问Pygame外星人入侵 子弹设置？

#主程序
import sys
import pygame
from setting import Settings
from ship import Ship
from bullet import Bullet

class AlienInvasion:
    def __init__(self):
        pygame.init()
        self.settings = Settings()
        self.screen = pygame.display.set_mode(
            (self.settings.screen_width,self.settings.screen_height))
        pygame.display.set_caption("Alien Invasion")

        self.ship = Ship(self)
        self.bullets = pygame.sprite.Group()

        self.open_fire =False

    def _check_events(self):
        for event in pygame.event.get():
            if event.type == pygame.QUIT:
                sys.exit()
            elif event.type == pygame.KEYDOWN:
                self._check_keydown_event(event)
            elif event.type == pygame.KEYUP:
                self._check_keyup_event(event)
    
    def _check_keydown_event(self,event):
        if event.key == pygame.K_RIGHT:
            self.ship.moving_right = True
        elif event.key == pygame.K_LEFT:
            self.ship.moving_left = True
        elif event.key == pygame.K_UP:
            self.ship.moving_up = True
        elif event.key == pygame.K_DOWN:
            self.ship.moving_down = True
        elif event.key == pygame.K_q:
            sys.exit()
        elif event.key == pygame.K_SPACE:
            self.open_fire = True

    def _check_keyup_event(self,event):
        if event.key == pygame.K_RIGHT:
            self.ship.moving_right = False
        elif event.key == pygame.K_LEFT:
            self.ship.moving_left = False
        elif event.key == pygame.K_UP:
            self.ship.moving_up = False
        elif event.key == pygame.K_DOWN:
            self.ship.moving_down = False
        elif event.key == pygame.K_SPACE:
            self.open_fire = False

    def _fire_bullet(self):
        if self.open_fire and len(self.bullets) < self.settings.bullets_allowed:
            new_bullet = Bullet(self)
            self.bullets.add(new_bullet)
    def _update_bullets(self):
        self.bullets.update()

        for bullet in self.bullets.copy():#实时复制bullets中元素，删除的不会遍历
            if bullet.rect.bottom <= 0:
                self.bullets.remove(bullet)


    def _update_screen(self):
        self.screen.fill(self.settings.bg_color)
        self.ship.blitme()
        for bullet in self.bullets.sprites():
            bullet.draw_bullet()

    def run_game(self):
        while True:
            self._check_events()
            self.ship.update()
            self._update_bullets()
            self._update_screen()
            self._fire_bullet()
            pygame.display.flip()

if __name__ == '__main__':
    ai = AlienInvasion()
    ai.run_game()
    
#bullet文件
import pygame
from pygame.sprite import Sprite

class Bullet(Sprite):

    def __init__(self,ai_game):
        super().__init__()

        self.screen = ai_game.screen
        self.settings = ai_game.settings
        self.color = ai_game.settings.bullet_color

        self.rect = pygame.Rect(0,0,
                self.settings.bullet_width,self.settings.bullet_height)
        self.rect.midtop = ai_game.ship.rect.midtop

        self.y = float(self.rect.y)

    def update(self):
        self.y -= self.settings.bullet_speed
        self.rect.y = self.y

    def draw_bullet(self):
        pygame.draw.rect(self.screen,self.color,self.rect)

#ship文件

import pygame

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

        self.settings = ai_game.settings

        self.image = pygame.image.load('images/ship.bmp')
        self.rect = self.image.get_rect()
        self.rect.midbottom = self.screen_rect.midbottom
        

        self.x = float(self.rect.x)
        self.y = float(self.rect.y)

        self.moving_right = False
        self.moving_left = False
        self.moving_up = False
        self.moving_down = False
    
    def update(self):
        if self.moving_right and self.rect.right < self.screen_rect.right:
            self.x += self.settings.ship_speed
        if self.moving_left and self.rect.left > 0:
            self.x -= self.settings.ship_speed
        if self.moving_down and self.rect.bottom < self.settings.screen_height:
            self.y += self.settings.ship_speed
        if self.moving_up and self.rect.y > 0:
            self.y -= self.settings.ship_speed
        
        self.rect.x = self.x
        self.rect.y = self.y

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

#settings文件

class Settings:
    def __init__(self):
        self.screen_width = 1200
        self.screen_height = 500
        self.bg_color = (230,230,230)

        self.ship_speed = 0.5

        self.bullet_speed = 0.5
        self.bullet_width = 3
        self.bullet_height = 15
        self.bullet_color = (60,60,60)
        self.bullets_allowed = 5