[Python] Kivy animation - decreasing duration?

I have a widget class in kivy, which I animate on screen. By the time I want to slowly decrease animation duration so the widget will move faster. My code changes the duration, but it didn't affect on screen. Why?

My widget class:

class Obstacle3(Widget):
def __init__(self, **kwargs):
super().__init__(**kwargs)
self.animation = Animation(x =-self.width, duration=2)
Clock.schedule_interval(self.faster, 2)
self.anim = True
self.animation.bind(on_complete=self.vanish)
self.animation.start(self)
def faster(self, *args):
if self.animation._duration <= 0.4:
self.animation._duration = self.animation._duration
else:
self.animation._duration = self.animation._duration - 0.4

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