新人请教

import sys
import pygame

from setting import Setting
from ship import Ship

class Game:
    def __init__(self):
        self.setting = Setting(self)
        self.screen = pygame.display.set_mode((1200, 800))
        self.screen_rect = self.screen.get_rect()
        pygame.display.set_caption("外星人入侵")
        self.ship = Ship(self)

        self.clock=pygame.time.Clock()


    def run_game(self):
        while True:
            for event in pygame.event.get():
                if event.type == pygame.QUIT:
                    pygame.quit()
                    sys.exit()
                if event.type == pygame.KEYDOWN:
                    self.check_keydown(event)
                if event.type == pygame.KEYUP:
                    self.check_keyup(event)
            self.ship.update()
            self.screen.fill(self.setting.bg_color)
            self.ship.blitme()
            pygame.display.flip()
            self.clock.tick(60)

    def check_keydown(self,event):
        if event.key == pygame.K_LEFT:
            self.ship.moving_left = True
        if event.key == pygame.K_RIGHT:
            self.ship.moving_right = True
        if event.key == pygame.K_UP:
            self.ship.moving_up = True
        if event.key == pygame.K_DOWN:
            self.ship.moving_down = True
        if event.key == pygame.K_q:
            sys.exit()

    def check_keyup(self,event):
        if event.key == pygame.K_LEFT:
            self.ship.moving_left = False
        if event.key == pygame.K_RIGHT:
            self.ship.moving_right = False
        if event.key == pygame.K_UP:
            self.ship.moving_up = False
        if event.key == pygame.K_DOWN:
            self.ship.moving_down = False



ai=Game()
ai.run_game()

import pygame
class Ship:
    def __init__(self,ai):
        self.ai = ai
        self.setting=ai.setting
        self.screen = ai.screen
        self.screen_rect = ai.screen.get_rect()
        self.image=pygame.image.load('images/3.png')
        self.rect = self.image.get_rect()
        self.rect.midbottom=self.screen_rect.midbottom
        self.moving_right=False
        self.moving_left=False
        self.moving_up=False
        self.moving_down=False
        self.x=float(self.rect.x)
        self.y=float(self.rect.y)
    def update(self):
        if self.moving_right and self.rect.right < self.screen_rect.right:
            self.x+=self.setting.ship_speed
        if self.moving_left and self.rect.left>0:
            self.x-=self.setting.ship_speed
        if self.moving_up and self.rect.top>0:
            self.y-=self.setting.ship_speed
        if self.moving_down and self.rect.bottom<self.screen_rect.bottom:
            self.y+=self.setting.ship_speed
        self.rect.x=self.x
        self.rect.y=self.y

    def blitme(self):
        self.screen.blit(self.image,self.rect)
```新手提问，这是我写的代码，为什么飞机运动时会有部分会飞出屏幕？请各位大神指教