Demilich cipher codec.

demilich.py

#!/usr/bin/env python3
"""
The Finnish death metal band Demilich released an album in 1993 called
"Nespithe". The liner notes, as well as the album title and one song title,
are written in code: start at the end of the phrase, group the letters in
three, and reverse the groups' order.
"""

from sys import argv, stdin

def encode(string):
    string = string.replace(" ","")
    newstr = ""
    for i in range(0,len(string),3):
        newstr = string[i:i+3] + newstr
    return newstr.upper()


def decode(string):
    string = string.replace(" ","")
    newstr = ""
    for i in range(len(string),2,-3):
        newstr += string[i-3:i]
    newstr += string[0:i%3]
    return newstr.upper()


def test():
    plain = "THESPINE"
    cipher = "NESPITHE"
    assert encode(plain) == cipher
    assert decode(cipher) == plain


OPS = {"-d": decode, "-e": encode}


def main():
    if len(argv) < 2 or argv[1] not in OPS:
        raise RuntimeError(f"Argument must be one of: {', '.join(OPS.keys())}")
    oper = OPS[argv[1]]

    for line in stdin:
        line = line.rstrip().replace(" ", "")
        print(oper(line), end="")


if __name__ == "__main__":
    main()                                                                                                                                                    

Nice! Could you specify the interpreter on the first line, at least for us Linux geeks?
#!/usr/bin/env python3

Damn, how'd I forget that, especially having written this on a Linux machine?

Kiitos, it's fixed now.

Came back to make this behave more like a Unix text filter, and also attempt some genexpr nonsense—

def encode_stupid(string):
    length = len(string)
    start = length - length % 3
    return "".join(string[i : min(i + 3, length)] for i in range(start, -1, -3)).upper()

def decode_stupid(string):
    length = len(string)
    return "".join(string[max(0, i - 3) : i] for i in range(length, -1, -3)).upper()

—that is, until I saw no difference with hyperfine and a worse result with timeit.

>>> timeit.timeit(lambda: decode_stupid("nespithe"))
2.2625253699952736
>>> timeit.timeit(lambda: decode("nespithe"))
1.0990409349906258