a.py
from kivy.app import App
from kivy.uix.dropdown import DropDown
from kivy.lang import Builder
class CustDrop(DropDown):
    def __init__(self, **kwargs):
        super(CustDrop, self).__init__( **kwargs)
        self.select('')
kv_str = Builder.load_string('''
BoxLayout:
    orientation: 'vertical'
    BoxLayout:
        canvas.before:
            Rectangle:
                pos: self.pos
                size: self.size
            Color:
                rgb: (1,1,1)
        size_hint_y:1
        Button:
            id: btn
            text: 'test'
            on_release: dropdown.open(self)
            #size_hint_y: None
            #height: '48dp'  
        CustDrop:
            id: dropdown
            Button:
                text: 'Run another script'
                size_hint_y: None
                height: '48dp'
        Label:
            size_hint_x: 4
    Label:
        size_hint_y: 9
''')
class ExampleApp(App):
    def build(self):
        return kv_str
if __name__ =='__main__':
    ExampleApp().run()
b.py
import kivy
kivy.require('1.0.6') # replace with your current kivy version !
from kivy.app import App
from kivy.uix.label import Label
class MyApp(App):
    def build(self):
        return Label(text='Hello world')
if __name__ == '__main__':
    MyApp().run()
when i run a.py file after that i click on test then dropdown show 'run another script'.when i click on 'run another script'(sub menu of test) then how to run b.py(MyApp().run()).It should be print 'Hello world' in new window.
 
    

 
    