A basic Vigenere encryption utility

vigenere.py

# usage: vigenere.py [-h] [--key KEY] [--decrypt] string
# 
# exmaple:
#   >vigenere.py HELLO
#   output: DEWVP
#
#   >vigenere.py --decrypt DEWVP
#   output: HELLO
#
# positional arguments:
#   string      the str to encrypt or decrypt
# 
# options:
#   -h, --help  show this help message and exit
#   --key KEY   use a different key
#   --decrypt   decrypt text

english_alphabet = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'

def vigenere(text, key, decrypt=False):
    text = text.upper()
    key = key.upper()
    encrypted_text = ""
    KEYLEN = len(key)
    keyIndex = 0
    
    for i in range(len(text)):
        t = ord(text[i]) - 65
        k = ord(key[keyIndex]) - 65
        if text[i] == " ":
            encrypted_text += " "
            continue
        
        if decrypt == False:
            encrypted_text += english_alphabet[(t + k) % 26 ]
        else:
            encrypted_text += english_alphabet[(t - k) % 26 ]
        keyIndex += 1
        if keyIndex == KEYLEN: keyIndex = 0
    
    return encrypted_text

if __name__ == "__main__":
    import argparse
    
    parser = argparse.ArgumentParser()
    parser.add_argument("string", help="the str to encrypt or decrypt")
    parser.add_argument("--key", help="use a different key")
    parser.add_argument("--decrypt", help="decrypt text", action="store_true")
    args = parser.parse_args()
    
    text = args.string
    key  = args.key or "WALKBYME"
    
    if args.decrypt:
        clear_text = vigenere(text, key, True)
        print(clear_text)
    else:
        cypher_text = vigenere(text, key)
        print(cypher_text)