CodeWars: Rot13

Source

Description

Given a string, replace each letter with the one that comes 13 letters after it in the alphabet. Ignore non-English aphabetical characters.

The Code

Imports

# python
from string import ascii_lowercase as lowercase
from string import ascii_uppercase as uppercase

# pypi
from expects import equal, expect

The Submitted Function

This is the version I submitted to CodeWars. It uses the dict update method to build a dict (although the solutions below seem neater) and the get method to handle the cases where the letter in the message isn't in the dictionary.

def rot13(message: str) -> str:
    """Implement a Caesar Cipher by shifting letters 13 places

    Non-english letters are left as-is

    Args:
     message: string to encode

    Return:
     the encoded version of the input string
    """
    code = {letter: lowercase[(index + 13) % 26] 
             for index, letter in enumerate(lowercase)}
    code.update((letter, uppercase[(index + 13) % 26])
                  for index, letter in enumerate(uppercase))
    return "".join(code.get(letter, letter) for letter in message)

A Test

def tester(encoder):
    inputs = ("test", "Test", "Test5")
    expecteds = ("grfg", "Grfg", "Grfg5")

    for message, expected in zip(inputs, expecteds):
        encoded = encoder(message)
        expect(encoded).to(equal(expected))
        expect(encoder(encoded)).to(equal(message))
    return

tester(rot13)

Alternatives

Quite a few of the other solutions (on the first page, anyway) used the built in str.maketrans and str.translate methods (they complement each other). I didn't see anything in the documentation about how defaults are handled so I'd have to look into it more. The top answer also used slicing instead of modulo (lower[13:] + lower[:13]) which might be better. The comments mention that the top answer actually won't work anymore since the maketrans and translate functions got moved out of string (which is where it's importing it from).

The top solutions seem to have a mix of current python and deprecated python (python 2?) so you'd have to be careful in using any of them.

Using the Slicing

If you were use slicing instead of the modulo I think it might look like this (from here on out I'm going to declare the code-books outside the function the way I would normally do it, I kind of didn't really think about it when submitting the solution above so I'll leave it as is).

CODE = dict(zip(lowercase + uppercase,
                lowercase[13:] + lowercase[:13] +
                uppercase[13:] + uppercase[:13]))

def rot13_2(message: str) -> str:
    """Implement a Caesar Cipher by shifting letters 13 places

    Non-english letters are left as-is

    Args:
     message: string to encode

    Return:
     the encoded version of the input string
    """
    return "".join(CODE.get(letter, letter) for letter in message)

tester(rot13_2)

This is more compact, although I'm not sure that the slicing is as immediately obvious as the use of the modulo is.

Using translate and maketrans

Here's a version using the built-in maketrans and translate functions.

CODE = str.maketrans(lowercase + uppercase,
                     lowercase[13:] + lowercase[:13] +
                     uppercase[13:] + uppercase[:13])


def rot13_3(message: str) -> str:
    """Implement a Caesar Cipher by shifting letters 13 places

    Non-english letters are left as-is

    Args:
     message: string to encode

    Return:
     the encoded version of the input string
    """
    return message.translate(CODE)

tester(rot13_3)
coded = rot13_3("I have been to paradise 3 times, but I have never been to me. "
                "Oh, the humanity!")
print(coded)
print(rot13_3(coded))

V unir orra gb cnenqvfr 3 gvzrf, ohg V unir arire orra gb zr. Bu, gur uhznavgl!
I have been to paradise 3 times, but I have never been to me. Oh, the humanity!

It kind of seems too much to use translate for this exercise, but it does feel cleaner than the dictionary, so I'll have to keep it in mind for the future.