Mono-function

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)

To explain the translation: we use a dict comprehension in which we reverse the ASCII lowercase digits, and enumerate through them - that is, z is 0, y is 1, and so on. We access the character at that index and set it to the value of c - so z translates to a.

In the calculation of the result, we try to obtain the value of the character using dict.get, which accepts a default parameter. In this case, the character itself is the default - that is, numbers won't be found in the translation key, and thus should remain as numbers.

We use a ternary operator to check if we actually mean to decode the function, in which case we return the result as is. If not, we chunk the result by joining every five characters with a space.

Another possible way to solve this would be to use a function that returns a function that encodes or decodes based on the parameters:

from string import ascii_lowercase as alc

lowercase = {chr: alc[id] for id, chr in enumerate(alc[::-1])}

def code(decode=False):
    def func(text):
        line = "".join(lowercase.get(chr, chr) for chr in text.lower() if chr.isalnum())
        return line if decode else " ".join(line[index:index+5] for index in range(0, len(line), 5))
    return func

    
encode = code()
decode = code(True)

The logic is the same - we've instead used one function that generates two other functions based on the boolean value of its parameter. encode is set to the function that's returned, and performs encoding. decode is set a function that decodes.