title: on_press event in kivy button not working as expected
body: <p>Below is small snippet from my code . </p>
  
  <p>issue:- get function is call when a screen "X" is loaded and some params are passed. In get i am create buttons with some names and assigning  on_press event. But issue is on_press event is called automatically without pressing button and next screen comes into picture. </p>
  
  <pre><code>   def get(self, service):
              """ some code"""
              but = Button(size_hint=(1, None))
              but.text = str(i['name'][0][:10])
              but.bind(on_press = self.change_screen(dict))
              print "adding widget to home  "  + str(but)
              self.home_box.add_widget(but)
  
      def change_screen(self, dict):
          self.screen_manager.current = 'Per_settings'
  </code></pre>
  
  <p>Any idea whats happening ?</p>