大大解释一下为什么图像不能上下移动

import sys

import pygame

from settings import Settings
from ship import Ship

class AlienInvasion:
    """管理游戏资源和行为的类"""

    def __init__(self):
        """"初始化游戏并创建游戏资源"""
        pygame.init()
        self.setting = Settings()

        self.screen = pygame.display.set_mode(
            (0,0),pygame.FULLSCREEN)
        self.setting.screen_width = self.screen.get_rect().width
        self.setting.screen_height = self.screen.get_rect().height
        pygame.display.set_caption("外星人入侵")

        self.ship = Ship(self)

    def run_game(self):
        """开始游戏主循环"""
        while True:
            self._check_events()
            self._update_screen()
            self.ship.update()
    def _check_events(self):
        """" 监视键盘和鼠标事件 """
        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):
        """"响应按键"""
        if event.key == pygame.K_d:
            self.ship.moving_right = True
        elif event.key == pygame.K_a:
            self.ship.moving_left = True
        elif event.key == pygame.K_w:
            self.moving_up = True
        elif event.key == pygame.K_s:
            self.moving_down = True
        elif event.key == pygame.K_q:
            sys.exit()


    def _check_keyup_events(self,event):
        """响应松开"""
        if event.key == pygame.K_d:
            self.ship.moving_right = False
        elif event.key == pygame.K_a:
            self.ship.moving_left = False
        elif event.key == pygame.K_w:
            self.moving_up = False
        elif event.key == pygame.K_s:
            self.moving_down = False
    def _update_screen(self):
        """ 更新屏幕上的图像，并切换到新屏幕 """
        self.screen.fill(self.setting.bg_color)
        self.ship.blitme()
        pygame.display.flip()

if __name__ == '__main__':
    # 创建游戏实例并运行游戏
    ai = AlienInvasion()
    ai.run_game()

class Settings:
    """储存游戏中所有设置的类"""

    def __init__(self):
        """初始化游戏的设置"""
        # 屏幕设置
        self.screen_width = 1200
        self.screen_height = 800
        self.bg_color = (230,230,230)

        # 飞船设置
        self.ship_speed = 1.5

import pygame


class Ship:
    """A class to manage the ship."""

    def __init__(self, ai_game):
        """Initialize the ship and set its starting position."""
        self.screen = ai_game.screen
        self.settings = ai_game.setting
        self.screen_rect = ai_game.screen.get_rect()

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

        # Start each new ship at the bottom center of the screen.
        self.rect.midbottom = self.screen_rect.center

        # 在飞船的属性X中储存小数值
        self.x = float(self.rect.x)
        self.y = float(self.rect.y)

        # 移动标志
        self.moving_right = False
        self.moving_left = False
        self.moving_up = False
        self.moving_down = False

    def update(self):
        """根据移动标志调整飞船的位置"""
        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 > self.screen_rect.left:
            self.x -= self.settings.ship_speed
        if self.moving_up and self.rect.top > 0:
            self.y -= self.settings.ship_speed
        if self.moving_down and self.rect.bottom > self.screen_rect.top:
            self.y += self.settings.ship_speed

        # 根据self.x更新self.rect.x
        self.rect.x = self.x
        self.rect.y = self.y


    def blitme(self):
        """Draw the ship at its current location."""
        self.screen.blit(self.image, self.rect)

测试了一下，左右能动，上下不行。

""""响应按键"""
        if event.key == pygame.K_d:
            self.ship.moving_right = True
        elif event.key == pygame.K_a:
            self.ship.moving_left = True
        elif event.key == pygame.K_w:
            self.ship.moving_up = True
        elif event.key == pygame.K_s:
            self.ship.moving_down = True
        elif event.key == pygame.K_q:
            sys.exit()

这儿少了.ship