Tutorial 18/20 — Defensive Design

Python: Input Validation

The Bouncer pattern. Protecting your code from bad data.

The Hook

Imagine a nightclub without a bouncer. Anyone—or anything—enters. This is how software crashes.

Validation is the digital bouncer. We use a while loop to keep asking the user for input UNTIL they enter something sensible.

Bad Data (Crash Risk):

"Age: -50", "Password: [empty]"

Validated Data:

Inside range, right length, right type.

Validating an Age range (Must be 0-120).

# Validation Protocol

# 1. Get initial input

age = int(input("Age: "))

# 2. Loop while INVALID

while age < 0 or age > 120:

print("Error: Impossible Age!")

# 3. Ask Again inside loop

age = int(input("Try again: "))

print("Access Granted")

Logic Gate

What happens if you enter "1234"?

pin = input("Set PIN: ")
while len(pin) < 4:
print("Too short.")
pin = input("Set PIN: ")
print("Saved.")

Level 49: Bronze

"Add a loop to force the user to type a name. If they press enter without typing, len(name) is 0."

name = input("Name: ")
# Loop while name was left blank
while len(name) == 0:
    print("Required!")
    name = ...
print("Hi " + name)

> presence_detector_standby

Level 50: Silver

"We forgot to ask for input INSIDE the loop. It stays infinite. Fix it!"

num = int(input("Enter positive: "))
while num < 1:
    print("Must be positive")
    # Fix: Ask for input again here!
    
print("Accepted")

> loop_safety_active

Level 51: Gold

Relational Logic Gate

> range_vector_online