~cpp 
class Background(pygame.Surface):
    def __init__(self, aSize):
        pygame.Surface.__init__(self, aSize)
    def clearSurface(self):
        self.fill((0,0,0))

def createBlackScreen(aSize):
    back = Background(aSize)
    back.clearSurface()           <-- 여기서 Attribute 에러
    return back