锚点布局anchorlayout在kv中的引用

from kivy.app import App
from kivy.uix.anchorlayout import AnchorLayout
from kivy.uix.button import Button

class AnchorLayoutWidget(AnchorLayout):
    def __init__(self,**kwargs):
        super().__init__(**kwargs)

class AnchorLayoutApp(App):
    def build(self):
        return AnchorLayoutWidget()

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

==========================kv文件=========================
<AnchorLayoutWidget>:
    # 设置间距
    padding:20
    AnchorLayout:
        anchor_x:'left'
        anchor_y:'top'
        Button:
            text:'bt1'
            size_hint:.3,.2
            # 颜色
            background_color:.2,.6,.3,1
        Button:
            text:'bt2'
            size_hint:.3,.2
            background_color:.3,.5,.4,1
    AnchorLayout:
        anchor_x:'right'
        anchor_y:'bottom'
        Button:
            text:'bt2'
            size_hint:.3,.2
            background_color:1,.5,.7,2