python - 不鼓励使用 kivy 动态添加属性——为什么？

Question

kivy.event.EventDispatcher 类有一个名为 apply_property() 的方法它允许在运行时向类添加属性。 文档包含警告:

This method is not recommended for common usage because you should declare the properties in your class instead of using this method.

我不清楚为什么不鼓励这种用法，而首选在类中声明。谁能赐教一下？

Answer 1

apply_property() 用于将属性添加到单个类实例，但也有污染其他实例属性的副作用。考虑以下代码:

#!/usr/bin/kivy
import kivy
kivy.require('1.9.1')

from kivy.app import App
from kivy.lang import Builder
from kivy.properties import NumericProperty
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.button import Button

Builder.load_string('''
<MyButton>:
    on_press: print('np' in self.properties())
''')

class MyButton(Button):
    pass

class MainApp(App):
    def build(self):
        mb1 = MyButton(text="mb1")
        mb2 = MyButton(text="mb2")
        mb1.apply_property(np=NumericProperty(100))

        layout = BoxLayout()
        layout.add_widget(mb1)
        layout.add_widget(mb2)
        return layout

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

如您所见，即使我在 mb1 实例上使用了 apply_property，mb2 报告它也有这样的属性。这是因为属性被添加到类中，而不是实例中。但是，只有 mb1 具有实际值:

#!/usr/bin/kivy
import kivy
kivy.require('1.9.1')

from kivy.app import App
from kivy.lang import Builder
from kivy.properties import NumericProperty
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.button import Button

Builder.load_string('''
<MyButton>:
    on_press: print(self.np)
''')

class MyButton(Button):
    pass

class MainApp(App):
    def build(self):
        mb1 = MyButton(text="mb1")
        mb2 = MyButton(text="mb2")
        mb1.apply_property(np=NumericProperty(100))

        layout = BoxLayout()
        layout.add_widget(mb1)
        layout.add_widget(mb2)
        return layout

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

尝试从 mb2 实例获取 np 属性的值会产生一个 AttributeError。一个安全的选择:

#!/usr/bin/kivy
import kivy
kivy.require('1.9.1')

from kivy.app import App
from kivy.lang import Builder
from kivy.properties import NumericProperty
from kivy.uix.boxlayout import BoxLayout
from kivy.uix.button import Button

Builder.load_string('''
<MyButton>:
    on_press: print(self.np)
''')

class MyButton(Button):
    np=NumericProperty(100)

class MainApp(App):
    def build(self):
        mb1 = MyButton(text="mb1")
        mb2 = MyButton(text="mb2")

        layout = BoxLayout()
        layout.add_widget(mb1)
        layout.add_widget(mb2)
        return layout

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

来源:this issue .