Extremely simple Caesar cipher

caesar.py

import argparse

parser = argparse.ArgumentParser(description='Shift a string n chars based on step.')
parser.add_argument('--shift', help='shift a string of text')
parser.add_argument('--deshift', help='deshift a string of text')
parser.add_argument('--steps', type=int)
args = parser.parse_args()

steps = args.steps

shift = lambda txt,sft=1: ''.join([[ch,chr((ord(ch) - ord(['A','a'][ch.islower()]) + sft)%26+ord(['A','a'][ch.islower()]))][ch.isalpha()] for ch in txt])
deshift = lambda s,n: shift(s,n) if n < 0 else shift(s,-n)

if args.shift:
    string = args.shift
    print shift(string,steps)

if args.deshift:
    string = args.deshift
    print deshift(string,steps)