问用python做一个飞机大战，敌机能发射子弹，做出来了，但是敌机发射不了子弹，不知道哪错了？

import pygame
import random
CREATE_ENEMY_EVENT = pygame.USEREVENT
HERO_FIRE_EVENT = pygame.USEREVENT + 1
ENEMY_FIRE_EVENT = pygame.USEREVENT + 2
class BaseSprite(pygame.sprite.Sprite):
    def __init__(self,image,speed=0):
        super().__init__()
        self.image = pygame.image.load(image)
        self.rect = self.image.get_rect()
        self.speed = speed

    def update(self):
        self.rect.y -= self.speed
class BackGround(BaseSprite):
    def __init__(self,image,isUP=True,speed=1):
        super().__init__(image,speed)
        if isUP:
            self.rect.y = -self.rect.height
            self.rect.x = 0

        else:
            self.rect.y = 0
            self.rect.x = 0
    def update(self):
        self.rect.y += self.speed
        if self.rect.y > self.rect.height:
            self.rect.y = -self.rect.height

class Enemy(BaseSprite):
    def __init__(self,image,speed=1):
        super().__init__(image,speed)
        self.rect.y = -self.rect.height
        self.speed = random.randint(2,4)
        self.rect.x = random.randint(0,423)
        self.bullet_group = pygame.sprite.Group()
        pass
    def update(self):
        self.rect.y += self.speed
        if self.rect.y > 700:
            self.kill()

    def fire(self):
        # print("敌机发射子弹")
        bullet2 = Bullet('./images/ebullet.png')
        bullet2.rect.centerx = self.rect.centerx
        bullet2.rect.y = self.rect.y + 35
        bullet2.speed = self.speed -1
        self.bullet_group.add(bullet2)
        pass
class Hero(BaseSprite):
    def __init__(self,image,speed=0):
        super().__init__(image,speed)
        self.rect.centerx = 240
        self.rect.y = 700-120-126
        self.bullet_group = pygame.sprite.Group()
    def update(self):
        self.rect.x += self.speed
        if self.rect.x < 0:
            self.rect.x = 0
        elif self.rect.x > 480-102:
            self.rect.x = 480-102
    def fire(self):
        #print('英雄发射了两颗子弹')
        bullet = Bullet('./images/hbullet.png')
        bullet.rect.centerx = self.rect.centerx
        bullet.rect.y = self.rect.y - 35
        bullet1 = Bullet('./images/hbullet.png')
        bullet1.rect.centerx = self.rect.centerx
        bullet1.rect.y = self.rect.y - 45
        self.bullet_group.add(bullet,bullet1)

        pass
class Bullet(BaseSprite):
    def __init__(self,image,speed=1):
        super().__init__(image,speed)
        pass
    def update(self):
        self.rect.y -= self.speed
        if self.rect.y < - self.rect.height:
            self.kill()

class MainGame():

    def create_sprites(self):
        bg1 = BackGround('./images/bg.png')
        bg2 = BackGround('./images/bg.png',isUP=False)
        self.backgroup = pygame.sprite.Group(bg1,bg2)
        self.hero = Hero('./images/hero.png')
        pygame.time.set_timer(HERO_FIRE_EVENT,500)
        self.herogroup = pygame.sprite.Group(self.hero)
        self.enemy = Enemy('./images/enemy.png')
        pygame.time.set_timer(ENEMY_FIRE_EVENT,3000)
        pygame.time.set_timer(self.CREATE_ENEMY_EVENT, 1000)
        self.enemygroup = pygame.sprite.Group()

    def __init__(self):
        screen_rect = pygame.Rect(0,0,480,700)
        self.screen = pygame.display.set_mode((screen_rect.width,screen_rect.height),0,32)
        self.clock = pygame.time.Clock()
        self.CREATE_ENEMY_EVENT = pygame.USEREVENT
        self.create_sprites()

        print('游戏初始化完成')
    def event_handler(self):
        for event in pygame.event.get():
            if event.type == pygame.QUIT:
                pygame.quit()
                exit()
            elif event.type == self.CREATE_ENEMY_EVENT:
                #print('创建敌机')
                enemy = Enemy('./images/enemy.png')
                self.enemygroup.add(enemy)
            if event.type == pygame.KEYDOWN:
                if event.key == pygame.K_LEFT:
                    self.hero.speed = -3
                elif event.key == pygame.K_RIGHT:
                    self.hero.speed = 3
            else:
                self.hero.speed = 0
            if event.type == HERO_FIRE_EVENT:
                self.hero.fire()
            else:
                self.enemy.speed = 0
            if event.type == ENEMY_FIRE_EVENT:
                self.enemy.fire()
    def check_collide(self):
        pygame.sprite.groupcollide(self.hero.bullet_group,self.enemygroup,True,True)
        pygame.sprite.groupcollide(self.hero.bullet_group, self.enemy.bullet_group, True, True)
        enemy_count = pygame.sprite.spritecollide(self.hero,self.enemygroup,True)
        for event in self.enemy.bullet_group:
            bullet_count = pygame.sprite.spritecollide(self.hero,self.enemy.bullet_group,True)
            if len(bullet_count) > 0:
                self.hero.kill()
                pygame.quit()
                exit()
        if len(enemy_count)>0:
            self.hero.kill()
            pygame.quit()
            exit()

        pass
    def update_draw_sprites(self):
       for group in [self.backgroup,self.enemygroup,self.enemy.bullet_group,self.herogroup,self.hero.bullet_group]:
           group.update()
           group.draw(self.screen)
    def start_game(self):
        while True:
            self.clock.tick(60)
            self.event_handler()
            self.check_collide()
            self.update_draw_sprites()
            pygame.display.update()


if __name__=='__main__':
    game = MainGame()
    game.start_game()