title: Kivy ObjectProperty to update label text
body: <p>I am working on creating a kivy user interface to display the values generated by a data model I've written as a standard python object.  In essence, I would like the user to be able to press a button, which would change the underlying data model and the results of this change would be automatically updated and displayed.  It is my understanding that this can be implemented using kivy properties (in this case, ObjectProperty).</p>
  
  <p>Here is some example code:</p>
  
  <pre><code>import kivy
  kivy.require('1.7.0')
  
  from kivy.app import App
  from kivy.uix.gridlayout import GridLayout
  from kivy.properties import ObjectProperty
  from kivy.lang import Builder
  
  Builder.load_string("""
  &lt;RootWidget&gt;:
      cols: 2
      Label:
          text: "Attribute a:"
      Label:
          text: root.data_model.a
      Label:
          text: "Attribute b:"
      Label:
          text: root.data_model.b
      Label:
          text: "Attribute c:"
      Label:
          text: root.data_model.c
      Button:
          text: "Make data_model.a longer"
          on_press: root.button_press()
      Button:
          text: "Make data_model.b shorter"
          on_press: root.button_press2()
  """)
  
  
  class DataModel(object):
      def __init__(self):
          self.a = 'This is a'
          self.b ='This is b'
  
      @property
      def c(self):
          return self.a + ' and ' + self.b
  
  class RootWidget(GridLayout):
      data_model = ObjectProperty(DataModel())
  
      def button_press(self, *args):
          self.data_model.a = 'This is a and it is really long now'
          print self.data_model.c
  
      def button_press2(self, *args):
          self.data_model.b = 'B'
          print self.data_model.c
  
  class TestApp(App):
      def build(self):
          return RootWidget()
  
  app = TestApp()
  app.run()
  </code></pre>
  
  <p>The desired result is for when user presses either button, the labels would automatically update to show the new properties.  As can be seen by the print statements, the data_model is getting updated correctly.  However, none of the labels are updating.  Can someone clarify how to do this?</p>