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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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