kivy 官方 pong 教程:'NoneType' 对象没有属性 'center'

I'm trying to learn Kivy with this tutorial. I've copy pasted the code after "Here is the entire code for this step:" to main.py and main.kv as described. When trying to run, I'm getting:

Traceback (most recent call last):
  File "main.py", line 47, in <module>
   PongApp().run()
  File "/home/kivy/code/kivy/kivy/app.py", line 527, in run
   root = self.build()
  File "main.py", line 41, in build
   game.serve_ball()
  File "main.py", line 23, in serve_ball
   self.ball.center = self.center
AttributeError: 'NoneType' object has no attribute 'center'

我做错了什么?

main.kv:

#:kivy 1.0.9

<PongBall>:
    size: 50, 50 
    canvas:
        Ellipse:
            pos: self.pos
            size: self.size          

<PongGame>:
    ball: pong_ball

    canvas:
        Rectangle:
            pos: self.center_x-5, 0
            size: 10, self.height

    Label:
        font_size: 70  
        center_x: root.width / 4
        top: root.top - 50
        text: "0"

    Label:
        font_size: 70  
        center_x: root.width * 3 / 4
        top: root.top - 50
        text: "0"

    PongBall:
        id: pong_ball
        center: self.parent.center

main.py:

from kivy.app import App
from kivy.uix.widget import Widget
from kivy.properties import NumericProperty, ReferenceListProperty,
    ObjectProperty
from kivy.vector import Vector
from kivy.clock import Clock
from random import randint


class PongBall(Widget):
    velocity_x = NumericProperty(0)
    velocity_y = NumericProperty(0)
    velocity = ReferenceListProperty(velocity_x, velocity_y)

    def move(self):
        self.pos = Vector(*self.velocity) + self.pos


class PongGame(Widget):
    ball = ObjectProperty(None)

    def serve_ball(self):
        self.ball.center = self.center
        self.ball.velocity = Vector(4, 0).rotate(randint(0, 360))

    def update(self, dt):
        self.ball.move()

        #bounce off top and bottom
        if (self.ball.y < 0) or (self.ball.top > self.height):
            self.ball.velocity_y *= -1

        #bounce off left and right
        if (self.ball.x < 0) or (self.ball.right > self.width):
            self.ball.velocity_x *= -1


class PongApp(App):
    def build(self):
        game = PongGame()
        game.serve_ball()
        Clock.schedule_interval(game.update, 1.0 / 60.0)
        return game


if __name__ == '__main__':
    PongApp().run()

解决方案

您的 KV 文件名称错误.它应该是pong.kv".如果 KV 文件的名称与您的应用程序的名称匹配(减去 App)，则会自动使用它.