编写外星人入侵程序时出现了子弹始终在一个地方发射而不跟随飞船位置的变化而变化，如何解决？

如题

import pygame
import sys
from pygame.sprite import Sprite
class Bullet(Sprite):
    def __init__(self,ai_game):
        super().__init__()
        self.screen = ai_game.screen
        self.settings = Settings()
        self.ship = Ship(ai_game)
        self.bullet_rect = pygame.Rect(0, 0, self.settings.bullet_width, self.settings.bullet_height)
        self.bullet_rect.midtop = self.ship.ship_rect.midtop
    def show(self):
        pygame.draw.rect(self.screen,self.settings.bullet_color,self.bullet_rect)
    def move(self):
        self.bullet_rect.centery -= self.settings.bullet_speed


class Settings:
    """存储游戏设置的类"""

    def __init__(self):
        self.screen_width = 800
        self.screen_height = 800
        self.bg_color = (230, 230, 230)
        self.ship_speed = 1
        self.bullet_width = 10
        self.bullet_height = 10
        self.bullet_color = (60,60,60)
        self.bullet_speed = 0.1


class Ship:
    """控制飞船的类"""

    def __init__(self, ai_game):
        self.screen = ai_game.screen
        self.screen_rect = ai_game.screen.get_rect()
        self.settings = Settings()
        self.ship_image = pygame.image.load("D:\python\ship.bmp")
        self.ship_rect = self.ship_image.get_rect()
        self.ship_rect.midbottom = self.screen_rect.midbottom
        self.moving_right = False
        self.moving_left = False

    def show(self):
        self.screen.blit(self.ship_image, self.ship_rect)

    def move(self):
        if self.moving_right and self.ship_rect.right < self.screen_rect.right:
            self.ship_rect.centerx += self.settings.ship_speed
        if self.moving_left and self.ship_rect.left >self.screen_rect.left:
            self.ship_rect.centerx -= self.settings.ship_speed


class AlienInvasion:
    """操控游戏的主类"""

    def __init__(self):
        pygame.init()
        self.settings = Settings()
        self.bullets = pygame.sprite.Group()
        self.screen = pygame.display.set_mode((self.settings.screen_width, self.settings.screen_height))
        self.ship = Ship(self)
        pygame.display.set_caption("飞机大站")

    def check_events(self):
        for event in pygame.event.get():
            if event.type == pygame.QUIT:
                sys.exit()
            if event.type == pygame.KEYDOWN:
                if event.key == pygame.K_RIGHT:
                    self.ship.moving_right = True
                if event.key == pygame.K_LEFT:
                    self.ship.moving_left = True
                if event.key == pygame.K_SPACE:
                    self.fire_bullets()

            if event.type == pygame.KEYUP:
                if event.key == pygame.K_RIGHT:
                    self.ship.moving_right = False
                if event.key == pygame.K_LEFT:
                    self.ship.moving_left = False

    def fire_bullets(self):
        a_bullet = Bullet(self)
        self.bullets.add(a_bullet)

    def update_events(self):
        self.screen.fill(self.settings.bg_color)
        self.ship.show()
        for bullet in self.bullets.sprites():
            bullet.show()
            bullet.move()
        pygame.display.flip()

    def run_game(self):
        while True:
            self.check_events()
            self.ship.move()
            for bullet in self.bullets.copy():
                if bullet.bullet_rect.top < 0:
                    self.bullets.remove(bullet)
            self.update_events()


ai = AlienInvasion()
ai.run_game()

在子弹的类中，你初始化了一个 Ship 实例，而在整个游戏的生命周期内，它只会创建一次，因此不会更新与飞船的位置相关的信息。需要从 Ship 类中获取当前的位置并在每次发射子弹时将子弹的位置设置为飞船的位置，而不是在初始化时。在 fire_bullets 方法中，请更改如下代码：

def fire_bullets(self):
    a_bullet = Bullet(self)
    a_bullet.bullet_rect.midtop = self.ship.ship_rect.midtop
    self.bullets.add(a_bullet)