Card 18: Validation

Module 7: Expert Techniques

1 The Hook

Imagine a nightclub without a bouncer. Anyone (or anything) enters.

Validation is the Bouncer. It stops bad data (like Age: -50) from crashing your app.

We use a while loop to keep asking UNTIL it's right.

2 Worked Example

Validating an Age (Must be 0-120).

# 1. Get initial input

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

# 2. Loop while INVALID (Less than 0 OR > 120)

while age < 0 or age > 120:

print("Impossible!")

# 3. Ask Again inside loop

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

print("Accepted.")

PREDICT

What happens if you enter "1234"?

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

Interactive Editor

main.py

# Menu Validator
choice = int(input("Enter Option (1-3): "))

# Validate: Must be 1, 2, or 3
while choice < 1 or choice > 3:
    print("Invalid option!")
    choice = int(input("Try again (1-3): "))

print("Loading Option " + str(choice) + "...")

Terminal Output

> _

BRONZE (MODIFY)

🥉 Name Check

1. Ask for a name.
2. If user presses Enter without typing, len(name) is 0.
3. Add a loop to force them to type a name.

bronze.py

name = input("Name: ")
# Loop while name is empty
while len(name) == 0:
    print("Required!")
    name = ...
print("Hi " + name)

> _

SILVER (FIX)

🥈 Infinite Loop

1. We want a positive number (num > 0).
2. If user types -5, the loop starts.
3. BUT... we forgot to ask for input inside the loop. It crashes!
4. Fix the infinite loop.

silver.py

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

print("Done")

> _

GOLD (MAKE)

🥇 Range Validator

1. Ask for a test score.
2. It must be between 0 and 100.
3. Use while to reject anything < 0 OR > 100.
4. Print "Saved" when valid.

gold.py

score = int(input("Score: "))

# While INVALID...
while ...:
    print("Invalid")
    score = ...

print("Saved")

> _

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