title: kivy Cannot convert float to kivy.properties.Property
body: <p>I'm trying to have a button which can be clicked to add two buttons below the most recently added buttons.  I'm doing this with a float <code>height</code> which starts at .9 and decrements from there.  I'm getting the error TypeError: Cannot convert float to kivy.properties.Property.</p>
  
  <pre><code>class TasksWindow(FloatLayout):
      height = .9
  
      def __init__(self, **kwargs):
          super(TasksWindow, self).__init__(**kwargs)
          addBtn = Button(
              text="+", pos_hint={'x': .9, 'top': 1}, size_hint=(.1, .1))
          self.add_widget(addBtn)
          addBtn.bind(on_press=self.clkAdd)
  
      def clkAdd(self, obj):
          height = obj.height
          editBtn = Button(text="Tap to Edit", pos_hint={
              'x': 0.0, 'top': height}, size_hint=(.9, .1))
          delBtn = Button(text="X", pos_hint={
                          'x': .9, 'top': height}, size_hint=(.1, .1))
          height -= .1
          self.add_widget(editBtn)
          self.add_widget(delBtn)
  
  
  class TasksApp(App):
      def build(self): 
          window = TasksWindow() 
          return window
  
  
  if __name__ == '__main__':
      TasksApp().run()
  </code></pre>