Caesar Cipher

#!/usr/bin/env python

from sys import argv, exit
from string import lowercase, uppercase, maketrans
from random import randint
import check_args

def get_new_alpha(key):
    temp = []
    for each in range(0,key):
        temp.append(lowercase[each])

    rest = ''.join(temp)
    new =  lowercase[key:26] + rest
    return new

def get_new_ALPHA(key):
    temp = []
    for each in range(0,key):
        temp.append(uppercase[each])

    rest = ''.join(temp)
    new =  uppercase[key:26] + rest
    return new

def encrypt(source, key):
    intab = lowercase[:26]
    outtab = get_new_alpha(key)
    trantab = maketrans(intab, outtab)

    first_pass = source.translate(trantab)
    intab = uppercase[:26]
    outtab = get_new_ALPHA(key)
    trantab = maketrans(intab,outtab)

    return first_pass.translate(trantab)


def decrypt(source, key):
    outtab = lowercase[:26]
    intab = get_new_alpha(key)
    trantab = maketrans(intab, outtab)

    first_pass = source.translate(trantab)

    outtab = uppercase[:26]
    intab = get_new_ALPHA(key)
    trantab = maketrans(intab, outtab)

    return first_pass.translate(trantab)


args = check_args.check_args(4)

cmd = string = key = False
#--------------------------------------------------
#These check for present argv's and set them.
if args[1]:
    cmd = argv[1]

if args[2]:
    string = argv[2]

if args[3]:
    key = int(argv[3])
#--------------------------------------------------
#If no argvs are passed, then we set them here
if not cmd:
    print "1:\tEncrypt\n2:\tDecrypt"
    ask = raw_input("> ")
    if ask == '1' or ask == 'encrypt':
        cmd = 'encrypt'

    if ask == '2' or ask == 'decrypt':
        cmd = 'decrypt'

if not string:

    print "Input the name of the file that you would like to %s OR" % cmd
    print "Input the string you would like to %s." % cmd
    string = raw_input("> ")

#This checks if the string is a file or just a string, and saves it as a raw text for later use.
try:
    open(string,'r')
    print "WARNING:\n\tThis will encrypt the contents of this file."
    safety = raw_input("Are you sure about this?:\n")
    if safety == 'no':
        exit(0)
    file = True
except IOError:
    string = string
    file = False

if not key:
    print "What is the key?"
    key = int(raw_input("> "))
#--------------------------------------------------
def test_input():
    print "cmd = %r\nstring = %r\nkey = %r\nfile = %r" % (cmd, string, key,file)

def handle():
    if file == False:
        if cmd == 'encrypt':
            print encrypt(string, key)

        if cmd == 'decrypt' :
            print decrypt(string, key)

    if file == True:
        original = open(string, 'r').read()
        txt = open(string, 'w')

        if cmd == 'encrypt':
            encrypted_text = encrypt(original,key)
            txt.write(encrypted_text)

        if cmd == 'decrypt':
            decrypted_text = decrypt(original, key)
            txt.write(decrypted_text)

        txt.close
if __name__ == '__main__':
    handle()