python - 在 Python 中访问其他类的函数

Question

我是 Python 的新手，很抱歉这个明显的问题。我希望 Computer_paddle 类能够调用 Ball 的函数 y_position()，并获取返回值。但它似乎并没有这样做，告诉我:

"global name 'ball' is not defined". 

要在另一个函数中调用一个函数，我需要做些什么特别的事情吗？

class Ball(games.Sprite):
    """ A ball that bounces off walls and paddles. """
    image = games.load_image("ball.png")

    def __init__(self, game, x, y):
        """ Initialise ball sprite. """
        super(Ball, self).__init__(image = Ball.image,
                                   x = x, y = y,
                                   dx = -3, dy = 0)

    def update(self):
        """Check if ball has hit paddle or wall, and then bounce that ball. """
        super(Ball, self).update()

        # check if ball overlaps paddles
        if self.overlapping_sprites:
            self.dx = -self.dx
            self.dy += random.randint(-1, 1)

        # check if ball hits a wall
        if self.top < 0 or self.bottom > games.screen.height:
            self.dy = -self.dy
        if self.left < 0 or self.right > games.screen.width:
            self.dx = -self.dx

    def y_position(self):
        return self.y

class Paddle(games.Sprite):
    """ A paddle that can only partly leave the screen. """
    image = games.load_image("paddle.png")

    def __init__(self, game, x, y):
        """ Initialise paddle sprite."""
        super(Paddle, self).__init__(image = Paddle.image, x = x, y = y)

    def update(self):
        """ Prevent sprite from completely leaving the screen. """
        if self.top < -33:
            self.top = -33

        if self.bottom > games.screen.height + 33:
            self.bottom = games.screen.height + 33

class Human_paddle(Paddle):
    """ A paddle controlled by the player. """

    def update(self):
        """ Move paddle to mouse position. """
        super(Human_paddle, self).update()

        self.y = games.mouse.y

class Computer_paddle(Paddle):
    """ A paddle controlled by the computer. """
    MAX_SPEED = 1

    def update(self):
        """ Move paddle towards ball's position on Y-axis. """
        super(Computer_paddle, self).update()

        ball_y = ball.y_position()

        if ball_y > self.y:
            self.y += MAX_SPEED
        if ball_y < self.y:
            self.y -= MAX_SPEED

Answer 1

不，但是您需要有一个对象的引用才能访问它的方法。因为您从不将 ball 绑定(bind)到任何东西，所以没有对象可以调用方法。您是想在全局级别创建 ball 作为 Ball 的实例吗？