敌机左右移动不出边界

import pygame  # pygame2d游戏
import time
from pygame.locals import *

class HeroPlane(object):
    def __init__(self,screen_temp):
        self.x=210
        self.y=700
        self.screen=screen_temp
        self.image= pygame.image.load("./feiji/hero1.png")
        self.bullet_list=[]
    def display(self):
        self.screen.blit(self.image, (self.x, self.y))
        for bullet in self.bullet_list:
            bullet.display()
            bullet.move()
    def move_left(self):
        self.x-=5
    def move_right(self):
        self.x+=5
    def fire(self):
        self.bullet_list.append(Bullet(self.screen,self.x,self.y))

class EnemyPlane(object):
    def __init__(self,screen_temp):
        self.x=0
        self.y=0
        self.screen=screen_temp#显示敌机
        self.image= pygame.image.load("./feiji/enemy0.png")
        self.bullet_list=[]
        self.direction="right"#飞机默认的显示方向
    def display(self):
        self.screen.blit(self.image, (self.x, self.y))
        for bullet in self.bullet_list:
            bullet.display()
            bullet.move()
    def move(self):
        if self.direction=="right":#控制方向
            self.x+=5
        elif self.direction=="left":
            self.x-=5

        if self.x>480-50:#测试距离
            self.direction="left"
        elif self.x<0:#往右
            self.direction="right"
    def fire(self):
        self.bullet_list.append(Bullet(self.screen,self.x,self.y))


class Bullet(object):
    def __init__(self,screen_temp,x,y):
        self.x = x+40
        self.y = y-20
        self.screen = screen_temp
        self.image = pygame.image.load("./feiji/bullet.png")
    def display(self):
        self.screen.blit(self.image,(self.x,self.y))
    def move(self):
        self.y-=20


def key_control(herotemp):
    for event in pygame.event.get():  # 11
        if event.type == QUIT:  # 按下叉号
            print("exit")
            exit()  # 退出程序，循环结束是break
        elif event.type == KEYDOWN:  # 按下键盘键

            if event.key == K_a or event.key == K_LEFT:
                print("left")
                herotemp.move_left()
            elif event.key == K_d or event.key == K_RIGHT:
                print("right")
                herotemp.move_right()
            elif event.key == K_SPACE:
                print("space")
                herotemp.fire()
def main():  # 01
    screen = pygame.display.set_mode((480, 852), 0, 32)  # 1窗口
    background = pygame.image.load("./feiji/background.png")  # 2背景
    hero = HeroPlane(screen)
    enemy=EnemyPlane(screen)


    while True:  # 5死循环，一直重复
        screen.blit(background, (0, 0))  # 3背景贴到窗口00处
        hero.display()
        enemy.display()
        enemy.move()#调用敌机方法
        pygame.display.update()  # 4显示贴的效果
        key_control(hero)
        time.sleep(0.01)  # 6电脑配置不高的话，内存占用会很大，所以延时显示可以减少内存消耗

if __name__ == '__main__':  # 0
    main()