introduction.md

Introduction

Atbash cipher in Python can be solved in many ways.

General guidance

The first thing is to have a "key" mapping - possibly in a dict or str.maketrans, otherwise the value would have to be calculated on the fly. Then, you have to "clean" up the string to be encoded by removing numbers/whitespace. Finally, you break it up into chunks of five before returning it.

For decoding, it's similar - clean up (which automatically joins the chunks) and translate using the same key - the realization that the same key can be used is crucial in solving this in an idiomatic manner.

Approach: separate functions

We use str.maketrans to create the encoding. In encode, we use a generator expression in str.join.

from string import ascii_lowercase
ENCODING = str.maketrans(ascii_lowercase, ascii_lowercase[::-1])

def encode(text: str):
    res = "".join(chr for chr in text.lower() if chr.isalnum()).translate(ENCODING)
    return " ".join(res[index:index+5] for index in range(0, len(res), 5))

def decode(text: str):
    return "".join(chr.lower() for chr in text if chr.isalnum()).translate(ENCODING)

Read more on this approach here.

Approach: mono-function

Notice that there the majority of the code is repetitive? A fun way to solve this would be to keep it all inside the encode function, and merely chunk it if decode is False: For variation, this approach shows a different way to translate the text.

from string import ascii_lowercase as asc_low
ENCODING = {chr: asc_low[id] for id, chr in enumerate(asc_low[::-1])}

def encode(text: str, decode: bool = False):
    res = "".join(ENCODING.get(chr, chr) for chr in text.lower() if chr.isalnum())
    return res if decode else " ".join(res[index:index+5] for index in range(0, len(res), 5))

def decode(text: str):
    return encode(text, True)

For more detail, read here.