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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 update_dropdown_list_slot(self):
 """Keep updating the dropdown list. Say, don't let the user choose USB devices if none is available """

        self.dropdown_widget.clear() # this will trigger dropdown_changed_slot
        self.row_instance_by_index = []
        for i, key in enumerate(self.row_instance_by_name.keys()):
            row_instance = self.row_instance_by_name[key]
            if (row_instance.isActive()):
                self.row_instance_by_index.append(row_instance)
                display_name = row_instance.getName()
                self.dropdown_widget.insertItem(i, display_name)
            row_instance.updateWidget()