Cesaer cipher

from string import ascii_lowercase as lower, ascii_uppercase as upper

def rot(n: int) -> dict:
    return dict(zip(lower[n:]+lower[:n]+upper[n:]+upper[:n], lower+upper))

def decode(s: str, n: int) -> str:
    mapping = rot(n)
    return ''.join(mapping.get(c,c) for c in s)

print(decode('Jgnnq, yqtnf!', 2))