kivy 官方 pong 教程:'NoneType' 对象没有属性

问题描述

我正在尝试通过本教程来学习 Kivy.我已将这是此步骤的完整代码:"之后的代码复制粘贴到 main.py 和 main.kv，如所述.尝试运行时，我得到:

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()

推荐答案

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.

You have the wrong name for your KV file. It should be "pong.kv". If the name of the KV file matches the name of your app (minus App), then it will be automagically used.

您将您的 KV 文件命名为main.kv"，它与您的应用名称PongApp"不匹配，所以魔法没有发生.您可以使用 Builder.load_file() 手动加载 KV 文件.如果您回头看教程，您会发现它要求您将 KV 文件命名为pong.kv".

You named your KV file "main.kv" which did not match with your app name "PongApp", so the magic did not happen. You were able to manually load the KV file by using Builder.load_file(). If you look back at the tutorial, you can see it requires you to name the KV file as "pong.kv".

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