Kivy开发中ScreenManager仅接受Screen组件报错求助

问题场景 #

刚接触Kivy框架，复制教程代码运行时触发kivy.uix.screenmanager.ScreenManagerException: ScreenManager accepts only Screen widget错误，无法找到解决方案。

原代码 #

main.py

from kivy.app import App


class MainApp(App):
    pass


MainApp().run()

main.kv

#:include Screen1.kv
#:include Screen2.kv

ScreenManager:
    Screen:
        name: "main"
    BoxLayout:
        Button:
            text: "go to Screen1"
            on_release: app.root.current = "screen1"
        Button:
            text: "go to Screen2"
            on_release: app.root.current = "screen2"

    Screen1:
    Screen2:

Screen1.kv

<Screen1@Screen>:
    name: "screen1"
    BoxLayout:
        Button:
            text: "go to main"
            on_release: app.root.current = "main"

Screen2.kv

<Screen2@Screen>:
    name: "screen2"
    BoxLayout:
        Button:
            text: "go to main"
            on_release: app.root.current = "main"

完整报错信息 #

C:\Users\39377\AppData\Local\Microsoft\WindowsApps\python3.10.exe C:\Users\39377\Desktop\APP\BubiniProject\BubiniApp\main.py 
[INFO   ] [Logger      ] Record log in C:\Users\39377\.kivy\logs\kivy_23-03-19_33.txt
[INFO   ] [deps        ] Successfully imported "kivy_deps.angle" 0.3.3
[INFO   ] [deps        ] Successfully imported "kivy_deps.glew" 0.3.1
[INFO   ] [deps        ] Successfully imported "kivy_deps.sdl2" 0.4.5
[INFO   ] [Kivy        ] v2.1.0
[INFO   ] [Kivy        ] Installed at "C:\Users\39377\AppData\Local\Packages\PythonSoftwareFoundation.Python.3.10_qbz5n2kfra8p0\LocalCache\local-packages\Python310\site-packages\kivy\__init__.py"
[INFO   ] [Python      ] v3.10.10 (tags/v3.10.10:aad5f6a, Feb  7 2023, 17:20:36) [MSC v.1929 64 bit (AMD64)]
[INFO   ] [Python      ] Interpreter at "C:\Users\39377\AppData\Local\Microsoft\WindowsApps\PythonSoftwareFoundation.Python.3.10_qbz5n2kfra8p0\python.exe"
[INFO   ] [Logger      ] Purge log fired. Processing...
[INFO   ] [Logger      ] Purge finished!
[INFO   ] [Factory     ] 189 symbols loaded
[INFO   ] [Image       ] Providers: img_tex, img_dds, img_sdl2, img_pil (img_ffpyplayer ignored)
[INFO   ] [Window      ] Provider: sdl2
[INFO   ] [GL          ] Using the "OpenGL" graphics system
[INFO   ] [GL          ] GLEW initialization succeeded
[INFO   ] [GL          ] Backend used <glew>
[INFO   ] [GL          ] OpenGL version <b'4.6.0 - Build 26.20.100.7870'>
[INFO   ] [GL          ] OpenGL vendor <b'Intel'>
[INFO   ] [GL          ] OpenGL renderer <b'Intel(R) UHD Graphics 620'>
[INFO   ] [GL          ] OpenGL parsed version: 4, 6
[INFO   ] [GL          ] Shading version <b'4.60 - Build 26.20.100.7870'>
[INFO   ] [GL          ] Texture max size <16384>
[INFO   ] [GL          ] Texture max units <32>
[INFO   ] [Window      ] auto add sdl2 input provider
[INFO   ] [Window      ] virtual keyboard not allowed, single mode, not docked
 Traceback (most recent call last):
   File "C:\Users\39377\Desktop\APP\BubiniProject\BubiniApp\main.py", line 8, in <module>
     MainApp().run()
   File "C:\Users\39377\AppData\Local\Packages\PythonSoftwareFoundation.Python.3.10_qbz5n2kfra8p0\LocalCache\local-packages\Python310\site-packages\kivy\app.py", line 954, in run
     self._run_prepare()
   File "C:\Users\39377\AppData\Local\Packages\PythonSoftwareFoundation.Python.3.10_qbz5n2kfra8p0\LocalCache\local-packages\Python310\site-packages\kivy\app.py", line 923, in _run_prepare
     self.load_kv(filename=self.kv_file)
   File "C:\Users\39377\AppData\Local\Packages\PythonSoftwareFoundation.Python.3.10_qbz5n2kfra8p0\LocalCache\local-packages\Python310\site-packages\kivy\app.py", line 696, in load_kv
     root = Builder.load_file(rfilename)
   File "C:\Users\39377\AppData\Local\Packages\PythonSoftwareFoundation.Python.3.10_qbz5n2kfra8p0\LocalCache\local-packages\Python310\site-packages\kivy\lang\builder.py", line 305, in load_file
     return self.load_string(data,** kwargs)
   File "C:\Users\39377\AppData\Local\Packages\PythonSoftwareFoundation.Python.3.10_qbz5n2kfra8p0\LocalCache\local-packages\Python310\site-packages\kivy\lang\builder.py", line 407, in load_string
     self._apply_rule(
   File "C:\Users\39377\AppData\Local\Packages\PythonSoftwareFoundation.Python.3.10_qbz5n2kfra8p0\LocalCache\local-packages\Python310\site-packages\kivy\lang\builder.py", line 659, in _apply_rule
     widget.add_widget(child)
   File "C:\Users\39377\AppData\Local\Packages\PythonSoftwareFoundation.Python.3.10_qbz5n2kfra8p0\LocalCache\local-packages\Python310\site-packages\kivy\uix\screenmanager.py", line 987, in add_widget
     raise ScreenManagerException(
 kivy.uix.screenmanager.ScreenManagerException: ScreenManager accepts only Screen widget.

Process finished with exit code 1

错误原因 #

ScreenManager的子控件只能是Screen类型（或继承自Screen的自定义控件），原main.kv中BoxLayout直接作为ScreenManager的子元素，违反了这一规则，导致报错。需将BoxLayout放到名为"main"的Screen内部。

修正后的代码 #

仅需修改main.kv，将BoxLayout嵌套进name为"main"的Screen中：

#:include Screen1.kv
#:include Screen2.kv

ScreenManager:
    Screen:
        name: "main"
        BoxLayout:
            Button:
                text: "go to Screen1"
                on_release: app.root.current = "screen1"
            Button:
                text: "go to Screen2"
                on_release: app.root.current = "screen2"

    Screen1:
    Screen2:

其余文件（main.py、Screen1.kv、Screen2.kv）无需修改，运行修正后的代码即可正常切换屏幕。

内容的提问来源于stack exchange，提问作者coliso