如何在kivy中使用嵌套的for循环和on_parent

from kivy.app import App
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.button import Button

class NestedForLoopApp(App):
    def build(self):
        layout = BoxLayout(orientation='vertical')
        
        # 外层循环
        for i in range(3):
            # 内层循环
            for j in range(3):
                # 创建按钮并添加到布局中
                button = Button(text=f'Button {i}-{j}')
                layout.add_widget(button)
                
                # 设置on_parent属性，当按钮被添加到布局中时触发回调函数
                button.bind(on_parent=self.on_button_parent)
        
        return layout
    
    def on_button_parent(self, button, layout):
        # 当按钮被添加到布局中时，打印按钮的文本和布局的方向
        print(f'Button {button.text} added to layout with orientation {layout.orientation}')

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