Worked example: Testing and Debugging a Python Program

What this is

A complete, secure Year 11 answer, annotated so you can see which success criterion each part meets. It is also the answer key. Every result below was checked by running the program. The four faults are: line 7 uses > where it needs >=; line 13 uses or where it needs and; line 21 multiplies by 0.1 where it needs 0.9; lines 6 and 30 call int() without checking the input first, so words and empty boxes crash the program. Pupils may choose different test data; what matters is that every edge in R2, R3 and R4 is tested and the expected result comes from the requirements.

Steps 2 and 3: the test plan with results

Every age test uses 3 lessons. Every lessons test uses age 9 and the surname Jones.

No.	Input tested	Data	Type	Expected result	Actual result	Pass or fail
1	Age	9 (lessons 3)	Typical	Booked: JON9, total £19.50	Booked: JON9, total £19.50	Pass
2	Age	4	Extreme	Accepted: Booked: JON4, total £19.50	Age must be 4 to 16	Fail
3	Age	16	Extreme	Accepted: Booked: JON16, total £19.50	Booked: JON16, total £19.50	Pass
4	Age	3	Erroneous	Age must be 4 to 16	Age must be 4 to 16	Pass
5	Age	17	Erroneous	Age must be 4 to 16	Age must be 4 to 16	Pass
6	Age	ten	Erroneous	Age must be 4 to 16	Crash, ValueError at line 6	Fail
7	Age	(blank)	Erroneous	Age must be 4 to 16	Crash, ValueError at line 6	Fail
8	Lessons	1	Extreme	Booked: JON9, total £6.50	Booked: JON9, total £6.50	Pass
9	Lessons	10	Extreme	10% off: £65.00 × 0.9 = £58.50	Booked: JON9, total £6.50	Fail
10	Lessons	5	Typical, just under the discount	No discount: £32.50	Booked: JON9, total £32.50	Pass
11	Lessons	6	Extreme, first discounted block	10% off: £39.00 × 0.9 = £35.10	Booked: JON9, total £3.90	Fail
12	Lessons	0	Erroneous	Lessons must be 1 to 10	Booked: JON9, total £0.00	Fail
13	Lessons	11	Erroneous	Lessons must be 1 to 10	Booked: JON9, total £7.15	Fail
14	Lessons	six	Erroneous	Lessons must be 1 to 10	Crash, ValueError at line 30	Fail

Both inputs have typical, extreme and erroneous tests, and the discount is tested on both sides of the edge (5 and 6) and at the top (10). Expected results are worked out from R2 to R4, so the plan can catch the developer’s mistakes rather than copy them. Crashes are recorded with the error and the line. Evidences criteria 1 and 2.

Step 4: the faults and the fixes

Tests that failed	Line(s)	What is wrong, and which requirement it breaks	Corrected code
2	7	age > 4 is false when the age is exactly 4, so the youngest allowed child is rejected. Breaks R2.	if age >= 4 and age <= 16:
12, 13	13	With or, every whole number passes: 0 is <= 10 and 11 is >= 1. Both conditions must be true. Breaks R3.	if lessons >= 1 and lessons <= 10:
9, 11	21	Multiplying by 0.1 keeps 10% of the price instead of taking 10% off. Breaks R4.	total = total * 0.9
6, 7, 14	6 and 30	int() is called on whatever was typed, so a word or an empty box stops the program with ValueError. Breaks R2, R3 and R6.	Check with .isdigit() before converting. See the corrected functions below.

The corrected validation functions and main program:

def check_age(age_text): if not age_text.isdigit(): return False age = int(age_text) if age >= 4 and age <= 16: return True else: return False def check_lessons(lessons_text): if not lessons_text.isdigit(): return False lessons = int(lessons_text) if lessons >= 1 and lessons <= 10: return True else: return False # main program, lines 29 to 33 become: if check_age(age_text): lessons_text = input("Number of lessons: ") if check_lessons(lessons_text): reference = make_reference(surname, age_text) price = block_price(int(lessons_text), int(age_text))

Rerunning all 14 tests with these changes: every test passes. Age 4 is booked as JON4 for £19.50, 10 lessons cost £58.50, 6 lessons cost £35.10, and 0, 11, “ten”, “six” and a blank box each give the correct message.

One row per fault, each with the exact line, the reason in terms of how Python evaluates it, the requirement it breaks, and a specific change. Moving the int() after an .isdigit() check fixes the crash in both places, and the full retest proves the changes worked. Evidences criteria 2 and 3.

Step 5: report and refine

(a) Siw, I ran 14 tests and 6 passed. There are four faults. Age 4 is wrongly rejected because line 7 uses > instead of >=. Any number of lessons is accepted, including 0 and 11, because line 13 uses or instead of and. Every discounted block is charged 10% of the price instead of 90%, so 6 lessons cost £3.90 instead of £35.10. Typing a word or leaving a box empty crashes the program at lines 6 and 30. The developer only tried typical values such as age 9 and 3 lessons, which all pass. Before the program goes on the desk, the four fixes must be made and all 14 tests run again.

Written for a client, not a programmer: it gives the pass rate, names each fault by its effect on the pool, explains why the developer missed them (only typical data was tried), and says what must happen next. Evidences criterion 4.

(b) The new function, with the Step 4 fix to the discount included:

def block_price(lessons, age): if age <= 7: total = lessons * 5.00 else: total = lessons * PRICE_PER_LESSON if lessons >= DISCOUNT_LESSONS: total = total * 0.9 return total

Line 33 changes to price = block_price(int(lessons_text), int(age_text)). Age 4 is already allowed by check_age, so age <= 7 covers 4 to 7.

(c)

Age	Lessons	Type	Expected result
7	3	Extreme, oldest at the lower price	3 × £5.00 = Booked: JON7, total £15.00
8	3	Extreme, youngest at the full price	3 × £6.50 = Booked: JON8, total £19.50
5	6	Typical, lower price with discount	6 × £5.00 × 0.9 = Booked: JON5, total £27.00

The new function keeps the constant for the full price, fixes the discount, and puts the new rule in one place. The tests sit either side of the new edge (7 and 8) and combine the new price with the discount. Evidences criteria 3 and 5.

If you finish early: answers

ap Rhys, aged 12, gives AP 12, with a space, and Li, aged 9, gives LI9, only 2 letters. R5 assumes every surname has 3 letters and no spaces. A better rule: remove spaces and apostrophes first, then pad short names with X, so the references are APR12 and LIX9. Check the new rule with Siw before changing it, because it changes the requirement.
" 9".isdigit() is false, so the program says Age must be 4 to 16. That is validation: checking the input is sensible. Verification would be checking it is what was meant, such as asking the receptionist to confirm the age. Using age_text.strip() first removes the space without accepting anything R2 forbids.
A window with three labelled Entry widgets (surname, age, lessons), a Book button whose command runs the checks and the price, and a Label under the button that shows either the booking or the error message in the same place every time.

Why this response is secure

The test data was chosen from the requirements, before looking for bugs, so it tests what the pool asked for.
Every edge is tested from both sides, which is where three of the four faults were hiding.
Each fix is specific: the line, the change, the requirement, and a retest to prove it.
The commentary explains the results to someone who cannot read Python.
The change for the new requirement reuses the fixed code and comes with its own tests.