GS1 GTIN Check Digit and UDI Element String Python Generator

# GS1 GTIN-14 check digit and UDI element string builder
# Application identifiers: 01 GTIN, 17 expiry (YYMMDD), 10 lot, 21 serial

GTIN_BASE = "0123456789012"
EXPIRY = "281031"
LOT = "LOT24A017"
SERIAL = "[Serial number (blank if lot controlled)]"
FNC1 = "\x1d"  # group separator used between variable length elements in the encoded data


def gs1_check_digit(digits):
    """Return the GS1 mod 10 check digit for a string of digits.
    Weights alternate 3, 1, 3, 1 starting from the rightmost digit of the input."""
    if not digits.isdigit():
        raise ValueError("GTIN base must contain digits only")
    total = 0
    for position, char in enumerate(reversed(digits)):
        weight = 3 if position % 2 == 0 else 1
        total += int(char) * weight
    return (10 - total % 10) % 10


def build_gtin(base):
    if len(base) != 13:
        raise ValueError("GTIN base must be 13 digits to make a GTIN-14")
    return base + str(gs1_check_digit(base))


def build_strings(gtin, expiry, lot, serial):
    parts_hri = ["(01)" + gtin]
    parts_data = ["01" + gtin]
    if expiry:
        parts_hri.append("(17)" + expiry)
        parts_data.append("17" + expiry)
    if lot:
        parts_hri.append("(10)" + lot)
        parts_data.append("10" + lot)
    if serial:
        if lot:
            parts_data[-1] = parts_data[-1] + FNC1
        parts_hri.append("(21)" + serial)
        parts_data.append("21" + serial)
    return "".join(parts_hri), "".join(parts_data)


if __name__ == "__main__":
    gtin = build_gtin(GTIN_BASE)
    hri, data = build_strings(gtin, EXPIRY, LOT, SERIAL)
    print("GTIN-14      :", gtin)
    print("Human readable:", hri)
    print("Encoded data  :", data.replace(FNC1, "<GS>"))