Python + Kivy: Не работает DragDrop

Не могу понять почему не работает

import kivy
kivy.require('1.7.2') # replace with your current kivy version !

from kivy.app import App
from kivy.uix.screenmanager import ScreenManager, Screen
from kivy.properties import ObjectProperty
from kivy.uix.button import Button
from kivy.uix.dropdown import DropDown

class DropBut(Button):
    def __init__(self, **kwargs):
        super(DropBut, self).__init__(**kwargs)
        self.drop_list = None
        self.drop_list = DropDown()

        types = ['Item1', 'Item2', 'Item3', 'Item4', 'Item5', 'Item6']

        for i in types:
            btn = Button(text=i, size_hint=(0.5, 0.1), pos_hint = {'center_x': 0.5, 'center_y': 0.5})
            btn.bind(on_release=lambda btn: self.drop_list.select(btn.text))

            self.drop_list.add_widget(btn)

        self.bind(on_release=self.drop_list.open)
        self.drop_list.bind(on_select=lambda instance, x: setattr(self, 'text', x))

class dropdApp(App):

    def build(self):

        return DropBut()

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