code: def _check_digit(self):
          
          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)
docs: Calculate the check digit for ISBN-13.
          See https://en.wikipedia.org/wiki/International_Standard_Book_Number
          for calculation.