文字列をrot13変換する

my_rot13.py

_str = raw_input()
alphabets = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'

def rot13_chr(c):
    if c.isalpha():
        if c.islower():
            tmp_alphabets = alphabets.lower()
        elif c.isupper():
            tmp_alphabets = alphabets
        ind = tmp_alphabets.find(c)
        if ind + 13 <= 25:
            ind = ind + 13
        else:
            ind = (ind + 13) % 25
        new_c = tmp_alphabets[ind]
    else:
        new_c = c
    return new_c

def rot13_s(s):
    tmp = ""
    for c in s:
        tmp += rot13_chr(c)
    return tmp

print(rot13_s(_str))