pygame帮助保持黑屏

Question

import pygame, random, time

def main():
    pygame.init()
    size =(500,400)
    surface=pygame.display.set_mode(size)
    pygame.display.set_caption('Pong v1')
    game = Game(surface)
    game.play()
    pygame.quit()

class Game():
    def __init__(self,surface):
        self.surface=surface
        self.FPS=24
        self.bg_color=pygame.Color('black')
        screen_width = surface.get_width()
        screen_height = surface.get_height()
        ball_radius=10
        ball_pos = [random.randint(ball_radius, screen_width-ball_radius), 
                    random.randint(ball_radius, screen_height-ball_radius)]
        ball_color=pygame.Color('white')
        ball_velocity=[2,1]
        self.ball=Ball(ball_pos,ball_radius,ball_color,ball_velocity,surface)

    def draw_score(self):
        font_color = pygame.Color("white")
        font_bg    = pygame.Color("black")
        font       = pygame.font.SysFont("arial", 72)
        text_img   = font.render("Score: (not in this version) " + str(self.score), True, font_color, font_bg)     
        text_pos   = (0,0)
        self.surface.blit(text_img, text_pos)        

    def draw(self):
        self.surface.fill(self.bg_color)
        self.ball.draw()
        self.draw_score()
        pygame.display.update()

    def update(self):
        self.ball.move()
        self.score=0

    def play(self):
        while True:
            self.draw()
            self.update()

class Ball:
    def __init__(self,center,radius,color,velocity,surface):
        self.center=center
        self.radius=radius
        self.color=color
        self.velocity=velocity
        self.surface=surface

    def move(self):
        screen_width=self.surface.get_width()
        screen_height=self.surface.get_height()
        screen_size=(screen_width,screen_height)
        for i in range(0,len(self.center)):
            self.center[i]+=self.velocity[i]
            if (self.center[i]<=0 + self.radius or self.center[i]>=screen_size[i] - self.radius):
                self.velocity[i]=-self.velocity[i]

    def draw(self):
        pygame.draw.circle(self.surface,self.color,self.center,self.radius)





main()

因此，我正在尝试创建一个简单的乒乓球，球从窗口边缘弹起。我还需要在屏幕的右侧和左侧创建两个球拍，但是在此版本中，它们不需要反弹球，只有墙才可以。我不知道该怎么实现，由于某种原因，当前代码一直在给我黑屏。有人可以解释一下我将如何创建两个球拍并摆脱黑屏吗？