Python: luhn

python/luhn/.exercism/metadata.json

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+{"track":"python","exercise":"luhn","id":"5daf06c1ba214c66a86767020db309e9","url":"https://exercism.io/my/solutions/5daf06c1ba214c66a86767020db309e9","handle":"DmitryKokorin","is_requester":true,"auto_approve":false}

python/luhn/README.md

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+# Luhn
+
+Given a number determine whether or not it is valid per the Luhn formula.
+
+The [Luhn algorithm](https://en.wikipedia.org/wiki/Luhn_algorithm) is
+a simple checksum formula used to validate a variety of identification
+numbers, such as credit card numbers and Canadian Social Insurance
+Numbers.
+
+The task is to check if a given string is valid.
+
+Validating a Number
+------
+
+Strings of length 1 or less are not valid. Spaces are allowed in the input,
+but they should be stripped before checking. All other non-digit characters
+are disallowed.
+
+## Example 1: valid credit card number
+
+```text
+4539 1488 0343 6467
+```
+
+The first step of the Luhn algorithm is to double every second digit,
+starting from the right. We will be doubling
+
+```text
+4_3_ 1_8_ 0_4_ 6_6_
+```
+
+If doubling the number results in a number greater than 9 then subtract 9
+from the product. The results of our doubling:
+
+```text
+8569 2478 0383 3437
+```
+
+Then sum all of the digits:
+
+```text
+8+5+6+9+2+4+7+8+0+3+8+3+3+4+3+7 = 80
+```
+
+If the sum is evenly divisible by 10, then the number is valid. This number is valid!
+
+## Example 2: invalid credit card number
+
+```text
+8273 1232 7352 0569
+```
+
+Double the second digits, starting from the right
+
+```text
+7253 2262 5312 0539
+```
+
+Sum the digits
+
+```text
+7+2+5+3+2+2+6+2+5+3+1+2+0+5+3+9 = 57
+```
+
+57 is not evenly divisible by 10, so this number is not valid.
+
+
+## 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:
+
+```python
+raise Exception("Meaningful message indicating the source of the error")
+```
+
+## Running the tests
+
+To run the tests, run `pytest luhn_test.py`
+
+Alternatively, you can tell Python to run the pytest module:
+`python -m pytest luhn_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/luhn` 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](http://exercism.io/tracks/python/tests).
+
+## Source
+
+The Luhn Algorithm on Wikipedia [http://en.wikipedia.org/wiki/Luhn_algorithm](http://en.wikipedia.org/wiki/Luhn_algorithm)
+
+## Submitting Incomplete Solutions
+
+It's possible to submit an incomplete solution so you can see how others have completed the exercise.

python/luhn/luhn.py

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+import re
+
+class Luhn:
+    def __init__(self, card_num):
+
+        pattern = re.compile(r'\s+')
+        card_num = re.sub(pattern, '', card_num)
+
+        if not card_num.isnumeric() or len(card_num) < 2:
+            self.is_valid = False
+        else:
+
+            checksum = 0
+
+            for i, ch in enumerate(reversed(card_num)):
+
+                x = int(ch)
+
+                if i % 2:
+                    x *= 2
+
+                    if x > 9:
+                        x -= 9
+
+                checksum += x
+
+            self.is_valid = not (checksum % 10)
+
+    def valid(self):
+        return self.is_valid

python/luhn/luhn_test.py

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+import unittest
+
+from luhn import Luhn
+
+# Tests adapted from `problem-specifications//canonical-data.json`
+
+
+class LuhnTest(unittest.TestCase):
+    def test_single_digit_strings_can_not_be_valid(self):
+        self.assertIs(Luhn("1").valid(), False)
+
+    def test_a_single_zero_is_invalid(self):
+        self.assertIs(Luhn("0").valid(), False)
+
+    def test_a_simple_valid_sin_that_remains_valid_if_reversed(self):
+        self.assertIs(Luhn("059").valid(), True)
+
+    def test_a_simple_valid_sin_that_becomes_invalid_if_reversed(self):
+        self.assertIs(Luhn("59").valid(), True)
+
+    def test_a_valid_canadian_sin(self):
+        self.assertIs(Luhn("055 444 285").valid(), True)
+
+    def test_invalid_canadian_sin(self):
+        self.assertIs(Luhn("055 444 286").valid(), False)
+
+    def test_invalid_credit_card(self):
+        self.assertIs(Luhn("8273 1232 7352 0569").valid(), False)
+
+    def test_invalid_long_number_with_an_even_remainder(self):
+        self.assertIs(Luhn("1 2345 6789 1234 5678 9012").valid(), False)
+
+    def test_valid_number_with_an_even_number_of_digits(self):
+        self.assertIs(Luhn("095 245 88").valid(), True)
+
+    def test_valid_number_with_an_odd_number_of_spaces(self):
+        self.assertIs(Luhn("234 567 891 234").valid(), True)
+
+    def test_valid_strings_with_a_non_digit_added_at_the_end_become_invalid(self):
+        self.assertIs(Luhn("059a").valid(), False)
+
+    def test_valid_strings_with_punctuation_included_become_invalid(self):
+        self.assertIs(Luhn("055-444-285").valid(), False)
+
+    def test_valid_strings_with_symbols_included_become_invalid(self):
+        self.assertIs(Luhn("055# 444$ 285").valid(), False)
+
+    def test_single_zero_with_space_is_invalid(self):
+        self.assertIs(Luhn(" 0").valid(), False)
+
+    def test_more_than_a_single_zero_is_valid(self):
+        self.assertIs(Luhn("0000 0").valid(), True)
+
+    def test_input_digit_9_is_correctly_converted_to_output_digit_9(self):
+        self.assertIs(Luhn("091").valid(), True)
+
+    def test_using_ascii_value_for_non_doubled_non_digit_isn_t_allowed(self):
+        self.assertIs(Luhn("055b 444 285").valid(), False)
+
+    def test_using_ascii_value_for_doubled_non_digit_isn_t_allowed(self):
+        self.assertIs(Luhn(":9").valid(), False)
+
+    # Additional tests for this track
+
+    def test_is_valid_can_be_called_repeatedly(self):
+        # This test was added, because we saw many implementations
+        # in which the first call to valid() worked, but the
+        # second call failed().
+        number = Luhn("055 444 285")
+        self.assertIs(number.valid(), True)
+        self.assertIs(number.valid(), True)
+
+
+if __name__ == "__main__":
+    unittest.main()