Simple Caesar cipher with one rotation

caesar_n1.py

import string

text = "Y29uZ3JhdHMgb24gc29sdmluZyB0aGlzIQ=="

new_t = ""
al = ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m",
      "m", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z"]


def encode(char: str) -> str:
    ret_char = ""

    for i, character in enumerate(al):
        if char.lower() == character:
            new_char = ""

            if i == 0:
                new_char = al[len(al)-1]
            else:
                new_char = al[i-1]
            ret_char = new_char
            break
        else:
            ret_char = char

    isLower = string.ascii_lowercase.find(char) != -1

    if isLower:
        ret_char = ret_char.lower()
    else:
        ret_char = ret_char.capitalize()

    return ret_char


for c in text:
    replacement = encode(c)

    new_t += replacement

print(new_t)