code: def pre_save(self, model_instance, add):
          
          value = getattr(model_instance, self.attname)
          if self.clean_isbn and value not in EMPTY_VALUES:
              cleaned_isbn = value.replace(' ', '').replace('-', '').upper()
              setattr(model_instance, self.attname, cleaned_isbn)
          return super(ISBNField, self).pre_save(model_instance, add)
docs: Remove dashes, spaces, and convert isbn to uppercase before saving
          when clean_isbn is enabled