如何在 pygame 中添加敌人？

我正在用 pygame 制作一个简单的游戏，目标是躲避来袭的敌人。我想要一个领带战士的形象随机下降，xwing 必须躲避他们，否则你会死。我如何在我的脚本中实现随机平局战士？在我的代码中，我收到一个错误，指出 x 未定义。

import pygame
import time
import random
pygame.init()

display_width = 1280
display_height= 800


pygame.display.set_mode((display_width, display_height), pygame.FULLSCREEN)

gameDisplay= pygame.display.set_mode((display_width,display_height))
pygame.display.set_caption('X-Wing Game')
clock= pygame.time.Clock()

black= (0,0,0)
white= (255,255,255)
red = (255,0,0)
blue_violet = (138,43,226)


xwing_width = 65
xwing_height = 130

tie_width = 80
tie_height =64

#images
xwingImg = pygame.image.load('X-Wing.bmp')
tieImg= pygame.image.load('tiefighter.png')

def things(thingx, thingy, thingw, thingh, color):
    pygame.draw.rect(gameDisplay, color, [thingx, thingy, thingw, thingh])
    gameDisplay.blit(tieImg,(x,y))

def tieImg (x,y):
    pygame.image.load('tieImg')


def xwing (x,y):
    gameDisplay.blit (xwingImg,(x,y))

def text_objects(text, font):
    textSurface = font.render(text, True, red)
    return textSurface, textSurface.get_rect()

def crash ():
    message_display ('Ouch, You Crashed!')

def message_display(text):
    largeText = pygame.font.Font('freesansbold.ttf',25)
    TextSurf, TextRect = text_objects(text, largeText)
    TextRect.center = ((display_width/2), (display_height/2))
    gameDisplay.blit(TextSurf, TextRect)

    pygame.display.update()

    time.sleep(1.3)

    game_loop()


def game_loop():

    x = (display_width * 0.45)
    y = (display_height * .75)

    x_change = 0
    y_change = 0

    thing_startx = random.randrange(0, display_width)
    thing_starty = -600
    thing_speed = 7
    thing_width = 81
    thing_height =65


    gameEXIT = False

    while not gameEXIT:

        for event in pygame.event.get() :
            if event.type == pygame.QUIT:
                pygame.quit()
                quit()

            if event.type == pygame.KEYDOWN:
                if event.key == pygame.K_LEFT:
                    x_change = -5
                if event.key == pygame.K_UP:
                     y_change= -5
                if event.key == pygame.K_DOWN:
                    y_change =5
                elif event.key == pygame.K_RIGHT:
                    x_change = 5


            if event.type == pygame.KEYUP:
                if event.key == pygame.K_LEFT or event.key == pygame.K_RIGHT:
                    x_change=0



        x += x_change
        y += y_change

        gameDisplay.fill(black)

        things(thing_startx, thing_starty, thing_width, thing_height, black )
        thing_starty += thing_speed
        xwing (x,y)

        if x > display_width - xwing_width or x < 0:
             x_change=0
        if y>display_height-xwing_height or y<0:
            y_change=0


        pygame.display.update()
        clock.tick(60)


game_loop()
pygame.quit()
quit()