Python入门与实践中，删除消失的子弹为何要遍历copy后的bullets呢？

为什么删除超出屏幕的子弹中，要遍历所有bullets的副本？直接遍历删除超出屏幕元素后的bullets列表不行么？

def _update_bullets(self):
    """Update position of bullets and get rid of old bullets."""
    # Update bullet positions.
    self.bullets.update()

    # Get rid of bullets that have disappeared.
    for bullet in self.bullets.copy():
         if bullet.rect.bottom <= 0:
             self.bullets.remove(bullet)

```