code: def add_isbn(self, isbn, medium=None):
          
          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)
docs: :param isbns: the isbns of the book
          :type isbns: object