pygame 外星人入侵, 出现AttributeError, 但是找不到错误？

Question

《python 编程：从入门到实践》书中例子：外星人入侵,在飞船可以左右移动下，子弹设置好，每次按空格键都会出现： （使用python2.7）
AttributeError: 'Bullet' object has not attribute 'draw_bullet'
检查了实参或形参顺序，但还是找不到。

麻烦各位大佬了！！

代码：

一、alien_invasion.py

import pygame
from pygame.sprite import Group
from settings import Settings
from ship import Ship
import game_functions as gf
def run_game():
    # 初始化游戏并创建屏幕
    pygame.init()
    ai_settings = Settings()
    screen = pygame.display.set_mode(
    (ai_settings.screen_width, ai_settings.screen_height))
    pygame.display.set_caption("Alien Invasion")
    #创建一艘飞船
    ship = Ship(ai_settings, screen)
    #创建一个用于存储子弹的编组
    bullets = Group()
    #开始游戏主循环
    while True:
        gf.check_events(ai_settings, screen, ship, bullets)
        #监视键盘和鼠标事件
        ship.update()
        bullets.update()
        gf.update_screen(ai_settings, screen, ship, bullets)
        
run_game()

二、bullet.py

import pygame
from pygame.sprite import Sprite

class Bullet(Sprite):
    #一个对飞船发射的子弹进行管理的类
    def __init__(self, ai_settings, screen, ship):
        #在飞船所处的位置创建一个子弹对象
        super(Bullet, self).__init__()
        self.screen = screen
        
        #在（0,0）处创建一个表示子弹的矩形，再设置正确的位置
        self.rect = pygame.Rect(0, 0, ai_settings.bullet_width, 
            ai_settings.bullet_height)
        self.rect.centerx = ship.rect.centerx
        self.rect.top = ship.rect.top
        #存储用小数表示的子弹位置
        self.y = float(self.rect.y)
        
        self.color = ai_settings.bullet_color
        self.speed_factor = ai_settings.bullet_speed_factor
def update(self):
    #向上移动子弹
    #更新表示子弹位置的小数值
    self.y -= self.speed_factor
    #更新表示子弹的rect位置
    self.rect.y = self.y
def draw_bullet(self):
    #在屏幕上绘制子弹
    pygame.draw.rect(self.screen, self.color, self.rect)

三、game_functions.py

import sys
import pygame
from bullet import Bullet


def check_keydown_events(event, ai_settings, screen, ship, bullets):
    #响应按键
    if event.key == pygame.K_RIGHT:
        ship.moving_right = True
    elif event.key == pygame.K_LEFT:
        ship.moving_left = True
    elif event.key == pygame.K_SPACE:
        #创建一颗子弹，并将其加入到编组bullets中
        new_bullet = Bullet(ai_settings, screen, ship)
        bullets.add(new_bullet)
        
def check_keyup_events(event, ship):
    #响应松开
    if event.key == pygame.K_RIGHT:
        ship.moving_right = False
    elif event.key == pygame.K_LEFT:
        ship.moving_left = False
    
def check_events(ai_settings, screen, ship, bullets):
    #响应按键和鼠标事件
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            sys.exit()
        elif event.type == pygame.KEYDOWN:
            check_keydown_events(event, ai_settings, screen, ship, bullets)                
        elif event.type == pygame.KEYUP:
            check_keyup_events(event, ship)
def update_screen(ai_settings, screen, ship, bullets):
    #更新屏幕上的图像并切换到新图像
    #每次循环时都重绘屏幕
    screen.fill(ai_settings.bg_color)
    #在飞船和外星人后面重绘所有子弹
    for bullet in bullets.sprites():
        bullet.draw_bullet()
    ship.blitme()
    #让最近绘制的屏幕可见
    pygame.display.flip()

四、settingds.py

class Settings():
    #存储游戏的所有设置的类
    
    def __init__(self):
        #初始化游戏设置
        #屏幕设置
        self.screen_width = 1200
        self.screen_height = 800
        self.bg_color = (255, 255, 255)
        #飞船移动速度控制
        self.ship_speed_factor = 1.5
        #子弹设置
        self.bullet_speed_factor = 1
        self.bullet_width = 3
        self.bullet_height = 15
        self.bullet_color = 60, 60, 60

五、ship.py

import pygame
class Ship():
    def __init__(self, ai_settings, screen):
        #初始化飞船并设置初始位置
        self.screen = screen
        self.ai_settings = ai_settings
        #加载飞船图像并获取外接矩形
        self.image = pygame.image.load('images/ship.bmp')
        self.rect = self.image.get_rect()
        self.screen_rect = screen.get_rect()
        #将每艘飞船放到屏幕底部中央
        self.rect.centerx = self.screen_rect.centerx
        self.rect.bottom = self.screen_rect.bottom
        
        #在飞船的新属性center中存储小数值
        self.center = float(self.rect.centerx)
        #移动标志
        self.moving_right = False
        self.moving_left = False
        
    def update(self):
        #根据移动标志调整飞船位置
        #跟新飞船的center值
        if self.moving_right and self.rect.right < self.screen_rect.right:
            self.center += self.ai_settings.ship_speed_factor
        if self.moving_left and self.rect.left > 0:
            self.center -= self.ai_settings.ship_speed_factor
        #根据self.center更新rect对象
        self.rect.centerx = self.center
    def blitme(self):
        #在指定位置绘制飞船
        self.screen.blit(self.image, self.rect)