问为什么在我的kv文件中，即使列出了类名称，kivy也不能处理规则？

EN

Root:
    ThoughtsClass:
    Distortion:


<ThoughtsClass>:
    cols: 1
    thought: thought
    id: thoughtclass

    TextInput:
        id: thought
        multiline: False

    BoxLayout:
        size_hint_y: 0.25
        orientation: 'horizontal'

        Button:
            text: 'Next Thought'
            on_press: thoughtclass.nextthought()

        Button:
            text: 'Done'
            on_press: thoughtclass.donethought()

<Distortion>:
    cols: 1
    disttext: disttext
    thoughtdisplay: thoughtdisplay

    Label:
        id:thoughtdisplay
        text: ''

    BoxLayout:
        orientation: 'horizontal'
        SelectDist:
            id: disttext

#!/usr/bin/env python3

from kivy.app import App
from kivy.uix.gridlayout import GridLayout
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.widget import Widget
from kivy.uix.label import Label
from kivy.uix.textinput import TextInput
from kivy.properties import StringProperty, ObjectProperty, DictProperty
from kivy.storage.dictstore import DictStore
from kivy.uix.dropdown import DropDown
from kivy.uix.button import Button
from kivy.uix.screenmanager import ScreenManager, Screen

class Root(ScreenManager):

    def __init(self, **kwargs):
        super(Root, self).__init__(self,**kwargs)


class ThoughtsClass(Screen):
    thought = ObjectProperty()
    negstatements = DictProperty()
    distmeanings = DictProperty()

    def __init__(self, **kwargs):
       super(ThoughtsClass, self).__init__(**kwargs)
       self.distmeanings = distortions
       self.name = 'Thoughts'


    def nextthought(self):
        print ('Thought: ' + self.thought.text)
        if self.thought.text:
            self.negstatements[self.thought.text] = {'distortion':'', 'rational':''}
        self.thought.text = ''

    def donethought(self):
        print ('Done pressed')
        if self.thought.text not in self.negstatements:
            self.nextthought()
        root.current = 'Distortions'
        root.current_screen.thoughts = self.negstatements


class Distortion(Screen):
    disttext = ObjectProperty()
    thoughts = DictProperty()
    thoughtdisplay = ObjectProperty()

    def __init__(self, **kwargs):
        super(Distortion, self).__init__(**kwargs)
        self.name = 'Distortions'

    def display_thought(self):
        for thoughttext in self.thoughts:
            self.thoughtdisplay.text = thoughttext
        dist_dd = SelectDist()
        dist_dd.build_dd()
        distbutton = Button(text='Choose a distortion:')
        distbutton.bind(on_release=dist_dd.open)
        dist_dd.bind(on_select=self.choose_dist())

    def choose_dist(self, instance, value):
        print ("Chose " + value)

class SelectDist(DropDown):

    def __init__(self, **kwargs):
        super(SelectDist, self).__init__(**kwargs)

    def build_dd(self):
        for diststring in distortions:
            btn = Button(text = diststring)
            btn.bind(on_release=lambda btn: self.select(btn.text))
            self.add_widget(btn)


class ThreeColumnApp(App):

    def build(self):
        global root
        root = Root()
        root.add_widget(ThoughtsClass())
        root.add_widget(Distortion())
        return root

if __name__ == '__main__':
    distortions = {}
    cbtinstance = ThreeColumnApp()
    cbtinstance.run()