Python turtle, что не так с кодом

 from turtle import Turtle, tracer, done, update 
 
st = Turtle() 
tracer(0) 
st.color('maroon') 
st.pensize(2) 
st.pu() 
st.goto(-350, 300) 
st.pd() 
 
st.goto(-350, -300) 
st.goto(350, -300) 
st.goto(350, 300) 
st.ht() 
 
shar = Turtle() 
shar.shape('circle') 
shar.color('blue') 
shar.turtlesize(2) 
shar.goto(-340, 200) 
v_x = 0.27 
v_y = 0 
a = -0.0005 
i = 0 
while True: 
    shar.clear() 
    shar.goto(shar.xcor() + v_x, shar.ycor() + v_y) 
    v_y += a 
    update() 
    if shar.ycor() < -290: 
        v_y *= -1 
    if shar.xcor() > 340 or shar.xcor() < -340: 
        v_x *= -1 
    if i > 100000: 
        break 
    i += 1 
done()