Create kivy app by multiple .kv files and .py files

My files:

1. main.py - main.kv
2. login.py - login.kv
3. workarea.py - workarea.kv
4. welcome.py - welcom.kv

How to open welcome.kv with the push of a button made in main.py and main.kv?

I want to not put all the interface code in one main.kv file.

main.py:

from login import Login
from welcome import Welcome

    class Screen_manager(ScreenManager):
        def __init__(self,**kwargs):
            super(ScreenManager,self).__init__(**kwargs)
            self.current='login-page'# not screen by name 'login-page' error
            self.addWidget(Login(name='login-page') #'nonetype' object has no attribute 'stop'


    class MainApp(App):
        Build(self):
            return ScreenManager()

    if __name__=="__main__":
        MainApp.run()

login.py:

from kivy.uix.screenmanger import Screen
    class Login(Screen):
        def openwelcome(self):
            ?????

login.kv:

<Login>:
    name:login-page
    Button:
        text:"press me"
        on_press:root.openwelcome()

In order to work with multiple .kv & .py files, lets assume we have all the .py file in libs/baseclass & all the .kv files in libs/kvs. In order to integrate all the files, we can create a main.kv file whose content looks like below:

#  import the python files defining the Screens
#: import Screen1 libs.baseclass.screen1.Screen1
#: import Screen2 libs.baseclass.screen2.Screen2

#  include the kv files for the other Screens
#: include libs/kvs/screen1.kv
#: include libs/kvs/screen2.kv

ScreenManager:
    Screen1:
    Screen2:

Finally load this file through Builder in an App. To link different screens, let's assume you are currently on Screen1 and want to go to Screen2, create your screen1.kv & screen2.kv as below:

<Screen1>:
    name: 'Screen1'

# And in screen2.kv file

<Screen2>:
    name: 'Screen2'

Finally on your main.kv should look like this

ScreenManager:
    Screen:
        name: "main_screen"

        BoxLayout:
            orientation: "vertical"

            Label:
                text: "main screen"
            Button:
                text: "to screen 1"
                on_press: app.root.current = "Screen1"
            Button:
                text: "to screen 2"
                on_press: app.root.current = "Screen2"
    Screen1:
    Screen2: