A2 · Practice questions · Exception handling

Catch the exception.

Six original Cambridge-style questions on exceptions and exception handling, including tracing and writing handler code, with mark-scheme solutions.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Get the marks

Try, catch, recover.

Define: an exception is an unexpected event / error that occurs at run time and disrupts normal execution.
Importance: no crash; meaningful message; files closed and data saved; the program can recover or retry.
Code: risky statements in try; a handler for each expected exception; finally for tidying up.
Appropriate when the cause is outside the program (files, devices, networks, conversions); use IF validation for predictable rules.

Explain

[3 marks]

Explain what is meant by an exception and give two examples of situations that cause one.

An unexpected event / error that occurs while the program is running and disrupts the normal flow of execution. ✓
Example 1: trying to open a file that does not exist. ✓
Example 2: division by zero / converting non-numeric text to a number / array index out of range / loss of a network connection. ✓

Explain

[3 marks]

Explain why exception handling is important in a program that saves customer orders to a file.

Without it, a run-time error (e.g. disk full or file locked) would crash the program. ✓
The handler can give the user a meaningful message and let them retry or save elsewhere. ✓
Data already entered is not lost and the file can be closed safely, so it is not corrupted. ✓

Trace

[4 marks]

A Python program processes the list values = ["8", "0", "x", "4"]. For each item it runs, in a try block, n = int(item) followed by print(24 / n). The handlers are except ValueError: print("not a number") and except ZeroDivisionError: print("cannot divide"). State the output.

"8": 24 / 8 = 3.0 is output. ✓
"0": int succeeds but 24 / 0 raises ZeroDivisionError: cannot divide. ✓
"x": int("x") raises ValueError: not a number. ✓
"4": 24 / 4 = 6.0 is output. ✓

Write

[5 marks]

Write program code in a language of your choice that asks the user for the name of a file, opens it for reading and outputs every line. If the file cannot be found, the program must output a suitable message and ask for another file name until one can be opened.

A loop that repeats until a file has been opened successfully (e.g. a Boolean flag). ✓
Input of the file name inside the loop. ✓
The open statement inside a try block, setting the flag when it succeeds. ✓
An except / catch block for the file-not-found exception that outputs a meaningful message. ✓
After the loop, every line is read and output and the file is closed. ✓

Explain

[2 marks]

State the purpose of a finally block and give an example of a statement that should be placed in one.

It contains code that runs whether or not an exception was raised. ✓
Example: closing a file / releasing a resource / closing a network connection. ✓

Analysis

[4 marks]

A student says: "I will put my whole program in one try block with one handler that catches every exception, so it can never crash." Evaluate this approach.

It would stop the program crashing, but after any error the rest of the try block is skipped, so the program cannot carry on with its task. ✓
A single general handler cannot give a meaningful message or a suitable recovery for each different error. ✓
It can hide logic errors (bugs) that should be found and fixed, making testing harder. ✓
Better: put only risky statements in small try blocks with specific handlers, and use validation for predictable errors. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.