Project: a tiny shop

What you will be able to do

After this lesson you can

• plan a program that keeps its data in dictionaries,
• handle the three cases of an item: sold, out of stock, and not sold at all,
• update a dictionary as the program runs, and
• test the unusual cases and extend the program.

The goal

A small shop sells pens, books, bags and rulers. The shop has a price for each item, and a stock: how many are on the shelf. A customer brings a basket, a list of the items that the customer wants. The program should

1. add the price of each item that can be sold to the total,
2. take one item off the shelf for each one sold,
3. tell the customer about an item that is out of stock, and about an item that the shop does not sell,
4. show the total to pay, and the stock that is left.

Step 1: the data, and the expected answers

Two dictionaries, with the same keys, and a list:

• prices: pen 10, book 80, bag 450, ruler 15.
• stock: pen 20, book 5, bag 0, ruler 12.
• basket: pen, pen, book, bag, eraser, ruler.

Work out the answer by hand first.

The total to pay must be 115. The stock must become: pen 18, book 4, bag 0, ruler 11.

Step 2: the program

This is a general idea: put the check that protects the later lines first.

Notice also that we use the dictionaries as tables of facts (prices, stock), and a list as an ordered sequence of events (the basket). The data decides which one to use.

Step 3: test the unusual cases

The example had one of each case. Now change the data and work out the answers by hand first.

The second row is the edge: the stock goes to 0 in the middle of the basket. The elif stock[item] == 0 must then refuse the sixth book. Does it?

The order of the tests matters: first is the item sold at all, then is there enough of it.

Step 4: extend it

Pick one or two, and do them without looking at a solution. Write the expected answers first.

1. A receipt. Count how many of each item were bought, with the counting pattern. Show pen x 2, book x 1, and so on.
2. A discount. If the total is 500 or more, take 10 percent off.
3. More stock than 1. A basket as a dictionary: {"pen": 3, "book": 1}, where the value is the number wanted. What changes in the loop?
4. A restock. A function restock(stock, item, amount) that adds to the stock of an item, or makes a new item.
5. Cheapest item. Find the item with the lowest price, with the "best so far" pattern.

Common mistakes

• Looking up an item before checking that it exists. A KeyError.
• Forgetting to take the item off the shelf. The stock never goes down.
• Testing stock[item] == 0 and forgetting a negative stock. Use <= 0 if a stock can go below 0.
• The prices and the stock with different keys. An item in one but not in the other gives a KeyError.
• Testing only the example. An empty basket and a sold-out item are the real tests.

Practice

Start with two quick questions. Each New numbers gives you a fresh one, so you can practise as long as you like.

Then the labs and the problems:

1. Warm-up. Open the lab Project: the tiny shop. The checker runs your shop on a basket that is different from the example.
2. Build. Do Step 4 for one extension, and add rows to the test table.
3. Stretch. Open the lab A phone book again, and write down which lines use the same ideas as the shop.

Remember these

• Use a dictionary for facts that you look up by name, and a list for an ordered sequence.
• Put the check that protects the later lookups first.
• A dictionary can be updated as the program runs.
• Test the unusual cases: empty, sold out, not sold.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Why does item not in prices come first in the if?

2. What should the stock of a pen be after two pens are sold from 20?

3. What does the shop show for an item that is not in prices?