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to ISBN Verifier in the Python Track

Published at Nov 15 2020 · 0 comments
Test suite

The ISBN-10 verification process is used to validate book identification numbers. These normally contain dashes and look like: 3-598-21508-8


The ISBN-10 format is 9 digits (0 to 9) plus one check character (either a digit or an X only). In the case the check character is an X, this represents the value '10'. These may be communicated with or without hyphens, and can be checked for their validity by the following formula:

(x1 * 10 + x2 * 9 + x3 * 8 + x4 * 7 + x5 * 6 + x6 * 5 + x7 * 4 + x8 * 3 + x9 * 2 + x10 * 1) mod 11 == 0

If the result is 0, then it is a valid ISBN-10, otherwise it is invalid.


Let's take the ISBN-10 3-598-21508-8. We plug it in to the formula, and get:

(3 * 10 + 5 * 9 + 9 * 8 + 8 * 7 + 2 * 6 + 1 * 5 + 5 * 4 + 0 * 3 + 8 * 2 + 8 * 1) mod 11 == 0

Since the result is 0, this proves that our ISBN is valid.


Given a string the program should check if the provided string is a valid ISBN-10. Putting this into place requires some thinking about preprocessing/parsing of the string prior to calculating the check digit for the ISBN.

The program should be able to verify ISBN-10 both with and without separating dashes.


Converting from strings to numbers can be tricky in certain languages. Now, it's even trickier since the check digit of an ISBN-10 may be 'X' (representing '10'). For instance 3-598-21507-X is a valid ISBN-10.

Bonus tasks

  • Generate a valid ISBN-13 from the input ISBN-10 (and maybe verify it again with a derived verifier).

  • Generate valid ISBN, maybe even from a given starting ISBN.

Exception messages

Sometimes it is necessary to raise an exception. When you do this, you should include a meaningful error message to indicate what the source of the error is. This makes your code more readable and helps significantly with debugging. Not every exercise will require you to raise an exception, but for those that do, the tests will only pass if you include a message.

To raise a message with an exception, just write it as an argument to the exception type. For example, instead of raise Exception, you should write:

raise Exception("Meaningful message indicating the source of the error")

Running the tests

To run the tests, run pytest isbn_verifier_test.py

Alternatively, you can tell Python to run the pytest module: python -m pytest isbn_verifier_test.py

Common pytest options

  • -v : enable verbose output
  • -x : stop running tests on first failure
  • --ff : run failures from previous test before running other test cases

For other options, see python -m pytest -h

Submitting Exercises

Note that, when trying to submit an exercise, make sure the solution is in the $EXERCISM_WORKSPACE/python/isbn-verifier directory.

You can find your Exercism workspace by running exercism debug and looking for the line that starts with Workspace.

For more detailed information about running tests, code style and linting, please see Running the Tests.


Converting a string into a number and some basic processing utilizing a relatable real world example. https://en.wikipedia.org/wiki/International_Standard_Book_Number#ISBN-10_check_digit_calculation

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.


import unittest

from isbn_verifier import is_valid

# Tests adapted from `problem-specifications//canonical-data.json`

class IsbnVerifierTest(unittest.TestCase):
    def test_valid_isbn(self):
        self.assertIs(is_valid("3-598-21508-8"), True)

    def test_invalid_isbn_check_digit(self):
        self.assertIs(is_valid("3-598-21508-9"), False)

    def test_valid_isbn_with_a_check_digit_of_10(self):
        self.assertIs(is_valid("3-598-21507-X"), True)

    def test_check_digit_is_a_character_other_than_x(self):
        self.assertIs(is_valid("3-598-21507-A"), False)

    def test_invalid_character_in_isbn(self):
        self.assertIs(is_valid("3-598-P1581-X"), False)

    def test_x_is_only_valid_as_a_check_digit(self):
        self.assertIs(is_valid("3-598-2X507-9"), False)

    def test_valid_isbn_without_separating_dashes(self):
        self.assertIs(is_valid("3598215088"), True)

    def test_isbn_without_separating_dashes_and_x_as_check_digit(self):
        self.assertIs(is_valid("359821507X"), True)

    def test_isbn_without_check_digit_and_dashes(self):
        self.assertIs(is_valid("359821507"), False)

    def test_too_long_isbn_and_no_dashes(self):
        self.assertIs(is_valid("3598215078X"), False)

    def test_too_short_isbn(self):
        self.assertIs(is_valid("00"), False)

    def test_isbn_without_check_digit(self):
        self.assertIs(is_valid("3-598-21507"), False)

    def test_check_digit_of_x_should_not_be_used_for_0(self):
        self.assertIs(is_valid("3-598-21515-X"), False)

    def test_empty_isbn(self):
        self.assertIs(is_valid(""), False)

    def test_input_is_9_characters(self):
        self.assertIs(is_valid("134456729"), False)

    def test_invalid_characters_are_not_ignored(self):
        self.assertIs(is_valid("3132P34035"), False)

    def test_input_is_too_long_but_contains_a_valid_isbn(self):
        self.assertIs(is_valid("98245726788"), False)

if __name__ == "__main__":
#!/usr/bin/env python3

Check if strings represent valid ISBN numbers.

from typing import Optional

def _isbn_character_to_int(character: str) -> Optional[int]:
    if character.isdecimal():
        return int(character)

    if character == "X":
        return 10

    return None

def is_valid(isbn: str) -> bool:
    Return True if the string isbn represents a valid ISBN number.

    digits = [d for d in (_isbn_character_to_int(c) for c in isbn) if d is not None]

    if len(digits) != 10:
        return False

    if 10 in digits[:-1]:
        return False

    return sum(d * i for d, i in enumerate(reversed(digits), 1)) % 11 == 0

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