Python凱撒密碼，若向後移動N位，怎麽修改？

def caesar_code(n, plain_text) -> str:

plain_text = str.upper(plain_text)

alphabets = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'

mapping = dict((char, i) for i, char in enumerate(alphabets))

encrypted = []

for token in plain_text.split():

for char in token:

encrypted.append(alphabets[(mapping[char] + n) % 26])

encrypted.append(' ')

return str.join('', encrypted[:-1])