Command line Python program for encoding/decoding Caesar Ciphers

CaesarCipher.py

#!/usr/bin/env python2.7

"""Command line script to encode/decode Caesar Ciphers"""

__author__ = "Palmer Paul"
__version__ = "1.0.0"
__email__ = "[email protected]"

def caesar(pt, n):
    from string import lowercase, uppercase
    
    ct = ""
    for x in range(len(pt)):
        if pt[x].isalpha():
            if pt[x].islower():
                ct = ct + lowercase[(lowercase.find(pt[x]) + n) % 26]
            else:
                ct = ct + uppercase[(uppercase.find(pt[x]) + n) % 26]
        else:
            ct = ct + pt[x]
            
    return ct

def main():
    from sys import argv

    if len(argv) != 3 or not argv[2].isdigit():
        print "Usage: CaesarCipher.py ( <filename> | <string> ) <shift_val>"
        raise SystemExit

    try:
        with open(argv[1]) as fp:
            data = fp.read()
    except IOError:
        data = argv[1]

    print caesar(data, int(argv[2]))

if __name__ == "__main__":
    main()