此代碼中的“行星”精靈不會左右移動，不會動畫。錯誤在哪里？實際上，該列表來自 google play 的應用程序“Programming Hero”，其中應用程序的作者一步一步地指導學生完成視頻 - 使用 python 制作游戲的過程......import pygamescreen_size = [360,640]screen =     pygame.display.set_mode(screen_size)background = pygame.image.load('background.png')planet =     pygame.image.load('one.png')spaceship =  pygame.image.load('spaceship.png')bullet =     pygame.image.load('bullet.png')keep_alive = Trueplanet_x = 140           # I've edited thismove_direction = 'right' # out of the loop, too# according to the same logic "I've saved the planet" to go infinitely right# thanx while keep_alive:    pygame.event.get()    keys = pygame.key.get_pressed()    if keys[pygame.K_SPACE] == True:        print('Space key pressed')    screen.blit(background, [0,0])    screen.blit(bullet,[180,500])    screen.blit(spaceship,[160,500])    # I put this out off loop:           planet_x = 140    # wrong, must go out of a loop, too: move_direction = 'right'    if move_direction == 'right':        planet_x = planet_x + 5        if planet_x == 270:            move_direction = 'left'    else:        planet_x = planet_x - 5        if planet_x == 0:            move_direction = 'right'    screen.blit(planet, [planet_x, 50])    pygame.display.update()