30 min read

Putting It Together: the Shop Till

A complete program using variables, input, decisions, a loop and functions — planned with a flowchart, then coded and tested.

The Brief

Step 1 — Plan

Inputs, processing, outputs
Inputs, processing, outputs

Step 2 — the flowchart

Read this flowchart as text

Terminator: Start (then → total = 0)
Process: total = 0 (then → INPUT price)
Input/output: INPUT price (then → price > 0?)
Decision: price > 0? (Yes → INPUT quantity; No → to_pay = apply_discount(total))
Input/output: INPUT quantity (then → total = total + line_total(price, quantity))
Predefined process: to_pay = apply_discount(total) (then → print_receipt(total, to_pay))
Predefined process: total = total + line_total(price, quantity) (then → INPUT price)
Predefined process: print_receipt(total, to_pay) (then → Stop)
Input/output: INPUT price (then → price > 0?)
Terminator: Stop

The double-sided boxes are calls to functions (predefined processes).

Step 3 — The Code

till.pyPython

1# till.py - Perera Corner Mart till2# Adds up items until a price of 0, applies a 10% discount over LKR 5,000,3# and prints a receipt.4 5DISCOUNT_LIMIT = 50006DISCOUNT_RATE = 0.107 8 9def line_total(price, quantity):10    return price * quantity11 12 13def apply_discount(total):14    if total > DISCOUNT_LIMIT:15        return total * (1 - DISCOUNT_RATE)16    return total17 18 19def print_receipt(subtotal, to_pay):20    print("-" * 30)21    print(f"{'Subtotal':<18}{subtotal:>12,.2f}")22    print(f"{'Discount':<18}{subtotal - to_pay:>12,.2f}")23    print(f"{'AMOUNT TO PAY':<18}{to_pay:>12,.2f}")24    print("-" * 30)25    print("Thank you for shopping!")26 27 28subtotal = 029price = float(input("Price (0 to finish): "))30 31while price > 0:32    quantity = int(input("Quantity: "))33    subtotal += line_total(price, quantity)34    price = float(input("Price (0 to finish): "))35 36to_pay = apply_discount(subtotal)37print_receipt(subtotal, to_pay)

Inputs (one per line — used by input() in order)

Expected output

Price (0 to finish): 1450
Quantity: 3
Price (0 to finish): 480
Quantity: 2
Price (0 to finish): 0
------------------------------
Subtotal              5,310.00
Discount                531.00
AMOUNT TO PAY         4,779.00
------------------------------
Thank you for shopping!

Step 4 — Test

Test plan
Test plan

Run each test by changing the Inputs box. Record the actual result and a screenshot.

Practice

The subtotal is exactly 5,000. What does apply_discount return?

More lessons in Python Programming · Next: Debugging Your Programs · Previous: Functions