# Debug an ISBN Validator - Debug an ISBN Validator **URL:** \ **Category:** Curriculum Help\ **Tags:** python\ **Created:** [April 16, 2026, 5:54pm UTC](https://forum.freecodecamp.org/t/debug-an-isbn-validator-debug-an-isbn-validator/786052 "2026-04-16T17:54:54Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![Mester](https://avatars.discourse-cdn.com/v4/letter/m/35a633/32.png) [@Mester](https://forum.freecodecamp.org/u/Mester)\ **Post date:** [April 16, 2026, 5:54pm UTC](https://forum.freecodecamp.org/t/debug-an-isbn-validator-debug-an-isbn-validator/786052/1 "2026-04-16T17:54:54Z")
### Tell us what’s happening: I am only missing task 12 and 13 but have no idea how to do them. If I add “if” statements, the other completed tasks will just fail. ### Your code so far ```py def validate_isbn(isbn, length): if length == 10: if not isbn[:9].isdigit(): print('Invalid character was found.') return if not (isbn[-1].isdigit() or isbn[-1] == 'X'): print('Invalid character was found.') return elif length == 13: if not isbn.isdigit(): print('Invalid character was found.') return main_digits = isbn[:-1] given_check_digit = isbn[-1] print("DEBUG main_digits:", main_digits) print("DEBUG given_check_digit:", given_check_digit) main_digits_list = [int(digit) for digit in main_digits] # Calculate the check digit from other digits if length == 10: expected_check_digit = calculate_check_digit_10(main_digits_list) else: expected_check_digit = calculate_check_digit_13(main_digits_list) # Check if the given check digit matches with the calculated check digit if given_check_digit == expected_check_digit: print('Valid ISBN Code.') else: print('Invalid ISBN Code.') def calculate_check_digit_10(main_digits_list): # Note: You don't have to fully understand the logic in this function. digits_sum = 0 # Multiply each of the first 9 digits by its corresponding weight (10 to 2) and sum up the results for index, digit in enumerate(main_digits_list): digits_sum += digit * (10 - index) # Find the remainder of dividing the sum by 11, then subtract it from 11 result = 11 - digits_sum % 11 # The calculation result can range from 1 to 11. # If the result is 11, use 0. # If the result is 10, use upper case X. # Use the value as it is for other numbers. if result == 11: expected_check_digit = '0' elif result == 10: expected_check_digit = 'X' else: expected_check_digit = str(result) return expected_check_digit def calculate_check_digit_13(main_digits_list): # Note: You don't have to fully understand the logic in this function. digits_sum = 0 # Multiply each of the first 12 digits by 1 and 3 alternately (starting with 1), and sum up the results for index, digit in enumerate(main_digits_list): if index % 2 == 0: digits_sum += digit * 1 else: digits_sum += digit * 3 # Find the remainder of dividing the sum by 10, then subtract it from 10 result = 10 - digits_sum % 10 # The calculation result can range from 1 to 10. # If the result is 10, use 0. # Use the value as it is for other numbers. if result == 10: expected_check_digit = '0' else: expected_check_digit = str(result) return expected_check_digit def main(): user_input = input('Enter ISBN and length: ') if "," not in user_input: print("Enter comma-separated values.") return values = user_input.split(',') isbn = values[0] length_str = values[1] if not length_str.isdigit(): print("Length must be a number.") return length = int(length_str) if length == 10 or length == 13: validate_isbn(isbn, length) else: print('Length should be 10 or 13.') #main() ``` ### Your browser information: User Agent is: `Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:149.0) Gecko/20100101 Firefox/149.0` ### Challenge Information: Debug an ISBN Validator - Debug an ISBN Validator > **[freeCodeCamp.org](https://www.freecodecamp.org/learn/python-v9/lab-isbn-validator/debug-an-isbn-validator)** > > Learn to Code — For Free ---
**Author:** ![dhess](https://sea3.discourse-cdn.com/freecodecamp/user_avatar/forum.freecodecamp.org/dhess/32/406235_2.png) [@dhess](https://forum.freecodecamp.org/u/dhess)\ **Post date:** [April 16, 2026, 7:40pm UTC](https://forum.freecodecamp.org/t/debug-an-isbn-validator-debug-an-isbn-validator/786052/2 "2026-04-16T19:40:34Z")
Hi @Mester, It looks like you have completely rewritten this area of the code that checks the length of the ISBN and returns the expected message if the length doesn’t match: ```auto if len(isbn, length) != length: print(f'ISBN-{length} code should be {length} digits long.') return main_digits = isbn[0:length] given_check_digit = isbn[length] main_digits_list = [int(digit) for digit in main_digits] ``` Also, the theory lecture preceding this challenge talks about exception handling, but I don’t see that being used in your code. You may want to review. Happy coding! ---
**Author:** ![system](https://global.discourse-cdn.com/freecodecamp/original/3X/e/1/e1c433102e34b46dfaa3e15d13ab9a514b127078.svg) [@system](https://forum.freecodecamp.org/u/system)\ **Post date:** [May 14, 2026, 7:41pm UTC](https://forum.freecodecamp.org/t/debug-an-isbn-validator-debug-an-isbn-validator/786052/3 "2026-05-14T19:41:17Z")
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