fix passing default arguements

examples/widgets/bubble_test.py

@@ -23,8 +23,7 @@ class BubbleShowcase(FloatLayout):
         self.add_widget(self.but_bubble)
 
     def show_bubble(self, *l):
-        bubb = Bubble(size_hint = (.27, .0792), arrow_pos = 'bottom_mid')
-        #bubb.content.rows = 1
+        bubb = Bubble(size_hint = (.27, .0792), arrow_pos = 'bottom_right')
         bubb.content.cols = 3
         bubb.add_widget(Button(text = 'Cut',
                         background_normal = 'data/images/bubble_btn.png',

kivy/uix/bubble.py

@@ -76,7 +76,7 @@ class Bubble(GridLayout):
         self.padding = 2
         self.add_widget(self.content)
         self.bk_img.bind(on_texture = self._on_texture)
-        self.arrow_pos    = 'bottom_mid'
+        self.on_arrow_pos()
 
     def _on_texture(self, *l):
         self.background_texture = self.bk_img.texture
@@ -95,6 +95,9 @@ class Bubble(GridLayout):
         self.arrow_img.source = self.arrow_image
 
     def on_arrow_pos(self, *l):
+        if not self.content:
+            return
+        print self.content
         self.arrow_layout.clear_widgets()
         self.clear_widgets()
         self.arrow_img.size_hint = (1, None)