Python Turtle實現彈球

import turtle as t
import time
t.setup(800,600)
t.bgcolor("yellow")
t.ht()
t.tracer(0)
x=0
y=0
xv=5
yv=-5
while True:
    t.clear()
    t.up()
    t.goto(x,y)
    t.down()
    t.fillcolor("green")
    t.begin_fill()
    t.circle(30)
    t.end_fill()
    x+=xv
    y+=yv
    if x>800//2-30 or x<-800//2+30:
        xv=-xv
    if y > 600 // 2 - 30 or y < -600 // 2 + 30:
        yv = -yv
    t.update()
    time.sleep(0.01)
t.done()