vigenere.py

from cs50 import get_string
from sys import argv

# Correct usage :)
if len(argv) == 2:

    # Length of key and Variable to track our key
    key_len = 0
    key_index = 0

    # Check if argv[1] is alphabetic
    for c in argv[1]:
        # Non-alphabetic
        if c.isalpha() == False:
            print("Key should be alphabetic")
            sys.exit(2)
        # Increment key count
        else:
            key_len+=1


    # Gets plain text
    plain_text = get_string("plaintext: ")

    print("ciphertext: ", end="")

    # Iterate through each char of plain_text.
    for c in plain_text:
        if c.isalpha():
            # Uppercase
            if c.isupper():
                print(chr(((ord(c) - 65) + ord((argv[1][key_index % key_len]).upper()-65))%26+65), end="")
                key_index+=1
            # Lowercase
            else:
                print(chr(((ord(c) - 97) + ord((argv[1][key_index % key_len]).upper()-97))%26+97), end="")
                key_index+=1

        # Non-alphabetic
        else:
            print(c, end="")



# Incorrect usage :(
else:
    print("Usage: python vigenere.py key")
    sys.exit(1)