def _check_digit(self):
        """ Calculate the check digit for ISBN-13.
        See https://en.wikipedia.org/wiki/International_Standard_Book_Number
        for calculation.
        """
        weights = (1 if x % 2 == 0 else 3 for x in range(12))
        body = ''.join([self.ean, self.group, self.registrant,
                        self.publication])
        remainder = sum(int(b) * w for b, w in zip(body, weights)) % 10
        diff = 10 - remainder
        check_digit = 0 if diff == 10 else diff
        return str(check_digit)