code: def define_url(self):
          
          current_index = self.city_combo_box.currentIndex()
          city_code = self.city_combo_box.itemData(current_index, Qt.UserRole)
          source = (self.source).format(city_code=city_code)
          return source
docs: Define API url based on which source is selected.
  
          :return: Valid url of selected source.
          :rtype: str