title: Create kivy dropdown list but only a part of them can be opened
body: <p>I am using kivy as my GUI tools for my python program.
  when I want to create a table,in which there is a column containing dropdown list to make select value easier.
  However, I can not make it works correctly.
  The following is my code.</p>
  
  <pre><code>import kivy
  kivy.require('1.10.0')
  
  from kivy.app import App
  from kivy.uix.label import Label
  from kivy.uix.button import Button
  from kivy.uix.dropdown import DropDown
  from kivy.uix.gridlayout import GridLayout
  
  sel =["A","B","C"]
  
  class MyGrid(GridLayout):
      def __init__(self, **kwargs):
          super(MyGrid, self).__init__(**kwargs)
          self.redraw()
  
      def redraw(self):
  
          self.clear_widgets()
          self.rows = 5
          self.cols =2
  
          for i in range(5):
  
              label = Label(text="cell"+str(i+1))
              self.add_widget(label)
  
              drpName = DropDown()
              btnName = Button(text="B",size_hint=(None, None))
              for e in sel:
                  btn=Button(text=e, size_hint_y=None, height=btnName.height)
                  btn.bind(on_release=lambda btn:drpName.select(btn.text))
                  drpName.add_widget(btn)
              btnName.bind(on_release=drpName.open)
              drpName.bind(on_select=lambda instance, x: setattr(btnName, 'text', x))
              self.add_widget(btnName)
  
  class testApp(App):
      def build(self):
          return MyGrid()
  
  if __name__=="__main__":
      testApp().run()
  </code></pre>
  
  <p>Only a part of button open the pull down list and all of the selected value will replace the text of last button.
  Could you help me out. Thanks in advance.  </p>