10. Input Validation Fundamentals

Never trust user input. Validation ensures data is safe before it enters your logic or storage.

Example

def register_user(username):
    if not username.isalnum() or len(username) < 3:
        raise ValueError("Invalid username.")
    print("User registered:", username)

register_user("user123")

✅ Lesson: Always define what valid looks like — do not just look for what’s invalid.