Kivy: referencing ids in canvas

canvaserror.kv

FloatLayout:
    Widget:
        canvas:
            Color:
                rgba: 1, 0, 0, 1
            Line:
                points: [btn.center_x, btn.center_y, 500, 500]  # NameError: name 'btn' is not defined

    Button:
        id: btn
        size_hint: .3, .3

canvaswork.kv

FloatLayout:
    Button:
        id: btn
        size_hint: .3, .3

    Widget:
        canvas:
            Color:
                rgba: 1, 0, 0, 1
            Line:
                points: [btn.center_x, btn.center_y, 500, 500]  # works just fine