UPC-A to UPC-E converter in Python

upca_to_upce.py

def upca_to_upce(upca):
    """
    Takes a upc-a code (12 digits), including check digit, 
        as a string and returns upc-e representation.
    UPC-E representation will be eight digits, including check digit
    Returns None if no upc-e representation is possible

    Rules from http://mdn.morovia.com/kb/UPCE-Specification-10634.html
    
    Test for all numbers 0-9 (last digit of upce) in number system 0
    Then test 1 number system 1 and one number system 2
    >>> upca_to_upce("042100005264")
    '04252614'
    >>> upca_to_upce("020200004417")
    '02044127'
    >>> upca_to_upce("020600000019")
    '02060139'
    >>> upca_to_upce("040350000077")
    '04035747'
    >>> upca_to_upce("020201000050")
    '02020150'
    >>> upca_to_upce("020204000064")
    '02020464'
    >>> upca_to_upce("023456000073")
    '02345673'
    >>> upca_to_upce("020204000088")
    '02020488'
    >>> upca_to_upce("020201000098")
    '02020198'
    >>> upca_to_upce("043000000854")
    '04308504'
    >>> upca_to_upce("127200002013")
    '12720123'
    >>> upca_to_upce("212345678992")
    """
    start_digit = upca[0]
    check_digit = upca[11]
    manufacturer_code = upca[1:6]
    product_code = upca[6:11]
    upce = None

    if not start_digit in ['0', '1']:
        return None


    if manufacturer_code[-3:]  in ["000", "100", "200"] and int(product_code) <= 999:
        upce = manufacturer_code[:2] + product_code[-3:] + manufacturer_code[2]
    elif manufacturer_code[-2:] == '00' and int(product_code) <= 99:
        upce = manufacturer_code[:3] + product_code[-2:] + "3"
    elif manufacturer_code[-1] == "0" and int(product_code) <= 9:
        upce = manufacturer_code[:4] + product_code[-1] + "4"
    elif manufacturer_code[-1] != "0" and int(product_code) in [5,6,7,8,9]:
        upce = manufacturer_code + product_code[-1]

    if upce is not None:
        upce = start_digit + upce + check_digit

    return upce