AttributeError: не видит существующий атрибут

Пишу учебный проект на pygame. По кнгие Э.Метиз "Изучаем Python".
Столкнулся с такой ошибкой: AttributeError: 'pygame.Surface' object has no attribute 'ship_speed'
Хотя атрибут определён в файле с настройками, не могу понять в чём проблема.
game.py

import pygame
from settings import Settings
from ship import Ship
from bullet import Bullet

class AlienInvasion():
    """A class for managing resources and game behavior"""

    def __init__(self):
        """Inizialization game and create games resources"""
        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.screen)
        self.bullets = pygame.sprite.Group()    

    def run_game(self):
        """Launch main cycle game"""
        while True:
            self._check_events()
            self.ship.update()
            self.bullets.update()
            self._update_screen()

    def _check_events(self):
        """keyboard and mouse"""
        for event in pygame.event.get():
            if event.type == pygame.QUIT:
                sys.exit()
            elif event.type == pygame.KEYDOWN:
                self._check_keydown_events(event) 
            elif event.type == pygame.KEYUP:
                self._check_keyup_events(event)


    def _check_keydown_events(self, event):
        """reacts to keydown"""
        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_F10:
            sys.exit()
        elif event.key == pygame.K_SPACE:
            self._fire_bullet()

    def  _check_keyup_events(self, event):
        """reacts to keyup."""           
        if event.key == pygame.K_RIGHT:
            self.ship.moving_right = False
        elif event.key == pygame.K_LEFT:
            self.ship.moving_left = False

    def _fire_bullet(self):
        """Create new bullet and including it in the bullets group."""
        new_bullet = Bullet(self)
        self.bullets.add(new_bullet)

    def _update_screen(self):
        """Refresh the screen and display a new screen."""
        self.screen.fill(self.settings.bg_color)
        self.ship.blitme()
        
        for bullet in self.bullets.sprites():
            bullet.draw_bullet()

        pygame.display.flip()


if __name__ == '__main__':
    # Create exemple and launch game.
    ai = AlienInvasion()
    ai.run_game()

ship.py

import pygame

class Ship():
    """Class for control ship"""
    def __init__(self, ai_game):
        """initialization ship and sets its initial position."""
        self.screen = ai_game
        self.settings = ai_game
        self.screen_rect = ai_game.get_rect()

        # Load image ship.
        self.image = pygame.image.load('images/ship.bmp')
        self.rect = self.image.get_rect()

        # Each new ship appears at the bottom of the screen.
        self.rect.midbottom = self.screen_rect.midbottom

        # Saving the real coordinate of the center of the ship.
        self.x = float(self.rect.x)

        # Move flag
        self.moving_right = False
        self.moving_left = False

    def update(self):
        """Updates the ship's position with the flag."""
        # Updatees atribut x, not rect.
        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

        # Update atribut rect based on self.x.
        self.rect.x = self.x

    def blitme(self):
        """Draw ship in current position."""
        self.screen.blit(self.image, self.rect)

settings.py

class Settings():
    """A class for save all settings game Alien Invasion"""
    
    def __init__(self):
        """Inicialization settings game"""
        # Display settings
        self.screen_width = 1200
        self.screen_height = 800
        self.bg_color = (230, 230, 230)

        # Ship settings
        self.ship_speed = 1.5

        # Bullet settings
        self.bullet_speed = 1
        self.bullet_width = 3
        self.bullet_height = 15
        self.bullet_color = (60, 60, 60)

Полный текст отчёта об ошибке:

pygame 1.9.6
Hello from the pygame community. https://www.pygame.org/contribute.html
Traceback (most recent call last):
  File "C:\Users\Gradysnik\Desktop\python_work\alien_invasion\game.py", line 79, in <module>
    ai.run_game()
  File "C:\Users\Gradysnik\Desktop\python_work\alien_invasion\game.py", line 27, in run_game
    self.ship.update()
  File "C:\Users\Gradysnik\Desktop\python_work\alien_invasion\ship.py", line 29, in update
    self.x += self.settings.ship_speed
AttributeError: 'pygame.Surface' object has no attribute 'ship_speed'
[Finished in 2.2s with exit code 1]