Kivy-ScreenManager的KeyError

Question

（请勿将其标记为重复项，其他类似的问题也不能回答我的特定问题）

我正在尝试通过我定义为change_screens的功能在应用程序中使用更改屏幕。在我的kv文件中，我的屏幕管理器的id为screen_manager。当我尝试在函数的python代码中获取id时，我总是收到KeyError: 'screen_manager'错误。

.kv文件

<ControlScreens>:
    id: screen_manager
    transition: CardTransition()

    HomeScreen:
        id: home
        name: 'home'

.py文件

def change_screen(self, screen_name):

    screen_manager = self.root.ids['screen_manager']
    screen_manager.current = screen_name

完全追溯

Traceback (most recent call last):
            File  "C:/Users/demar/OneDrive/Documents/kivy/Social_Media_App/metropolis.py", l ine  125, in <module>
         MetropolisApp().run()
       File "C:\Users\demar\AppData\Local\Programs\Python\Python37-32\lib\site-packages\kivy\app.py", line 826, in run
         runTouchApp()
       File "C:\Users\demar\AppData\Local\Programs\Python\Python37-32\lib\site-packages\kivy\base.py", line 502, in runTouchApp
         EventLoop.window.mainloop()
       File "C:\Users\demar\AppData\Local\Programs\Python\Python37-32\lib\site-packages\kivy\core\window\window_sdl2.py", line 727, in mainloop
         self._mainloop()
       File "C:\Users\demar\AppData\Local\Programs\Python\Python37-32\lib\site-packages\kivy\core\window\window_sdl2.py", line 460, in _mainloop
         EventLoop.idle()
       File "C:\Users\demar\AppData\Local\Programs\Python\Python37-32\lib\site-packages\kivy\base.py", line 340, in idle
         self.dispatch_input()
       File "C:\Users\demar\AppData\Local\Programs\Python\Python37-32\lib\site-packages\kivy\base.py", line 325, in dispatch_input
         post_dispatch_input(*pop(0))
       File "C:\Users\demar\AppData\Local\Programs\Python\Python37-32\lib\site-packages\kivy\base.py", line 231, in post_dispatch_input
         listener.dispatch('on_motion', etype, me)
       File "kivy\_event.pyx", line 707, in kivy._event.EventDispatcher.dispatch
       File "C:\Users\demar\AppData\Local\Programs\Python\Python37-32\lib\site-packages\kivy\core\window\__init__.py", line 1360, in on_motion
         self.dispatch('on_touch_down', me)
       File "kivy\_event.pyx", line 707, in kivy._event.EventDispatcher.dispatch
       File "C:\Users\demar\AppData\Local\Programs\Python\Python37-32\lib\site-packages\kivy\core\window\__init__.py", line 1376, in on_touch_down
         if w.dispatch('on_touch_down', touch):
       File "kivy\_event.pyx", line 707, in kivy._event.EventDispatcher.dispatch
       File "C:\Users\demar\AppData\Local\Programs\Python\Python37-32\lib\site-packages\kivy\uix\screenmanager.py", line 1191, in on_touch_down
         return super(ScreenManager, self).on_touch_down(touch)
       File "C:\Users\demar\AppData\Local\Programs\Python\Python37-32\lib\site-packages\kivy\uix\widget.py", line 460, in on_touch_down
         if child.dispatch('on_touch_down', touch):
       File "kivy\_event.pyx", line 707, in kivy._event.EventDispatcher.dispatch
       File "C:\Users\demar\AppData\Local\Programs\Python\Python37-32\lib\site-packages\kivy\uix\relativelayout.py", line 288, in on_touch_down
         ret = super(RelativeLayout, self).on_touch_down(touch)
       File "C:\Users\demar\AppData\Local\Programs\Python\Python37-32\lib\site-packages\kivy\uix\widget.py", line 460, in on_touch_down
         if child.dispatch('on_touch_down', touch):
       File "kivy\_event.pyx", line 707, in kivy._event.EventDispatcher.dispatch
       File "C:\Users\demar\AppData\Local\Programs\Python\Python37-32\lib\site-packages\kivy\uix\widget.py", line 460, in on_touch_down
         if child.dispatch('on_touch_down', touch):
       File "kivy\_event.pyx", line 707, in kivy._event.EventDispatcher.dispatch
       File "C:\Users\demar\AppData\Local\Programs\Python\Python37-32\lib\site-packages\kivy\uix\behaviors\button.py", line 151, in on_touch_down
         self.dispatch('on_press')
       File "kivy\_event.pyx", line 703, in kivy._event.EventDispatcher.dispatch
       File "kivy\_event.pyx", line 1214, in kivy._event.EventObservers.dispatch
       File "kivy\_event.pyx", line 1098, in kivy._event.EventObservers._dispatch
       File "C:\Users\demar\AppData\Local\Programs\Python\Python37-32\lib\site-packages\kivy\lang\builder.py", line 64, in custom_callback
         exec(__kvlang__.co_value, idmap)
       File "C:\Users\demar\OneDrive\Documents\kivy\Social_Media_App\metropolis.kv", line 201, in <module>
         on_press: app.login()
       File "C:/Users/demar/OneDrive/Documents/kivy/Social_Media_App/metropolis.py", line 113, in login
         self.change_screen('logged_in')
           File "C:/Users/demar/OneDrive/Documents/kivy/Social_Media_App/metropolis.py",   line 119, in change_screen
         screen_manager = self.root.ids['screen_manager']
     KeyError: 'screen_manager'