title: Kivy run function from kv button
body: <p>I'm new at kivy and try to run a do_login function inside my MyApp Class from a kv generated button.</p>
  
  <p>my kv layout with the button</p>
  
  <pre><code>RelativeLayout:
      FloatingActionButton:
          id:                 float_act_btn
          on_press:           ???how to call call do_login from MyApp
  </code></pre>
  
  <p>and my class with the containing do_login function</p>
  
  <pre><code>class MyApp(App):
  
      def build(self):
          main_widget = Builder.load_string(login_kv) 
  
  def do_login(self, *args):
      print'jo'
  </code></pre>
  
  <p>How to use on_press to call do_login?</p>
  
  <p>with on_press:do_login(login.text, password.text)' I get 'do_login' is not defined and the same with self.do_login I get MaterialRaisedButton' object has no attribute 'do_login'</p>