凯撒密码B

def encryption(str, n):
    cipher = []
    for i in range(len(str)):
        if str[i].islower():
            if ord(str[i]) < 123-n: #ord('z')=122
                c = chr(ord(str[i]) + n)
                cipher.append(c)
            else:
                c = chr(ord(str[i]) + n - 26)
                cipher.append(c)
        elif str[i].isupper():
            if ord(str[i]) < 91-n: #ord('Z')=90
                c = chr(ord(str[i]) + n)
                cipher.append(c)
            else:
                c = chr(ord(str[i]) + n - 26)
                cipher.append(c)
        else:
            c = str[i]
            cipher.append(c)
    cipherstr = ('').join(cipher)
    return cipherstr

#获得用户输入的明文
plaintext = input()
ciphertext = encryption(plaintext, 3)
print(ciphertext)