Re: UPC / EAN

[Arthur de Jong <arthur [at] arthurdejong.org>]

On Sun, 2011-06-19 at 12:05 +0200, Jochen Garcke wrote:
> Maybe of interest, feel free to use the code.
> 
> I needed to validate barcodes of the form UPC or EAN-13, with potential
> additional EAN-2 or EAN-5 for periodicals, this code validates it.

Thanks. For the calculation of the checksum I currently use in ISBN-13 I
have the following quite compact piece of code:

def _calc_isbn13_check_digit(number):
    """Calculate the ISBN check digit for 13-digit numbers. The number passed
    should not have the check bit included."""
    return str((10 - sum( (2 * (i % 2) + 1) * int(n) for i, n in 
enumerate(number))) % 10)

which can be easily turned into something that validates arbitrary
length EAN codes:

def calc_check_digit(number):
    """Calculate the EAN check digit for 13-digit numbers. The number passed
    should not have the check bit included."""
    return str((10 - sum( (3 - 2 * (i % 2)) * int(n) for i, n in 
enumerate(reversed(number)))) % 10)

> Not sure if for a general code one would/should do the checks to
> remove EAN5 and EAN2 though, one might get into trouble doing this
> always.

I've shied away from stripping the EAN-2 and EAN-5 supplements in my
code for now. Perhaps an extra parameter to do the stripping can be
introduced. Have a look:

http://arthurdejong.org/trac/python-stdnum/browser/python-stdnum/stdnum/ean.py

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