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to Rotational Cipher in the Python Track

Published at Jul 17 2018 · 0 comments
Test suite


This exercise has changed since this solution was written.

Create an implementation of the rotational cipher, also sometimes called the Caesar cipher.

The Caesar cipher is a simple shift cipher that relies on transposing all the letters in the alphabet using an integer key between 0 and 26. Using a key of 0 or 26 will always yield the same output due to modular arithmetic. The letter is shifted for as many values as the value of the key.

The general notation for rotational ciphers is ROT + <key>. The most commonly used rotational cipher is ROT13.

A ROT13 on the Latin alphabet would be as follows:

Plain:  abcdefghijklmnopqrstuvwxyz
Cipher: nopqrstuvwxyzabcdefghijklm

It is stronger than the Atbash cipher because it has 27 possible keys, and 25 usable keys.

Ciphertext is written out in the same formatting as the input including spaces and punctuation.


  • ROT5 omg gives trl
  • ROT0 c gives c
  • ROT26 Cool gives Cool
  • ROT13 The quick brown fox jumps over the lazy dog. gives Gur dhvpx oebja sbk whzcf bire gur ynml qbt.
  • ROT13 Gur dhvpx oebja sbk whzcf bire gur ynml qbt. gives The quick brown fox jumps over the lazy dog.

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 the appropriate command below (why they are different):

  • Python 2.7: py.test rotational_cipher_test.py
  • Python 3.4+: pytest rotational_cipher_test.py

Alternatively, you can tell Python to run the pytest module (allowing the same command to be used regardless of Python version): python -m pytest rotational_cipher_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/rotational-cipher 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 the help page.


Wikipedia https://en.wikipedia.org/wiki/Caesar_cipher

Submitting Incomplete Solutions

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


import unittest

import rotational_cipher

# Tests adapted from `problem-specifications//canonical-data.json` @ v1.2.0

class RotationalCipherTest(unittest.TestCase):
    def test_rotate_a_by_0(self):
        self.assertEqual(rotational_cipher.rotate('a', 0), 'a')

    def test_rotate_a_by_1(self):
        self.assertEqual(rotational_cipher.rotate('a', 1), 'b')

    def test_rotate_a_by_26(self):
        self.assertEqual(rotational_cipher.rotate('a', 26), 'a')

    def test_rotate_m_by_13(self):
        self.assertEqual(rotational_cipher.rotate('m', 13), 'z')

    def test_rotate_n_by_13_with_wrap_around_alphabet(self):
        self.assertEqual(rotational_cipher.rotate('n', 13), 'a')

    def test_rotate_capital_letters(self):
        self.assertEqual(rotational_cipher.rotate('OMG', 5), 'TRL')

    def test_rotate_spaces(self):
        self.assertEqual(rotational_cipher.rotate('O M G', 5), 'T R L')

    def test_rotate_numbers(self):
            rotational_cipher.rotate('Testing 1 2 3 testing', 4),
            'Xiwxmrk 1 2 3 xiwxmrk')

    def test_rotate_punctuation(self):
            rotational_cipher.rotate("Let's eat, Grandma!", 21),
            "Gzo'n zvo, Bmviyhv!")

    def test_rotate_all_letters(self):
            rotational_cipher.rotate("The quick brown fox jumps"
                                     " over the lazy dog.", 13),
            "Gur dhvpx oebja sbk whzcf bire gur ynml qbt.")

if __name__ == '__main__':
def rotate(text, key):
    """Encode phrase with Caesar cipher"""

    # shifts characters
    encoded_str = ''
    for char in text:
        if char.isalpha():
            if char.isupper():
                offset = ord('A')
            if char.islower():
                offset = ord('a')
            encoded_char = ((ord(char) + key - offset) % 26) + offset  # that some voodoo
            encoded_str += chr(encoded_char)
            encoded_str += char

    return encoded_str

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