python实现飞船游戏的纵向移动

import sys
import pygame

 # 监视键盘和鼠标事件 
def check_keydown_events(event,ship):
 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_UP:
  ship.moving_up=True
 elif event.key==pygame.K_DOWN:
  ship.moving_down=True

def check_keyup_events(event,ship):
 if event.key==pygame.K_RIGHT:
  ship.moving_right=False
 elif event.key==pygame.K_LEFT:
  ship.moving_left=False
 elif event.key==pygame.K_UP:
  ship.moving_up=False
 elif event.key==pygame.K_DOWN:
  ship.moving_down=False
 
def check_events(ship):
 for event in pygame.event.get():
  if event.type==pygame.QUIT:
   sys.exit()
  elif event.type==pygame.KEYDOWN:
   check_keydown_events(event,ship)
  elif event.type==pygame.KEYUP:
   check_keyup_events(event,ship)
 
def update_screen(ai_settings,screen,ship):
 screen.fill(ai_settings.bg_color)
 ship.blitme()

 pygame.display.flip() #让最近回执的屏幕可见（刷新）

import pygame
class Ship():

def __init__(self,ai_settings,screen):
 self.screen=screen
 self.ai_settings=ai_settings

#加载飞船图像
  self.image=pygame.image.load('chengyan_ship.bmp')
  self.rect=self.image.get_rect() 
  self.screen_rect=screen.get_rect()

 self.rect.centerx=self.screen_rect.centerx #x的坐标
 self.rect.centery=self.screen_rect.centery #y的坐标
 
 self.rect.bottom=self.screen_rect.bottom
 
 self.moving_right=False
 self.moving_left=False
 self.moving_up=False
 self.moving_down=False
 
 #得到飞船移动到最下面的值（我不知道有没有表述清楚...就是只能飞到界面的最下面）
 self.screen_top=self.rect.top
 
def update(self):
 #横向移动
 if self.moving_right and self.rect.right<self.screen_rect.right:
  self.rect.centerx+=self.ai_settings.ship_speed_factor
  #纵向移动
 if self.moving_left and self.rect.left>0:
  self.rect.centerx-=self.ai_settings.ship_speed_factor
 if self.moving_up and self.rect.top>0:
  self.rect.centery-=self.ai_settings.ship_speed_factor
 if self.moving_down and self.rect.top<self.screen_top:
  self.rect.centery+=self.ai_settings.ship_speed_factor

 self.rect.centerx=self.rect.centerx
 self.rect.centery=self.rect.centery

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