upce.py

def expand(cls, digits, number_system=0):
    if len(digits) == cls.expected_length:
        barcode = digits[1:-1]
        number_system, checksum = digits[0], digits[-1]
    elif len(digits) == cls.expected_length - 2:
        barcode = digits
        checksum = None
    else:
        raise ValueError("Invalid barcode length")

    if barcode[-1] in '012':
        upc = barcode[:2] + barcode[-1] + '0' * 4 + barcode[2:5]
    elif barcode[-1] == '3':
        upc = barcode[:3] + '0' * 5 + barcode[3:5]
    elif barcode[-1] == '4':
        upc = barcode[:4] + '0' * 5 + barcode[4]
    else:
        upc = barcode[:5] + '0' * 4 + barcode[-1]

    upc_e = number_system + upc
    check = UpcA.calc_check_digit(upc_e)

    if checksum and checksum != check:
        raise ValueError("Cannot expand, invalid UPC-E")

    return UpcA(upc_e + check)