"Animation._instances" keeps getting fat

Animation._instances keeps getting fat.py

'''
https://youtu.be/P2WtjZAWc8I
'''
from kivy.app import App
from kivy.factory import Factory
from kivy.animation import Animation
from kivy.clock import Clock
import itertools

anim_id_iter = itertools.count()


def start_new_animation(dt):
    print("len(Animation._instances) ==", len(Animation._instances))
    print(f"start {next(anim_id_iter)}th animation")
    w = Factory.Widget()
    anim1 = Animation(x=100)
    anim2 = Animation(y=200)
    (anim1 & anim2).start(w)
    Clock.schedule_once(lambda dt: anim1.cancel(w), .5)

class SampleApp(App):
    def build(self):
        return Factory.Widget()
    def on_start(self):
        Clock.schedule_interval(start_new_animation, 1)

SampleApp().run()