Encryption: Encrypt an ASCII [a-zA-Z0-9] string with a Caeser Cipher (ROT-N) encryption

rot13.py

import argparse
import sys

# Author: Bradford Law
# Author Website: https://bradford.la
# Description: A simple rot13 implementation for Python 3.x

# get args with argparse library:
# https://docs.python.org/3/library/argparse.html
parser = argparse.ArgumentParser(description='A simple rot13 implementation in Python.', prog='rot13')
group = parser.add_mutually_exclusive_group(required=True)
group.add_argument('-e','--encode', action='store_true', help='encode stdin')
group.add_argument('-d','--decode', action='store_true', help='decode stdin')
parser.add_argument('rot', type=int, metavar='ROT', default=13, nargs='?', help='provide rotational offset (default: %(default)s)')
args = parser.parse_args()

# if we are decoding, we simply reverse the rot
if (args.decode):
    args.rot *= -1

# parse stdin and apply rot to each character
input = sys.stdin.read()
parseflag = False
for c in input:
    charnumber = ord(c)
    # check for A-Z, A=65, Z=90
    if charnumber >= 65 and charnumber <= 90:
        charnumber -= 65
        print(chr((charnumber + args.rot)%26 + 65), end="")
    # check for a-z, a=97, z=122
    elif charnumber >= 97 and charnumber <= 122:
        charnumber -= 97
        print(chr((charnumber + args.rot)%26 + 97), end="")
    else:
        print(c, end="")
        parseflag = True

# warn user if characters couldn't be encrypted
if parseflag:
    sys.stderr.write("Some characters could not be encrypted.\n\n")