title: Why does a Kivy property declared in init() generate an error when bind() is used?
body: <p>I'm testing Kivy v1.10.0, and don't understand why the location where I set a Kivy property makes a difference.</p>
  
  <p>This code works:</p>
  
  <pre><code>from kivy.app import App
  from kivy.properties import ListProperty
  from kivy.uix.boxlayout import BoxLayout
  from kivy.uix.widget import Widget
  
  
  class CustomBtn(Widget):
      pressed = ListProperty([0, 0])
  
      def __init__(self, **kwargs):
          super(CustomBtn, self).__init__(**kwargs)
          # self.pressed = ListProperty([0, 0])
  
      def on_touch_down(self, touch):
          if self.collide_point(*touch.pos):
              self.pressed = touch.pos
              return True
          return super(CustomBtn, self).on_touch_down(touch)
  
  
  class RootWidget(BoxLayout):
      def __init__(self, **kwargs):
          super(RootWidget, self).__init__(**kwargs)
          cb = CustomBtn()
          self.add_widget(cb)
          cb.bind(pressed=self.btn_pressed)
  
      def btn_pressed(self, instance, pos):
          print(pos)
  
  
  class MyApp(App):
      def build(self):
          return RootWidget()
  
  
  if __name__ == '__main__':
      MyApp().run()
  </code></pre>
  
  <p>However, if I replace the line currently at class level:</p>
  
  <pre><code>pressed = ListProperty([0, 0])
  </code></pre>
  
  <p>by the equivalent in <code>CustomBtn.__init__()</code>:</p>
  
  <pre><code>self.pressed = ListProperty([0, 0])
  </code></pre>
  
  <p>I get an error in instruction <code>cb.bind(pressed=self.btn_pressed)</code>:</p>
  
  <pre><code>File "kivy\_event.pyx", line 438, in kivy._event.EventDispatcher.bind (kivy\_event.c:6500)
  KeyError: 'pressed'
  </code></pre>
  
  <p>I believe declaring (assigning) an attribute at class level out of any method and doing the same in <code>__init__()</code> were equivalent. Kivy properties are not Python attributes, and maybe the sequence in which objects are built is different and makes a difference for Kivy?</p>