from pygame.sprite import Sprite, RenderUpdates

class DirtySprite(Sprite):
    def __init__(self):
        Sprite.__init__(self)
        self.dirty = True

class RenderDirty(RenderUpdates):
    def draw(self, surface):
        dirty = self.lostsprites
        self.lostsprites = []
        for s, r in self.spritedict.items():
            if not s.dirty:
                continue
            
            s.dirty = False
            newrect = surface.blit(s.image, s.rect)
            if r is 0:
                dirty.append(newrect)
            else:
                if newrect.colliderect(r):
                    dirty.append(newrect.union(r))
                else:
                    dirty.append(newrect)
                    dirty.append(r)
            spritedict[s] = newrect
        return dirty
