学习 Python，使用《 Python 编程：从⼊门到实践》，外星人入侵的主程序 alien_invasion.py 运行后，只能出现一个空白窗口，而不能出现程序预设的飞机等其他任何东西。非常迷惑，恳请高人指点一下，感谢！感谢！
主程序如下：
import pygame
from pygame.sprite import Group
from settings import Settings
from game_stats import GameStats
from scoreboard import Scoreboard
from button import Button
from ship import Ship
import game_functions as gf
def run_game():
# Initialize pygame, settings, and screen object.
pygame.init()
ai_settings = Settings()
screen = pygame.display.set_mode(
(ai_settings.screen_width, ai_settings.screen_height))
pygame.display.set_caption("Alien Invasion")
# Make the Play button.
play_button = Button(ai_settings, screen, "Play")
# Create an instance to store game statistics, and a scoreboard.
stats = GameStats(ai_settings)
sb = Scoreboard(ai_settings, screen, stats)
# Set the background color.
bg_color = (230, 230, 230)
# Make a ship, a group of bullets, and a group of aliens.
ship = Ship(ai_settings, screen)
bullets = Group()
aliens = Group()
# Create the fleet of aliens.
gf.create_fleet(ai_settings, screen, ship, aliens)
# Start the main loop for the game.
while True:
gf.check_events(ai_settings, screen, stats, sb, play_button, ship,
aliens, bullets)
if stats.game_active:
ship.update()
gf.update_bullets(ai_settings, screen, stats, sb, ship, aliens,
bullets)
gf.update_aliens(ai_settings, screen, stats, sb, ship, aliens,
bullets)
gf.update_screen(ai_settings, screen, stats, sb, ship, aliens,
bullets, play_button)
run_game()