Validate IBAN or Post Account

checksums.py

POST_TABLE = (0, 9, 4, 6, 8, 2, 7, 1, 3, 5)
IBAN_TABLE = dict([(c, str(idx))
                   for (idx, c) in enumerate('0123456789abcdefgh'
                                             'ijklmnopqrstuvwxyz')])


def validate_postaccount(value):
    """Check the Post Account."""
    if not value:
        raise ValueError('Post Account cannot be empty')
    # The format is a-b-c
    # in b, leading 0s may be stripped
    (a, b, c) = value.strip().split('-', 2)
    if (not (a + b + c).isdigit()) or len(a) != 2 or len(b) > 6 or len(c) != 1:
        raise ValueError('Post Account - invalid format')
    (a, b, checksum) = (int(a), int(b), int(c))
    key = 0
    for char in "%02d%06d" % (a, b):
        key = POST_TABLE[(key + int(char)) % 10]
    if checksum != -key % 10:
        raise ValueError('Post Account - invalid checksum')
    return "%02d-%06d-%d" % (a, b, checksum)


def validate_iban(value):
    """Check the IBAN number."""
    if not value:
        raise ValueError('IBAN number cannot be empty')
    # keep only alphanumeric chars and convert to lower case.
    iban = ''.join([c for c in value if c.isalnum()]).lower()
    # the first four digits are shifted to the end
    checkval = iban[4:] + iban[:4]
    # letters are transformed into numbers (a = 10, b = 11, ...)
    checkval = ''.join([IBAN_TABLE[c] for c in checkval])
    # iban is correct if modulo 97 == 1
    if int(checkval) % 97 != 1:
        raise ValueError('IBAN number is invalid')
    return iban