Pygame Surface.fill() not work but my code has not any errors

Question

import sys
import pygame


def run():
  pygame.init()

  screen = pygame.display.set_mode((1280, 720))

  while True:
    screen.fill((255,0,0)

    for event in pygame.event.get():
      if(event.type == pygame.QUIT):
        sys.exit()

    pygame.display.flip()

run()

I don't know why my screen (created by pygame) is still gray not (255,0,0)

I use:

• pygame version: 1.9.4
• python version: 3.7.1
• Platform: Mac OSX