The cash machine and the wrong PIN

You type the wrong PIN at a cash machine. It does not crash. It does not show a strange code. It says: "Wrong PIN. You have 2 tries left."

The machine expected a mistake. Good programs expect mistakes too. Good code is robust: it keeps working when something unexpected happens.

Key words

Word	What it means	Example
Robust	Keeps working, or stops safely, when something unexpected happens.
Exception	An error that Python raises while the program runs.	ValueError
try	Starts a block of code that might cause an exception.
except	Starts the code that runs if the try block raises that exception.
Unexpected input	Data the program did not plan for.	letters in an age box
Error message	Text that tells the user what went wrong and what to do.
Validation	A check that data is sensible.
Crash	The program stops because of an error it did not handle.

The cells students fill in are left blank in this preview.

Run it: a crash

Code: python

text = "twelve"
age = int(text)
print("Next year you will be", age + 1)

# Run it. Python raises an exception and the program stops.
# Read the last line of the message. What kind of exception is it?

Three signs of robust code

Robust code does three things.

It copes with unexpected input. A letter in an age box does not crash it.
It copes with unexpected endings. A missing file or a lost connection does not lose the user's work.
It gives specific, meaningful error messages. The user learns what to do next.

Robust and not robust

Situation	Not robust	Robust
The user types "abc" for an age	The program crashes	It says: please type a whole number
A data file is missing	A long error appears and the program stops	It says: budget.txt was not found. Check the name
The power fails while saving	The saved file is incomplete	The program saves often, so little is lost
The user divides by zero	The program stops	It says: you cannot divide by zero

The cells students fill in are left blank in this preview.

Catching an exception

Python has a tool for this. Put the risky code in a try block. Put the plan B in an except block.

If nothing goes wrong, Python skips the except block. If an exception happens, Python jumps to it.

Try it: try and except

Code: python

text = "twelve"

try:
    age = int(text)
    print("Next year you will be", age + 1)
except ValueError:
    print("Please type your age as digits, for example 12.")

# Change "twelve" to "12" and run it again.

Try it: two more exceptions

Code: python

try:
    print(10 / 0)
except ZeroDivisionError:
    print("You cannot divide by zero.")

try:
    file = open("budget_2026.txt")
    print(file.readline())
    file.close()
except FileNotFoundError:
    print("budget_2026.txt was not found. Check the file name.")

Name the exception

data-callouts.warning.label

Write except ValueError, not just except. A plain except hides every error, even your own bugs. Catch the exception you expect and let the rest show.

A good error message

Bad message	Why it is bad	Better message
Error 7	The user cannot tell what it means	Your age must be a whole number, such as 16.
Invalid input	It does not say which input or why	The postcode must have 5 to 8 characters.
Something went wrong	No clue what to do next	The file could not be saved. Check you have space, then try again.

The cells students fill in are left blank in this preview.