Testing, debugging, reviewing and refining

Types of error (spec 9.3.1)

Test data

Plan tests before running them, with the expected result for each:

• Normal data: typical valid values.
• Boundary data: values at the edges of the valid range and just outside.
• Erroneous data: wrong type or clearly invalid values, which should be rejected without crashing.

Compare actual results with expected results; any difference is a bug to fix.

Debugging techniques

• Read the error message: it names the exception and the line number.
• Add temporary print() statements to show variables at key points.
• Use the IDE debugger: set a breakpoint, step through line by line and watch variables.
• Dry run with a trace table: one column per variable, one row per change.

n = 5
f = 1
while n > 1:
    f = f * n
    n = n - 1
    print("n =", n, "f =", f)

Output:

n = 4 f = 5
n = 3 f = 20
n = 2 f = 60
n = 1 f = 120

Trace table for the loop above (first row: before the loop)

Reviewing and refining

Once it works, review the code against the task: does it meet every requirement, handle every case, and is the output formatted as asked? Then refine it: remove repetition into subprograms, replace literals with constants, improve names and comments, and reduce the number of passes. Re-test after every change. In the exam, do not add features that were not asked for.

Worked example

A program should accept ages 11 to 18. Plan normal, boundary and erroneous test data.

1. Normal: 14, accept
2. Boundary: 11 and 18, accept; 10 and 19, reject
3. Erroneous: "twelve", rejected with a message, no crash

Answer: Normal 14; boundary 10, 11, 18, 19; erroneous "twelve"

Check yourself

1. A program runs without crashing but calculates an average that is always slightly too high. What kind of error is this?
   • Syntax error
   • Runtime error
   • Logic error
   • Type conversion error
   Show answer

   Logic error. It runs and finishes, so it is not a syntax or runtime error; the result is wrong, so it is a logic error.

2. Which line contains a syntax error?
   • if total > 10
   • total = total + 1
   • print("Total:", total)
   • while total < 10:
   Show answer

   if total > 10. An if statement must end with a colon.

3. A program accepts quantities from 1 to 50. What type of test data is 50?
   • Normal
   • Erroneous
   • Boundary
   • Extreme invalid
   Show answer

   Boundary. 50 is the largest valid value, at the edge of the range.

4. What is displayed when this code runs? ```python a = 1 b = 1 for i in range(4): a, b = b, a + b print(a, b) ```
   Show answer

   5 8. Trace (a, b): (1, 1), (1, 2), (2, 3), (3, 5), (5, 8): 5 8.

5. This code should display 9 but it does not. Which line number contains the error? ``` 1 values = [9, 2, 6, 1] 2 largest = 0 3 for i in range(1, len(values)): 4 if values[i] > largest: 5 largest = values[i] 6 print(largest) ```
   Show answer

   3. The loop starts at index 1, so values[0] (9) is never checked. It should be for i in range(0, len(values)):, or simply for i in range(len(values)):.

6. In an IDE, what does setting a breakpoint do?
   • Deletes the line from the program
   • Pauses the program at that line so you can inspect variables and step on
   • Stops the program from ever reaching that line
   • Converts the line into a comment
   Show answer

   Pauses the program at that line so you can inspect variables and step on. A breakpoint pauses execution so you can watch variables and step through the following lines.

Practise with instant feedback Print this lesson

Sources

• Pearson Edexcel International Advanced Subsidiary/Advanced Level in Computer Science specification (XCP01/YCP01), first teaching 2026
• Pearson IAL Computer Science Unit 2 Programming Language Subset (PLS), Python 3
• Pearson IAL Computer Science IAS sample assessment materials (Unit 2 paper and mark scheme)

Last updated 8 November 2026. Spotted a mistake? Tell us.