def define_url(self):
        """Define API url based on which source is selected.

        :return: Valid url of selected source.
        :rtype: str
        """
        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