:param isbns: the isbns of the book
        :type isbns: object

def add_isbn(self, isbn, medium=None):
        """
        :param isbns: the isbns of the book
        :type isbns: object
        """
        isbn_dict = {}
        if isbn is not None:
            isbn_dict['value'] = normalize_isbn(isbn)
        if medium is not None:
            isbn_dict['medium'] = medium

        self._append_to('isbns', isbn_dict)