I would like to have an graphical interface in Kivy for my robot while ROS will works in background.

First will illustrate the project hierarchy.

rosworkscape                  <-catkinwrokspace
|--src
    |--gui                <---catkinpackage
        |--controlpanel
            |--_init.py
            |--graphicalinterface.py
        |--srv
            |-- __init.py
            |--LightsToggle.srv
        |--__init.py
        |--CMakeLists.txt
        |--package.xml
        |--start_method.py

I made a very simple interface in Kivy:

graphical_interface.py

from kivy.app import App
from kivy.uix.label import Label
from kivy.uix.button import Button
from kivy.uix.screenmanager import ScreenManager, Screen, SlideTransition
from kivy.uix.boxlayout import BoxLayout
import rospy
from gui.srv import LightsToggle

class MainMenuScreen(Screen):
    def __init__(self, **kwargs):
        super (MainMenuScreen, self).__init__(**kwargs)

layer = BoxLayout(orientation='vertical')
btn_lights = Button(text="Lights", size_hint_y=None, size_y=100)
btn_lights.bind(on_press=self.lights_toggle)
layer.add_widget(btn_lights)    

    lights = Label(text="Lights: OFF", font_size='18dp', size_hint=(1, .3), halign='left')       
    lights.bind(size=lights.setter('text_size'))      
    layer.add_widget(lights)

self.add_widget(layer)

def lights_toggle(self, *args):
try:
    rospy.wait_for_service('lights_toggle', timeout = 2.0)
    except rospy.ROSException:
        print("Failed. Is server running?")
    return
try:        
    lights_toggle = rospy.ServiceProxy('lights_toggle', LightsToggle)
    response = lights_toggle()
    print ('Lights are :' + response)
except rospy.ServiceException, e:
    print ('Service call failed: ' + e)

class ControlPanelApp(App):     
    def build(self):
        screen_manager = ScreenManager(transition=SlideTransition())
        screen_1 = MainMenuScreen(name='menu')
        screen_manager.add_widget(screen_1)
        return screen_manager


def graphical_interface(cmd_q):
    ControlPanelApp().run()

And this file is run by start_method.py

from control_panel.graphical_interface import graphical_interface
if __name__ == '__main__':
    graphical_interface()

Now the issue:
1) I run catkinmake in rosworkscape - done successfully.
2) source devel/setup.bash
3) rosrun gui start_method.py ( ! roscore is already running)

The code is ok and it works.

It will fails with:

...somepath/rosworkspace/src/gui/controlpanel/graphical_interface.py", line 6, in from kivy.app import App ImportError: No module named app

If I do not perform

source devel/setup.bash

then the Kivy works! But it fails on importing the

LightsToggle

If I run the code without Kivy just ROS and with service (with source devel/setup.bash) it works as well.

I guess during the source devel/setup.bash is some system path or Python path overwritten but I can't figure it out.