A simple rot13 implementation.

rot13.py

"""Apply Rot13 cipher to argument list"""

import string
import sys

def rot13(text):
    """Apply the rot13 cipher to the given text
    
    Args:
        text: The text to be shifted.	

    Returns:
        The content of `text` shifted by 13 chars.
    """

    map = ["abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ",
           "nopqrstuvwxyzabcdefghijklmNOPQRSTUVWXYZABCDEFGHIJKLM"]

    # Add non shifted chars to the conversion map.
    map[0] += string.digits + string.punctuation + string.whitespace
    map[1] += string.digits + string.punctuation + string.whitespace
	
    # This is probably not very pythonic but it remaps the chars.
    return "".join([map[1][x] for x in (map[0].find(s) for s in text)])

if __name__ == "__main__":
    if len(sys.argv) < 2:
        raise SyntaxError("Missing parameters.")

    print(rot13(" ".join(sys.argv[1:])))