kivy官方pong教程:"NoneType"对象没有属性"center" [英] kivy official pong tutorial: 'NoneType' object has no attribute 'center'

问题描述

我正在尝试通过本教程来学习Kivy.我已经将代码粘贴到"main.py"和"main.kv"之后，粘贴到"main.py"和"main.kv"之后.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'

我在做什么错了?

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(减去应用程序)相匹配，则会被自动使用.

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KV文件命名为"main.kv"，该文件与您的应用程序名称"PongApp"不匹配，因此魔术没有发生.您可以使用Builder.load_file()手动加载KV文件.如果您回顾一下该教程，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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