Error messaging

301.16 Defensive Design

Defensive Design — Note 5 of 5

Friendly feedback to users.

What you need to know

Good error messages tell the user what went wrong and how to fix it. They should be clear, polite, and specific — never show raw Python tracebacks to users.

What to know

Tell the user WHAT went wrong
Tell the user HOW to fix it
Be specific: "Enter a number between 1 and 10" not "Invalid"
Never show raw tracebacks to end users

Code examples

Friendly error messages

python

mark = input("Enter mark: ")
if not mark.isdigit():
    print("Please enter a whole number.")
elif int(mark) < 0 or int(mark) > 100:
    print("Mark must be between 0 and 100.")
else:
    print("Mark recorded: " + mark)

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Specification notes for programming are in Paper 2.