UnboundLocalError：赋值前引用的局部变量'lead_x'

Question

def gameLoop():
    gameExit = False

    lead_x_change = display_width/2
    lead_y_change = display_width/2

    lead_x_change = 0
    lead_y_change = 0

    while not gameExit:
        for event in pygame.event.get():
            if event.type == pygame.QUIT:
                gameExit = True
            if event.type == pygame.KEYDOWN:
                if event.key == pygame.K_LEFT:
                    lead_x_change = -block_size
                    lead_y_change = 0
                elif event.key == pygame.K_RIGHT:
                     lead_x_change = block_size
                     lead_y_change = 0
                elif event.key == pygame.K_UP:
                    lead_y_change = -block_size
                    lead_x_change = 0
                elif event.key == pygame.K_DOWN:
                    lead_y_change = block_size
                    lead_x_change = 0

        if lead_x >= display_width or lead_x < 0 or lead_y >= display_height or lead_y < 0 :
           gameExit = True

unboundlocalerror读取：

  

分配前引用的局部变量'lead_x'。

请帮助任何知道如何解决这个问题的人。提前谢谢！