Level 5 · Boss: build the checkout calculator

Putting the basics together

Real analysis code is rarely one clever line. It’s a short chain of plain steps, each easy to check: compute something, adjust it by a rule, present it. This boss level is exactly that chain, and it uses everything from the chapter: variables, arithmetic, rounding, string cleaning, conditions and f-strings.

Step 1: the subtotal

A cart line is price times quantity. A subtotal is the sum of the lines:

subtotal = lamp_price * lamp_qty + pillow_price * pillow_qty

Round once, at the end, to two decimals. Rounding each line first can make the total drift by a cent on larger carts.

Step 2: a coupon typed by a human

People type coupon codes in every possible way: pine10, PINE10, Pine10 . If you compare the raw text to "PINE10", most of them fail. Normalise first, then compare:

code = coupon.strip().upper()
if code == "PINE10":
    discounted = subtotal * 0.9
else:
    discounted = subtotal

A 10% discount means the customer pays 90%, so multiplying by 0.9 is the same as subtotal - subtotal * 0.10. Both are correct; pick the one that reads more clearly to you.

Notice the else branch. Without it, discounted would not exist when the coupon is wrong, and the next line would fail with a NameError. Every path through your if should leave the variables you use later defined.

Step 3: shipping and a readable total

The shipping rule depends on the discounted amount, which is a common business choice: the customer has to spend $75 after discounts to earn free shipping. Once you have the final number, format it for people:

total = discounted + shipping
f"Total: ${total:.2f}"   # 'Total: $181.78'

The :.2f format both rounds and pads, so 181.8 would print as 181.80. For support agents, that consistency matters: a total that sometimes shows one decimal looks like an error.

Thinking like an analyst

Before writing code for a rule like this, write the rule down in plain words and check the edge cases with the person who asked:

• Does the coupon apply before or after shipping? (Here: before, to the items only.)
• Is free shipping judged before or after the discount? (Here: after.)
• What happens at exactly $75? (Here: free.)

Most “bugs” in business calculations are not Python mistakes. They are rules that were never pinned down. Asking these questions early is a big part of the job.

Common mistakes

• Comparing the coupon without cleaning it, so " pine10 " never matches.
• Applying shipping to the subtotal before the discount, which changes who gets free shipping.
• Returning the number instead of the formatted text in the last step. The task asks for the string Total: $181.78, so result must be a str.