做到在屏幕上绘制飞船开始报错

import sys
import pygame
from settings import Setting
from  ship import Ship

def run_game():
    pygame.init()
    pygame.display.set_caption('Alien Invasion')
    ship = Ship(screen)

    ai_settings = Setting()
    screen = pygame.display.set_mode(
        (ai_settings.screen_width, ai_settings.screen_height))

    while True:#监视鼠标和键盘事件
        for event in pygame.event.get():
            if event.type == pygame.QUIT:
                sys.exit()
        screen.fill(ai_settings.bg_color)
        ship.blitme()
        pygame.display.flip()

run_game()

import pygame

class Ship():
    def __init__(self, screen):
        self.screen = screen

        self.image = pygame.image.load(
            'E:/PY/044_飞机大战/飞机大战素材包/hero2.png')
        self.rect = self.image.get_rect()
        self.screen_rect = screen.get_rect()

        self.rect.centerx = self.screen_rect.centerx
        self.rect.bottom = self.screen_rect.bottom

    def blitme(self):
        self.screen.blit(self.image, self.rect)

运行报错，在ship = Ship(screen)一行。发生异常: UnboundLocalError
local variable 'screen' referenced before assignment

谢谢，真的是这样