If you use this in production you are stupid.

substitution-cipher.py

from functools import cache
from typing import List, Tuple
from string import ascii_uppercase


def generate_sub(start: str = 'Z') -> Tuple[str]:
    _l: List[str] = list(ascii_uppercase)
    start_index: int = _l.index(start)
    _o_list: List[str] = list(_l)

    for i in range(0, start_index):
        _l.remove(_o_list[i])
        _l.append(_o_list[i])

    return tuple(_l)


class SubstitutionCipher:
    def __init__(self, key: str = 'Z'):
        self._ascii_uppercase: Tuple[str] = tuple(ascii_uppercase)
        self._sub: Tuple[str] = generate_sub(key)

    @cache
    def _lookup_encode(self, _char: str) -> str:
        return self._sub[self._ascii_uppercase.index(_char.capitalize())]

    @cache
    def _lookup_decode(self, _char: str) -> str:
        return self._ascii_uppercase[self._sub.index(_char.capitalize())]

    def encode(self, string: str) -> str:
        return ''.join(tuple(map(lambda char: self._lookup_encode(char), string)))

    def decode(self, string: str) -> str:
        return ''.join(tuple(map(lambda char: self._lookup_decode(char), string)))


if __name__ == '__main__':
    c = SubstitutionCipher('C')
    print(c.encode('UWSP'))
    print(c.decode('WYUR'))

    d = SubstitutionCipher('D')
    print(d.encode('DAVID'))
    print(d.decode('GDYLG'))

    k = SubstitutionCipher('K')
    print(k.encode('DAVID'))
    print(k.decode('NKFSN'))

    p = SubstitutionCipher('P')
    print(p.encode('DAVID'))
    print(p.decode('SPKXS'))