Retrieving the definition of properties from kv files on add_widget

kvpropertyinonwidget.py

from kivy.app import App
from kivy.uix.widget import Widget
from kivy.uix.boxlayout import BoxLayout
from kivy.properties import StringProperty
from kivy.clock import Clock
from kivy.lang import Builder

class Widget1(Widget):
    string_id = StringProperty('default')

class Widget2(BoxLayout):
    def __init__(self, **kwargs):
        super(Widget2, self).__init__(**kwargs)
        Clock.schedule_once(self.print_children_properties)

    def add_widget(self, widget):
        super(Widget2, self).add_widget(widget)
        print widget.string_id

    def print_children_properties(self, dt):
        for each in self.children:
            print each.string_id

Builder.load_string("""
Widget2:
    
<Widget2>:
    Widget1:
        string_id: 'widget 1'
    Widget1:
        string_id: 'widget 2'
""")
        
class TestApp(App):
    def build(self):
        pass

if __name__ == '__main__':
    TestApp().run()