Chapter 43: Credit Card Validator¶
🚀 Open Notebook¶
📺 Video Tutorial¶
Credit card validator in Python 💳 (8:49)
📚 What You’ll Learn¶
Implement the Luhn algorithm to validate credit card numbers - a real-world application of mathematical algorithms!
🎯 Learning Objectives¶
Understand the Luhn algorithm (mod 10 algorithm)
Convert strings to lists of integers
Implement mathematical validation logic
Reverse and manipulate lists
Create practical validation tools
📖 Concept Explanation¶
The Luhn Algorithm¶
The Luhn algorithm (also called mod 10 algorithm) is used to validate credit card numbers, IMEI numbers, and other identification numbers.
How It Works:¶
Remove the last digit (check digit)
Reverse the remaining digits
Double every second digit
If doubled digit > 9, subtract 9
Sum all digits
Add the check digit
If total % 10 == 0, the number is valid
Example:¶
Credit card: 4532015112830366
Check digit: 6
Remaining: 453201511283036
Reversed: 630382115102354
Double every 2nd: 6,6,0,6,8,4,1,2,5,2,0,4,3,10,5,8
Subtract 9 if > 9: 6,6,0,6,8,4,1,2,5,2,0,4,3,1,5,8
Sum: 61
Add check digit: 61 + 6 = 67
67 % 10 = 7 ≠ 0, so invalid (this is just an example)
💡 Examples¶
Basic Validator¶
def validate_card(card_number):
# Remove spaces and convert to string
card_number = card_number.replace(" ", "")
# Check if all characters are digits
if not card_number.isdigit():
return False
# Remove last digit (check digit)
check_digit = int(card_number[-1])
digits = [int(d) for d in card_number[:-1]]
# Reverse the digits
digits.reverse()
# Double every second digit
for i in range(1, len(digits), 2):
digits[i] *= 2
if digits[i] > 9:
digits[i] -= 9
# Sum all digits
total = sum(digits) + check_digit
# Check if divisible by 10
return total % 10 == 0
# Test
card = "4532015112830366"
if validate_card(card):
print("✅ Valid card number")
else:
print("❌ Invalid card number")
Interactive Validator¶
def validate_card(card_number):
# Remove spaces and hyphens
card_number = card_number.replace(" ", "").replace("-", "")
# Validation checks
if not card_number.isdigit():
return False, "Card number must contain only digits"
if len(card_number) < 13 or len(card_number) > 19:
return False, "Card number must be between 13 and 19 digits"
# Luhn algorithm
check_digit = int(card_number[-1])
digits = [int(d) for d in card_number[:-1]]
digits.reverse()
for i in range(1, len(digits), 2):
digits[i] *= 2
if digits[i] > 9:
digits[i] -= 9
total = sum(digits) + check_digit
if total % 10 == 0:
return True, "Valid card number"
else:
return False, "Invalid card number"
# Main program
print("💳 Credit Card Validator")
print("-" * 30)
card = input("Enter card number: ")
is_valid, message = validate_card(card)
if is_valid:
print(f"✅ {message}")
else:
print(f"❌ {message}")
With Card Type Detection¶
def get_card_type(card_number):
"""Identify the card type based on the first digits"""
if card_number.startswith('4'):
return "Visa"
elif card_number.startswith('5'):
return "Mastercard"
elif card_number.startswith('37'):
return "American Express"
elif card_number.startswith('6'):
return "Discover"
else:
return "Unknown"
def validate_card(card_number):
# Remove spaces and hyphens
card_number = card_number.replace(" ", "").replace("-", "")
if not card_number.isdigit():
return False, "Invalid format", None
if len(card_number) < 13 or len(card_number) > 19:
return False, "Invalid length", None
# Luhn algorithm
check_digit = int(card_number[-1])
digits = [int(d) for d in card_number[:-1]]
digits.reverse()
for i in range(1, len(digits), 2):
digits[i] *= 2
if digits[i] > 9:
digits[i] -= 9
total = sum(digits) + check_digit
card_type = get_card_type(card_number)
if total % 10 == 0:
return True, "Valid", card_type
else:
return False, "Invalid checksum", card_type
# Test
print("💳 Credit Card Validator")
print("-" * 40)
card = input("Enter card number: ")
is_valid, message, card_type = validate_card(card)
if is_valid:
print(f"✅ Valid {card_type} card")
else:
print(f"❌ {message}")
if card_type != "Unknown":
print(f" Appears to be {card_type} format")
✍️ Practice Exercises¶
Exercise 1: Basic Validator¶
Implement a simple Luhn algorithm validator that:
Takes a card number as input
Validates using Luhn algorithm
Prints valid or invalid
Exercise 2: Format Checker¶
Add validation for:
Only digits (or spaces/hyphens)
Correct length (13-19 digits)
No letters or special characters
Exercise 3: Batch Validator¶
Create a program that validates multiple cards from a file:
cards = [
"4532015112830366",
"6011514433546201",
"371449635398431",
"0000000000000000"
]
for card in cards:
is_valid, message, card_type = validate_card(card)
status = "✅" if is_valid else "❌"
print(f"{status} {card}: {card_type} - {message}")
🔍 Common Mistakes¶
1. Not Reversing Before Doubling¶
# ❌ Wrong - doubles wrong digits
for i in range(1, len(digits), 2):
digits[i] *= 2
# ✅ Correct - reverse first
digits.reverse()
for i in range(1, len(digits), 2):
digits[i] *= 2
2. Forgetting to Subtract 9¶
# ❌ Wrong - doesn't handle > 9
digits[i] *= 2
# ✅ Correct
digits[i] *= 2
if digits[i] > 9:
digits[i] -= 9
3. Including Check Digit in Sum Before Adding¶
# ❌ Wrong - check digit doubled
digits = [int(d) for d in card_number]
# ✅ Correct - exclude check digit
check_digit = int(card_number[-1])
digits = [int(d) for d in card_number[:-1]]
🎮 Real-World Applications¶
E-commerce: Validate cards before processing
Payment Gateways: Pre-validation before API calls
Form Validation: Client-side card number checking
Testing: Generate valid test card numbers
Security: Quick check before expensive validation
🚀 Challenge Projects¶
Challenge 1: Card Generator¶
Create valid test card numbers using the Luhn algorithm:
def generate_card(prefix, length):
# Generate random digits
# Calculate check digit using Luhn
# Return valid card number
pass
Challenge 2: Masked Display¶
Show card number with masking:
Input: 4532015112830366
Output: 4532 **** **** 0366
Challenge 3: Expiry Date Validator¶
Extend to validate:
Card number (Luhn)
Expiry date (MM/YY format, not expired)
CVV (3-4 digits)
📝 Key Takeaways¶
Luhn algorithm validates identification numbers
Reverse digits before doubling
Double every second digit (after reversing)
Subtract 9 if doubled digit > 9
Sum must be divisible by 10
Always validate format before applying algorithm
Card type can be identified from first digits
🎓 Key Takeaways from Video¶
Lists store multiple items in a single variable
Use loops to repeat actions
Use if-elif-else for conditional logic
💡 These points cover the main concepts from the video tutorial to help reinforce your learning.
🔗 Next Chapter¶
Continue to Chapter 44: Banking Program to build a complete banking application!
Security Note: This validates the format only. Real validation requires checking with the card issuer. Never store card numbers in plain text!