### main.py ###
from kivy.app import App

class MainScreen():
    def button_clicked(self):
        print("*")

class KivyApp(App):
    pass
KivyApp().run()


### kivy.kv ###
MainScreen:
<MainScreen@BoxLayout>:
    Button:
        on_press: root.button_clicked()

Why I can not call button_clicked() function from kivy? I know that I can call it if describe like

class MainScreen(BoxLayout)

in main.py. But why I can not with above codes?

CodePudding user response：

So, I am not 100% sure about my answer but here is what I think is happening...

The py file runs first and then the kv file, so if in the py file you have Class MainScreen(BoxLayout): the MainScreen class is defined and inherits from BoxLayout... Then the rules are defined in the kV file as normal. When you use the @BoxLayout in the kV file the class is created again, so this over rides the first definition of MainScreen. As the py MainScreen has been over ridden by the kV MainScreen the attribute root.button_clicked() no longer exists....

If you really need use the @BoxLayout in the kV file you could add the function to the app class:

class LayoutApp(App):

  def button_clicked(self):
    print("*")

And then in the kV file:

MainScreen:
<MainScreen@BoxLayout>:
    Button:
       on_press: app.button_clicked()