请问pygame关于绘制图像的问题

tree_image1 = pygame.image.load(tree_bush_image).subsurface((0, 0), (115, 128)..convert_alpha()
这句语句有什么问题哦?
报错
tree_image1 = pygame.image.load(tree_bush_image).subsurface((0, 0), (115, 128)..convert_alpha()
pygame.error: No video mode has been set

额,pygame.display.set_mode()没有写在这个语句前头的原因、