How do I move a Kivy widget's canvas items if the items are defined in .py and not kv lang?

Question

I need to be able to spawn arbitrary "Planets" of various shapes, sizes, and colors, and move them around in a multi-body simulation, add them, and remove them as things happen in the simulation.

I have added a number of operations to an instance of a kivy widget's canvas. In trying to find a solution to this problem, for comparison, I have also defined a shape in the kivy lang file for the corresponding derived Widget class (based on the Pong tutorial).

When changing the position of the widget, only the shape defined in the kv lang file moves with the widget. The one defined in the python 3 code does not move.

What is the proper way to move the widget's canvas items in Kivy in response to changes to the widget's position?

Here is some sample code, and using Python 3 with Kivy, you'll see the white dot (defined in the kv lang) move away from the red dot (defined in the widget's init function) once the event spawns the "planet"

from kivy.app import App
from kivy.uix.widget import Widget
from kivy.graphics import *
from kivy.properties import NumericProperty, ReferenceListProperty, ObjectProperty
from kivy.vector import Vector
from kivy.clock import Clock
from kivy.lang import Builder

class Planet(Widget):

    # velocity of the ball on x and y axis
    dx = NumericProperty(0)
    dy = NumericProperty(0)

    def init(self,  pos=(50,50), **kwargs):
        """ Initialize the planet"""
        self.pos = pos
        print("Init planet. pos:", self.pos)
        # These shapes do not move with the widget.
        #  Why?
        # Only the white circle in .kv lang moves with it.
        self.canvas.add(Color(0.8,0,0))
        self.canvas.add(Ellipse(pos=self.pos, size=(50,50)))


    def move(self):
        """ Move the planet. """
        self.pos = Vector(self.velocity) + self.pos
        print("Planet now at", self.pos)



class System(Widget):

    mars = ObjectProperty(None)

    def update(self, dt):
        print("Update! " , dt)
        if self.mars:
            self.mars.move()

    def spawn(self, dt):
        print("Insert!", dt)
        self.mars = Planet()
        self.mars.init()
        self.add_widget(self.mars)
        self.mars.velocity = (1,1)


class PlanetApp(App):
    def build(self):
        sys = System()
        Clock.schedule_interval(sys.update, 1/4)
        Clock.schedule_once(sys.spawn, 3)
        return sys

if __name__ == '__main__':
    Builder.load_string("""
#:kivy 1.0.9
<Planet>
    canvas:
        Ellipse:
            pos: self.pos
            size: self.size
""")

    PlanetApp().run()

Thank you!

Answer 1

After posting my own question, I figured it out...

Updating the pos of a widget does not update the pos of the objects in that widget's canvas. But you can update the pos in the widget's canvas separately.

I don't understand why the kv lang implementation updates the pos of the objects in its canvas, while the py implementation does not - so maybe someone else can provide a more thorough answer. The documentation regularly indicates that the kind of thing done in the code example's py implementation is the same as the kv lang. But that is apparently not quite true. So why? Can anyone explain?

However, this works...

class Planet(Widget):
    ...
    # store the shapes in the canvas in the object.
    # (could put in a list to iterate over them to update pos)
    ellipse = ObjectProperty(None)
    ...

    def init(self,  pos=(50,50), **kwargs):
        ...
        # save the element we're adding to the canvas
        self.ellipse = Ellipse(pos=self.pos, size=(50,50))
        self.canvas.add(self.ellipse)

    def move(self):
        self.pos = Vector(self.velocity) + self.pos
        self.ellipse.pos = self.pos  # move the canvas element too
        print("Planet now at", self.pos)