如何从本地创建全局变量并存储其先前的值？

在我的 pygame 游戏中，我希望子弹能够检测到它何时位于给定的命中框中。为此，我需要从本地创建一个全局变量。但是，每次出现新对象时，全局变量都会更新为新的碰撞盒。这不允许我跟踪之前的碰撞箱并检测子弹何时位于屏幕上的旧对象内。我该如何防止这种情况？我应该如何存储 a 的先前值？

这是我定义碰撞盒和对象的其他功能的类。

class Enemy:
    def __init__(self, y, width, height):
        self.width = width
        self.height = height
        self.vel = 1.5
        self.y = y
        self.x = random.randrange(screen_width - 64 * 2)
        self.index = random.choice(number)
        self.hitboxes = [(self.x + 68, self.y + 68, self.width - 10, self.height - 14),
                         (self.x + 38, self.y + 47, self.width + 20, self.height - 5),
                         (self.x + 18, self.y + 12, self.width + 32, self.height + 30),
                         (self.x + 20, self.y + 32, self.width + 16, self.height + 5),
                         (self.x + 4, self.y + 7, self.width - 24, self.height - 31)]  # hitbox list
        self.hitbox = self.hitboxes[self.index]  # selecting hitbox from list  

    def draw(self, win):
        win.blit(asteroids[self.index], (self.x, self.y))
        pygame.draw.rect(win, (255, 0, 0), self.hitbox, 2)

这里是问题所在的主循环（阅读代码中的注释）

asteroids = [pygame.image.load('rock0.png'), pygame.image.load('rock1.png'), pygame.image.load('rock2.png'),
             pygame.image.load('rock3.png'), pygame.image.load('rock4.png')]

number = [0, 1, 2, 3, 4]

asteroids_on_screen = []

rock = Enemy(-140, 64, 64)

run = True
clock = pygame.time.Clock()
while run:
    last = pygame.time.get_ticks()
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            run = False
        elif event.type == my_event_id:
            x = random.randrange(screen_width - 64 * 2)
            index = random.choice(number)
            asteroids_on_screen.append(Enemy(rock.y, rock.width, rock.height))
    global a  # if I define a as a global here I will be able to detect
              # when the bullet is within the hitbox of the 
              # newest added object, since a gets updated for each 
              # object that enters the screen, but not the other ones.
    for a in asteroids_on_screen:
        if -141 < a.y < 500:
            a.y += a.vel
            a.hitbox = (a.hitbox[0], a.hitbox[1] + a.vel, a.hitbox[2], a.hitbox[3])
        else:
            asteroids_on_screen.pop(asteroids_on_screen.index(a))

    for bullet in bullets:
        if bullet.x + bullet.width < a.hitbox[0] + a.hitbox[2] and bullet.x - bullet.width > a.hitbox[0]:
            if bullet.y - bullet.height < a.hitbox[1] + a.hitbox[3] and bullet.y + bullet.height > a.hitbox[1]:
                rock.hit()
                bullets.pop(bullets.index(bullet))
        if 0 < bullet.y < 500:
            bullet.y -= bullet.vel
        else:
            bullets.pop(bullets.index(bullet))

只需使用嵌套循环即可。你必须针对每颗小行星检查每颗子弹：

# move the asteroids
for a in asteroids_on_screen:
    if -141 < a.y < 500:
        a.y += a.vel
        a.hitbox = (a.hitbox[0], a.hitbox[1] + a.vel, a.hitbox[2], a.hitbox[3])
    else:
        asteroids_on_screen.pop(asteroids_on_screen.index(a))

# hit test for each combination of asteroid and bullet
for a in asteroids_on_screen:
    for bullet in bullets:
        if bullet.x + bullet.width < a.hitbox[0] + a.hitbox[2] and bullet.x - bullet.width > a.hitbox[0]:
            if bullet.y - bullet.height < a.hitbox[1] + a.hitbox[3] and bullet.y + bullet.height > a.hitbox[1]:
                rock.hit()
                bullets.pop(bullets.index(bullet))

# move the remaining bullets
for bullet in bullets:
    if 0 < bullet.y < 500:
        bullet.y -= bullet.vel
    else:
        bullets.pop(bullets.index(bullet))