Python screen.fill不'；好像不行

我已经编写了一个游戏，一切正常，所以现在我想要的是游戏开始前的屏幕。然后必须单击“开始”继续游戏。到目前为止，我有以下代码：

if menu == 'start':
    screen.fill((0,255,0))
    txt = font.render('Start',True,(255,255,0))
    txt_x = 480
    txt_y = 290
    buttonrect = pygame.Rect((txt_x,txt_y),txt.get_size())
    pygame.draw.rect(screen,(0,0,255),buttonrect)
    screen.blit(txt,(txt_x,txt_y))
    if pygame.mouse.get_pressed()[0] and buttonrect.collidepoint(pygame.mouse.get_pos()):
        menu = 'game'

---

我已将菜单设置为“开始”，但它不起作用。屏幕是黑色而不是绿色。但当我点击坐标时，我就去玩游戏了。我的代码有什么问题？

设置屏幕背景后，需要更新主显示：

if menu == 'start':
    screen.fill((0,255,0))
    txt = font.render('Start',True,(255,255,0))
    txt_x = 480
    txt_y = 290
    buttonrect = pygame.Rect((txt_x,txt_y),txt.get_size())
    pygame.draw.rect(screen,(0,0,255),buttonrect)
    screen.blit(txt,(txt_x,txt_y))

    pygame.display.update()

    if pygame.mouse.get_pressed()[0] and buttonrect.collidepoint(pygame.mouse.get_pos()):
        menu = 'game'

---

设置屏幕背景后，您需要更新主显示：

if menu == 'start':
    screen.fill((0,255,0))
    txt = font.render('Start',True,(255,255,0))
    txt_x = 480
    txt_y = 290
    buttonrect = pygame.Rect((txt_x,txt_y),txt.get_size())
    pygame.draw.rect(screen,(0,0,255),buttonrect)
    screen.blit(txt,(txt_x,txt_y))

    pygame.display.update()

    if pygame.mouse.get_pressed()[0] and buttonrect.collidepoint(pygame.mouse.get_pos()):
        menu = 'game'