rot13.py

#!/usr/bin/env python

import sys

try:
    text = sys.argv[1]
except:
    print "Usage: rot13 <text>"
    sys.exit(1)

def rotate_string(s, r=0):
    s2=""
    for c in s:
        if c >= 'a' and c <= 'z':
            c2 = chr(((ord(c)-ord('a')) + r) % 26 + ord('a'))
        elif c >= 'A' and c <= 'Z':
            c2 = chr(((ord(c)-ord('A')) + r) % 26 + ord('A'))
        else:
            c2 = c
        s2 += c2
    return s2

for i in range(26):
    print "%d:" % i, rotate_string(text, i)