Transform text with the vigenere cipher.

vigenere.py

LETTERS = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'

def vigenere(string, key, direction=1):
    """Transform text with the vigenere cipher.

    ``string`` is the text to transform.
    ``key`` is the key to use in the process.
    ``direction`` is the direction of transformation:
        1 for encoding, and -1 for decoding.

    Return value is the transformed text.
    """
    result = ''
    string, key = string.upper(), key.upper()
    key = (key * (len(string) // len(key) + 1))[:len(string)]
    j = 0
    for i in string:
        idx = LETTERS.find(i)
        if idx == -1:
            result += i
            continue
        idx += LETTERS.find(key[j]) * direction
        idx %= len(LETTERS)
        result += LETTERS[idx]
        j += 1
    return result

def envigenere(string, key):
    """Encode text with the vigenere cipher.

    ``string`` is the text to encode.
    ``key`` is the key to use in encoding.

    Equivalent to vigenere(string, key, 1)

    Return value is the encoded text.
    """
    return vigenere(string, key, 1)

def devigenere(string, key):
    """Decode text encoded with the vigenere cipher.

    ``string`` is the text to decode.
    ``key`` is the key used in encoding.

    Equivalent to vigenere(string, key, -1)

    Return value is the decoded text.
    """
    return vigenere(string, key, -1)

if __name__ == '__main__':
    edq = ''
    while edq != 'q':
        edq = input('Do you want to [e]ncode, [d]ecode, or [q]uit? ')
        if edq == 'e':
            print(envigenere(input('Text: '), input('Key: ')))
        elif edq == 'd':
            print(devigenere(input('Text: '), input('Key: ')))