실행을 시켜보면 공이 방향키로 움직일수 있는데요...(아직은 미완성)

저 공이 계속 화면으로 넘어가는걸 막을 수 있을까요?

%%code%%

import pygame

pygame.init()

x = 300

y = 400

screen = pygame.display.set_mode((y, x), 0, 32)

pygame.display.set_caption("ball")

WHITE = (255, 255, 255)

ball = pygame.image.load('ball.png')

ballx = 150

bally = 100

ballSize = ball.get_rect().size

ballWidth = ballSize[0]

ballHeight = ballSize[1]

ballXpos = (y / 2) - (ballWidth / 2)

ballYpos = x - ballHeight

done=False

while not done:

for event in pygame.event.get():

if event.type == pygame.QUIT:

done=True

elif event.type == pygame.KEYDOWN:

if event.key == pygame.K_RIGHT:

ballx+=5

elif event.key == pygame.K_LEFT:

ballx-=5

elif event.key == pygame.K_UP:

bally-=5

elif event.key == pygame.K_DOWN:

bally+=5

screen.fill(WHITE)

screen.blit(ball, (ballx, bally))

pygame.display.flip()