很简单的pygame游戏有一步我没懂

def花

import pygame,sys

pygame.init()
size = width,height = 600,400
speed = [1,1]
BLACK = 0,0,0
screen = pygame.display.set_mode(size）
pygame.display.set_caption("pygame壁球碰撞")
ball = pygame.image.load('PYG02-ball.gif')
ball_ract = ball.get_rect()
fps = 300
clock = pygame.time.Clock()
still = False

while True:
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            sys.exit()
        elif event.type == pygame.MOUSEBUTTONDOWN:
            if event.button == 1:
                still = True
        elif event.type == pygame.MOUSEBUTTONUP:
            still = False
            if event.button == 1:
                ball_ract = ball_ract.move(event.pos[0] - ball_ract.left,event.pos[1]-ball_ract.top)#就是这里，是为了防止鼠标点击到窗口旁边，卡在旁边不动，没懂为什么这么判断
        elif event.type == pygame.MOUSEMOTION:
            if event.buttons[0] == 1:
                ball_ract = ball_ract.move(event.pos[0]-ball_ract.left,event.pos[1]-ball_ract.top)
    if pygame.display.get_active() and not still:
        ball_ract = ball_ract.move(speed[0],speed[1])
    if ball_ract.right > width or ball_ract.left < 0:
        speed[0] = -speed[0]
        if ball_ract.right > width and ball_ract.right + speed[0] > ball_ract.right:
            speed[0] = -speed[0]
    if ball_ract.bottom > height or ball_ract.top <0:
        speed[1] = -speed[1]
        if ball_ract.bottom > height and ball_ract.bottom + speed[1] > ball_ract.bottom:
            speed[1] = -speed[1]

    screen.fill(BLACK)
    screen.blit(ball,ball_ract)
    pygame.display.update()
    clock.tick(fps)