Painting __init__ vs .kv init

main.py

from kivy.app import App 
from kivy.lang import Builder
from kivy.uix.scatterlayout import ScatterLayout
from kivy.uix.widget import Widget
from kivy.graphics import Line


kv = """ 
<Test>:
    orientation: 'vertical'
    width: 320
    height: 480
    MyWidget
        x: 20
        y: 20
        size_hint: (.9, .9)
    MyWidget
        pos_hint: dict(x=.5, y=.5)
        size_hint: (20 / self.parent.width, 20 / self.parent.height)
    """

Builder.load_string(kv)

thisworks = """ 
<MyWidget@Widget>:
    text: 'My Widget'
    canvas:
        Line:
            rectangle: self.x, self.y, self.width, self.height
"""

# Uncomment next line, and it works as expected:
#Builder.load_string(thisworks)


class MyWidget(Widget):
    """This definition of MyWidget does not work as expected.
    If I was using
        Builder.load_string(thisworks)
    it would work as expected.
    """

    def __init__(self, **kwargs):
        super(MyWidget, self).__init__(**kwargs)
        with self.canvas.after:
            Line(rectangle=[self.x, self.y, self.width, self.height])


class Test(ScatterLayout):
    def __init__(self, *args, **kwargs):
        super(Test, self).__init__(*args, **kwargs)


class TestApp(App):
    def build(self):
        return Test()

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