code: def _next_page(self):
          
          if not self._has_next_page():
              return None
  
          if self.next_page_token is not None:
              setattr(self._request, self._request_token_field, self.next_page_token)
  
          response = self._method(self._request)
  
          self.next_page_token = getattr(response, self._response_token_field)
          items = getattr(response, self._items_field)
          page = Page(self, items, self.item_to_value)
  
          return page
docs: Get the next page in the iterator.
  
          Returns:
              Page: The next page in the iterator or :data:`None` if
                  there are no pages left.