如何修复TypeError：必须使用Dropper实例作为第一个参数来调用未绑定方法

我正在写一个游戏，其中一块岩石掉下来，您用鼠标使厨师不被压碎。当岩石从屏幕上掉落时，又有两个岩石产生并掉落。这一直持续到角色被弄脏为止。

但是我得到了错误：

TypeError: unbound method additonal_drop() must be called with Dropper instance as first argument (got nothing instead)

我不确定应该放入什么()。有人可以向我解释这个错误吗？

另外，如何使Dropper子画面不可见？

这是我的代码：

from livewires import games, color
import random

games.init(screen_width = 640, screen_height = 480, fps = 50)


class Chef(games.Sprite):

    image = games.load_image("chef.bmp")

    def __init__(self):
        super(Chef, self).__init__(image = Chef.image,
                                  x = games.mouse.x,
                                  bottom = games.screen.height)
    def update(self):
        """ Move to mouse x position. """
        self.x = games.mouse.x

        if self.left < 0:
            self.left = 0

        if self.right > games.screen.width:
            self.right = games.screen.width

        self.check()

    def check(self):
        """ Check if hit by rocks. """
        for rock in self.overlapping_sprites:
            rock.end_game()

class Rock(games.Sprite):
    """
    A rock which falls to the ground.
    """ 
    image = games.load_image("rock.bmp")
    speed = 1

    def __init__(self, x = 320, y = 90):
        """ Initialize a rock object. """
        super(Rock, self).__init__(image = Rock.image,
                                    x = x, y = y,
                                    dy = Rock.speed)

    def end_game(self):
        """ End the game. """
        end_message = games.Message(value = "Game Over",
                                    size = 90,
                                    color = color.red,
                                    x = games.screen.width/2,
                                    y = games.screen.height/2,
                                    lifetime = 2 * games.screen.fps,
                                    after_death = games.screen.quit)
        games.screen.add(end_message)

    def update(self):
        """ Check if bottom edge has reached screen bottom. """
        if self.bottom > games.screen.height:
            self.destroy()
            Dropper.additonal_drop()


class Dropper(games.Sprite):
    """
    A invisible sprite that drops the rocks.
    """
    image = games.load_image("rock.bmp")
    def __init__(self, y = 55, speed = 2, odds_change = 200):
        """ Initialize the dropper object. """
        super(Dropper, self).__init__(image = Dropper.image,
                                      x = games.screen.width / 2, y = y,
                                      dx = speed)

        self.odds_change = odds_change
        self.time_til_drop = 0


    def update(self):
        """ Determine if direction needs to be reversed. """
        if self.left < 0 or self.right > games.screen.width:
            self.dx = -self.dx
        elif random.randrange(self.odds_change) == 0:
           self.dx = -self.dx

    def additonal_drop(self):
        new_rock = Rock(x = self.x)
        games.screen.add(new_rock)

        new_rock = Rock(x = self.x)
        games.screen.add(new_rock)


def main():
    """ Play the game. """
    wall_image = games.load_image("wall.jpg", transparent = False)
    games.screen.background = wall_image

    the_chef = Chef()
    games.screen.add(the_chef)

    the_rock = Rock()
    games.screen.add(the_rock)

    the_dropper = Dropper()
    games.screen.add(the_dropper)

    games.mouse.is_visible = False

    games.screen.event_grab = True
    games.screen.mainloop()

# start it up!
main()