Convert VH aircraft registration code to ICAO mode-S identifier

modescode.py

import sys

def getVal(ch):
    if ch >= 'A' and ch <= 'Z':
        return ord(ch) - ord('A')
    if ch >= '0' and ch <= '9':
        return ord(ch) - ord('0') + 26
    return -1


if len(sys.argv) <= 1:
    print ('Usage: modescode AAA')
    sys.exit()
code = sys.argv[1].upper()
if (len(code) != 3) or (not code.isalnum()):
    print ('Argument must be three alphanumeric characters')
    sys.exit()

total = 0x7C0000
total += getVal(code[2])
total += getVal(code[1]) * 36
total += getVal(code[0]) * 36 * 36
print('{} encoded in hex is {:X}'.format( code, total))