Anticipate misuse

301.16 Defensive Design

Defensive Design — Note 1 of 5

Predict invalid input.

What you need to know

Defensive design means writing code that handles unexpected or invalid input gracefully instead of crashing (e.g. wrapping int(input(...)) in try/except). Always assume the user will make mistakes.

What to know

Users may enter text when numbers are expected
Users may leave fields blank
Users may enter values outside the valid range
Plan for every possible wrong input

Code examples

Anticipating misuse

python

# Without defence — crashes on bad input
age = int(input("Age: "))  # Crashes if user types "abc"

# With defence
try:
    age = int(input("Age: "))
except ValueError:
    print("Please enter a number")

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Specification notes for programming are in Paper 2.