Kivy dropdown:你能让它“向上”而不是“向下”吗？

from kivy.app import App
from kivy.uix.button import Button
from kivy.uix.dropdown import DropDown

class CustomDropDown(DropDown):
    def open(self, widget):
        self.width = widget.width
        self.height = widget.height * len(self.children)
        self.top = widget.top
        super().open(widget)

class MyApp(App):
    def build(self):
        dropdown = CustomDropDown()
        for i in range(5):
            btn = Button(text='Option %d' % i, size_hint_y=None, height=40)
            btn.bind(on_release=lambda btn: dropdown.select(btn.text))
            dropdown.add_widget(btn)

        main_button = Button(text='Open Dropdown', size_hint=(None, None))
        main_button.bind(on_release=dropdown.open)

        return main_button

if __name__ == '__main__':
    MyApp().run()