The age that was "banana"

A student club has a sign-up form. Somebody types "banana" for their age. Somebody leaves their name empty. Somebody types an age of 400.

The program saves it all. Now the club list is full of wrong data. Good programs stop bad data before it gets in.

Key words

Word	What it means	Example
Validation	A check that data makes sense before the program uses it.
Valid	Data that passes the check.	an age of 16
Invalid	Data that fails the check.	an age of 400
Presence check	Checks that something was typed.
Length check	Checks how many characters there are.
Range check	Checks that a number is between a lowest and a highest value.
Type check	Checks that the data is the right kind, such as a whole number.
Format check	Checks that the data follows a pattern.	an email has an @
Check digit	An extra digit worked out from the other digits. It finds typing mistakes.	the last digit of a barcode

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

What validation does

Validation asks: does this data make sense? It runs when a user types something, before the program uses the data. If the data is invalid, the program asks again.

Validation cannot tell if data is true. An age of 25 passes a range check even if the user is 15.

Try it: presence and length

Code: python

name = ""
password = "abc123"

if len(name) == 0:
    print("Please type your name.")
if len(password) < 8:
    print("The password needs at least 8 characters.")

# Give name and password good values. The messages disappear.

The five checks

Check	The question it asks	Example
Presence	Is there something?	A name is not empty
Length	Is it long or short enough?	A password has 8 to 20 characters
Range	Is the number between the lowest and highest?	An age is from 0 to 120
Type	Is it the right kind of data?	An age is a whole number
Format	Does it follow the pattern?	An email has an @ and a dot
Check digit	Does the last number fit the others?	The next lesson

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

Try it: type and range

Code: python

text = "250"

if not text.isdigit():
    print("Type check failed: use digits only.")
elif int(text) > 120:
    print("Range check failed: the age is more than 120.")
else:
    print("Age accepted.")

# Try "abc", "16", "-5" and "250".

Try it: a format check

Code: python

email = "[email protected]"

if "@" in email and "." in email:
    print("The format looks right.")
else:
    print("An email needs an @ and a dot.")

# Try "sam.mail.com".

Try it: is it a real date?

Code: python

from datetime import datetime

text = "25/12/2026"

try:
    when = datetime.strptime(text, "%d/%m/%Y")
    print("A real date:", when.strftime("%A %d %B %Y"))
except ValueError:
    print("Please write the date like 25/12/2026.")

# Try "31/02/2026" and "tomorrow". Both fail the check.

isdigit has limits

data-callouts.warning.label

"16".isdigit() is True. But "-5".isdigit() and "12.5".isdigit() are False, because of the minus sign and the dot. isdigit() is True only when every character is a digit. For whole numbers from 0 up, that is just what you want.