Describe Game Genie codes

gamegenie.py

#!/usr/bin/env python3
import sys

def describe(code):
    """
    Return a written description of what a Game Genie code does.

    Args:
        code (str): A 6- or 8-letter Game Genie code like 'SXIOPO' or 'PEVXIYGA'.

    Returns:
        str: A description of the code.

    Raises:
        ValueError: If the given code is invalid.
    """

    # Translate the code into nibbles.
    try:
        nibbles = []
        for c in code:
            nibbles.append('APZLGITYEOXUKSVN'.index(c))
    except ValueError:
        raise ValueError('%s contains %r, which is invalid in a Game Genie code.' % (code, c))

    if len(nibbles) not in (6, 8):
        raise ValueError('%s is invalid: Game Genie codes are 6 or 8 letters long.' % code)

    # Unscramble the code.
    n0, n1, n2, n3, n4, n5 = nibbles[:6]
    address = ((n3 & 7) << 12) | ((n5 & 7) << 8) | ((n4 & 8) << 8) \
            | ((n2 & 7) <<  4) | ((n1 & 8) << 4) | (n4 & 7) | (n3 & 8)

    if len(nibbles) == 6:
        data = ((n1 & 7) << 4) | ((n0 & 8) << 4) | (n0 & 7) | (n5 & 8)
        return '{0} will make reads to 0x{1:04x} always yield {2} (0x{2:02x}).'.format(code, address, data)

    elif len(nibbles) == 8:
        n6, n7 = nibbles[6:8]
        data = ((n1 & 7) << 4) | ((n0 & 8) << 4) | (n0 & 7) | (n7 & 8)
        compare = ((n7 & 7) << 4) | ((n6 & 8) << 4) | (n6 & 7) | (n5 & 8)
        return '{0} will make reads to 0x{1:04x} always yield {2} (0x{2:02x}) ' \
                'when the actual value is {3} (0x{3:02x}).'.format(code, address, data, compare)

if __name__ == '__main__':
    if len(sys.argv) < 2:
        sys.exit('Usage: %s [Game Genie code]' % sys.argv[0])

    for code in sys.argv[1:]:
        try:
            print(describe(code.upper()))
        except ValueError as e:
            sys.stderr.write('{}\n'.format(e))