to calculate things, like how much 34 apples would cost if they were £0.42 each:

34 * 0.42

We can use variables to store results for later:

my_apples_cost = 34 * 0.42

We can use operators on other types, like strings. Strings support the use of the plus sign to join strings together:

greeting = "Hello, " + "Anna"

We can call the print function, which will then display what we give it to the terminal screen for us to see:

greeting = "Goodbye, " + "Anna"
print(greeting)

Strings don't, however, support using the plus sign to add them to numbers:

age = 39
print("I am " + age + " years old")

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: cannot concatenate 'str' and 'int' objects

But since we know the plus sign can join strings together, we just need to turn the number into a string first:

age_number = 39
age_string = str(age_number)
print("I am" + age_string + " years old")

Exercise

We have a list of items in a shopping cart. We want to find out the total cost, but it's quite complicated. We need to write a Python script to calculate the cost for us!

We have a shopping cart with:

  • 4 apples costing £0.21 each
  • 5 bananas costing £0.12 each

This information should be provided to your Python code like this:

>>> shopping_cart = [{'name': 'apple', 'price_per_item': 0.21, 'number': 4}, {'name': 'banana', 'price_per_item': 0.12, 'number': 5}]

To make life complicated, the following rules apply:

  • Apples are on sale at 50% off
  • There is an additional sales tax for the entire basket of 15%

We want an output of the following form (except with the actual prices instead of £0.00):

Cost of basket before sales and tax: £0.00
Cost of basket after sales, before tax: £0.00
Cost of basket after sales and tax: £0.00

Write the python code required to perform all the calculations - you'll find it helpful to use variables to store your intermediate results. Then, use the print() function to display the output.

It's okay if your code only works for this specific shopping cart. We will revisit this exercise after chapter 5 to handle a cart of any size and with any items!

Hints

  • I'm getting this error: TypeError: cannot concatenate 'str' and 'int' objects
    • You may be trying to use the + operator on a string and an integer - remember that + can only either add two numbers together (4 + 4), or join two strings together ("Hello, " + "Sarah"). You might need to use the str() function to convert your number into a string first.
    • You may prefer using an f-string instead (f"Hello, {name}").
  • To format a number, e.g. to two decimal places, it's best to use a format specifier in the format string. A fixed number of decimal places is a common requirement, so you may find the answer more easily outside of the official Python documentation.

Solution

Click to reveal each block

Expected output 
Cost of basket before sales and tax: £1.44
Cost of basket after sales, before tax: £1.02
Cost of basket after sales and tax: £1.17
Getting the data out of the shopping_cart 
>>> shopping_cart = [{'name': 'apple', 'price_per_item': 0.21, 'number': 4}, {'name': 'banana', 'price_per_item': 0.12, 'number': 5}]
>>> apples = shopping_cart[0]
>>> bananas = shopping_cart[1]
>>> price_of_apples = apples['price_per_item'] * apples['number']
>>> price_of_bananas = bananas['price_per_item'] * bananas['number']
Before discounts/tax 
>>> first_total = price_of_apples + price_of_bananas
>>> print(f'Cost of basket before sales and tax: £{first_total}')
After sales before tax 
>>> discounted_price_of_apples = price_of_apples * 0.5
>>> after_sales_before_tax = discounted_price_of_apples + price_of_bananas
>>> print(f'Cost of basket after sales, before tax: £{after_sales_before_tax}')
After sales and tax 
>>> after_sales_and_tax = after_sales_before_tax * 1.15
>>> print(f'Cost of basket after sales and tax: £{after_sales_and_tax:.2f}')