Create an implementation of the atbash cipher, an ancient encryption system created in the Middle East.
The Atbash cipher is a simple substitution cipher that relies on transposing all the letters in the alphabet such that the resulting alphabet is backwards. The first letter is replaced with the last letter, the second with the second-last, and so on.
An Atbash cipher for the Latin alphabet would be as follows:
Plain: abcdefghijklmnopqrstuvwxyz Cipher: zyxwvutsrqponmlkjihgfedcba
It is a very weak cipher because it only has one possible key, and it is a simple monoalphabetic substitution cipher. However, this may not have been an issue in the cipher's time.
Ciphertext is written out in groups of fixed length, the traditional group size being 5 letters, and punctuation is excluded. This is to make it harder to guess things based on word boundaries.
gsvjf rxpyi ldmul cqfnk hlevi gsvoz abwltgives
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")
To run the tests, run the appropriate command below (why they are different):
Alternatively, you can tell Python to run the pytest module (allowing the same command to be used regardless of Python version):
python -m pytest atbash_cipher_test.py
-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
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For more detailed information about running tests, code style and linting, please see Running the Tests.
It's possible to submit an incomplete solution so you can see how others have completed the exercise.
import unittest from atbash_cipher import decode, encode # Tests adapted from `problem-specifications//canonical-data.json` @ v1.2.0 class AtbashCipherTest(unittest.TestCase): def test_encode_no(self): self.assertMultiLineEqual(encode("no"), "ml") def test_encode_yes(self): self.assertMultiLineEqual(encode("yes"), "bvh") def test_encode_OMG(self): self.assertMultiLineEqual(encode("OMG"), "lnt") def test_encode_O_M_G(self): self.assertMultiLineEqual(encode("O M G"), "lnt") def test_encode_long_word(self): self.assertMultiLineEqual(encode("mindblowingly"), "nrmwy oldrm tob") def test_encode_numbers(self): self.assertMultiLineEqual( encode("Testing, 1 2 3, testing."), "gvhgr mt123 gvhgr mt") def test_encode_sentence(self): self.assertMultiLineEqual( encode("Truth is fiction."), "gifgs rhurx grlm") def test_encode_all_things(self): plaintext = "The quick brown fox jumps over the lazy dog." ciphertext = "gsvjf rxpyi ldmul cqfnk hlevi gsvoz abwlt" self.assertMultiLineEqual(encode(plaintext), ciphertext) def test_decode_word(self): self.assertMultiLineEqual(decode("vcvix rhn"), "exercism") def test_decode_sentence(self): self.assertMultiLineEqual( decode("zmlyh gzxov rhlug vmzhg vkkrm thglm v"), "anobstacleisoftenasteppingstone") def test_decode_numbers(self): self.assertMultiLineEqual( decode("gvhgr mt123 gvhgr mt"), "testing123testing") def test_decode_all_the_letters(self): ciphertext = "gsvjf rxpyi ldmul cqfnk hlevi gsvoz abwlt" plaintext = "thequickbrownfoxjumpsoverthelazydog" self.assertMultiLineEqual(decode(ciphertext), plaintext) def test_decode_with_too_many_spaces(self): self.assertMultiLineEqual(decode("vc vix r hn"), "exercism") def test_decode_with_no_spaces(self): ciphertext = "zmlyhgzxovrhlugvmzhgvkkrmthglmv" plaintext = "anobstacleisoftenasteppingstone" self.assertMultiLineEqual(decode(ciphertext), plaintext) # additional track specific test def test_encode_decode(self): self.assertMultiLineEqual( decode(encode("Testing, 1 2 3, testing.")), "testing123testing") if __name__ == '__main__': unittest.main()
import string class Atbash: PLAIN = 'abcdefghijklmnopqrstuvwxyz' PRIME = 'zyxwvutsrqponmlkjihgfedcba' CIPHER = dict(zip(list(PLAIN), list(PRIME))) EXCLUDE = set(string.punctuation + ' ') @staticmethod def encode(self, msg): return self.split_every_five(self, self.encoded(self, msg)) @staticmethod def split_every_five(self, encoded): return ' '.join([encoded[i:i + 5] for i in range(0, len(encoded), 5)]) @staticmethod def encoded(self, msg): return ''.join(([char if char.isdigit() else self.CIPHER[char] for char in self.clean(self, msg)])) @staticmethod def clean(self, msg): return (char for char in msg.lower() if char not in self.EXCLUDE) @staticmethod def decode(self, msg): return self.encoded(self, msg) def encode(msg): return Atbash.encode(Atbash, msg) def decode(msg): return Atbash.decode(Atbash, msg)
A huge amount can be learned from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.
Here are some questions to help you reflect on this solution and learn the most from it.