pygame.error: video system not initialized

在pygame写游戏出现pygame.error: video system not initialized

源代码

import sys
import pygame
def run_game():
    pygame.init()
    screen = pygame.display.set_mode((1200,800))
    pygame.display.set_caption("Alien Invasion")

#主循环
while True:
    #监视键盘和鼠标事件
    for event in pygame.event.get():
        if event.type==pygame.QUIT:
            sys.exit()

    #让最近屏幕可见
    pygame.display.flip()

run_game()
报错

Traceback (most recent call last):
File "D:/project/python/FirstGame/alien_invasion.py", line 11, in <module>
for event in pygame.event.get():
pygame.error: video system not initialized

原因在于while true没有放进主循环，没有缩进