我一直在学习“McGugan - 使用 Python 和 Pygame 开始游戏开发（Apress，2007）”教程，在涉及对象移动的第五章左右的代码中，我不断收到关于代码中使用的“-”的无效语法警报。它不是最新的，但由于其简单性和必要性，我认为在任何更新中都不会更改减法。
这是我的代码：
background_image_filename = 'sushiplate.jpg'
sprite_image_filename = 'fugu.png'
import pygame
from pygame.locals import *
from sys import exit
from gameobjects.vector2 import Vector2
pygame.init()
screen = pygame.display.set_mode((640, 480), 0, 32)
background = pygame.image.load(background_image_filename).convert()
sprite = pygame.image.load(sprite_image_filename).convert_alpha()
clock = pygame.time.Clock()
position = Vector2(100.0, 100.0)
speed = 250.
heading = Vector2()
while True:
for event in pygame.event.get():
if event.type == QUIT:
exit()
if event.type == MOUSEBUTTONDOWN:
destination = Vector2(*event.pos) – Vector2(*sprite.get_size())/2.
heading = Vector2.from_points(position, destination)
heading.normalize()
screen.blit(background, (0,0))
screen.blit(sprite, position)
time_passed = clock.tick()
time_passed_seconds = time_passed / 1000.0
distance_moved = time_passed_seconds * speed
position += heading * distance_moved
pygame.display.update()
我做错了什么还是只是过时了？
非常需要任何帮助。