How to increase size of a screen and center it in the monitor?

I am making a program with several screens and each one has its own size, so each time I change of screen I must re-size the App.
The problem is that, when I change the size of the window, Kivy only increase or reduce the lenght of the right and the bottom sides, not the four sides, so the screen always get “uncentered” after change the size.
I’ll try to make an example with lines:

--------------------------------   --------------------------------
|          Monitor             |   |                               |
|           --------           |   |           --------------      |
|          |   My   |          |   |          |              |     |
|          |   App  |          | > |          |              |     |
|           --------           |   |          |              |     |
|                              |   |          |______________|     |
--------------------------------   --------------------------------

Like you can see the program only change the lengh of the right and bottom sides.

How can I center again the screen or better change the size of the four sides and not only two?

This is an example of my code but I don’t have any idea of what to do:

This is an .py example:

from kivy.app import App
from kivy.uix.screenmanager import Screen, ScreenManager
from kivy.core.window import Window

class Screen_Manager(ScreenManager):
    pass

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

    def on_pre_enter(self):
    Window.size = (900, 500)    


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

    def on_pre_enter(self):
        Window.size = (400, 300)    


class MyApp(App):
    def build(self):
        return Screen_Manager()

MyApp().run()

This is an my.kv example (my in order to work with MyApp):

<Screen_Manager>:
    id: screen_manager
    Main:
    Login:

<Login>:
    name: 'login'
    Button:
        text: 'Go to Main'
        on_press: root.manager.current = 'main'

<Main>:
    name: 'main'
    Button:
        text: 'Go to Login'
        on_press: root.manager.current = 'login'

I have found a way but I don’t like it much:

    def on_pre_enter(self):
        # New size
        size = (NEW_X, NEW_Y)

        # Get the actual pos and knowing the old size calculate the new one
        top  = Window.top  * Window.size[1] / size[1]
        left = Window.left * Window.size[0] / size[0]

        # Change the size
        Window.size = size

        # Fixing pos
        Window.top  = top
        Window.left = left

Answer

I couldn’t find any hint that kivy was able to supply the screen size of the host device, so I hope if there is one, someone will tell us. In the meantime, I borrowed some setup code from another forum. I’ve only tested it on my macbook, and it seems to work, getting the screen size. I don’t know what will happen on windows, or linux, or multiple monitors. But here’s the new code:

import sys
"""
returns Monitor size x and y in pixels for desktop platforms, or None for
mobile platforms
Found at:
https://groups.google.com/forum/#!topic/kivy-users/uZYrghb87g0
"""
if sys.platform == 'linux2':
    import subprocess
    output = subprocess.Popen(
        'xrandr | grep "*" | cut -d" " -f4',
        shell=True,
        stdout=subprocess.PIPE).communicate()[0]
    screenx = int(output.replace('n', '').split('x')[0])
    screeny = int(output.replace('n', '').split('x')[1])
elif sys.platform == 'win32':
    from win32api import GetSystemMetrics
    screenx = GetSystemMetrics(0)
    screeny = GetSystemMetrics(1)
elif sys.platform == 'darwin':
    from AppKit import NSScreen
    frame_size = NSScreen.mainScreen().frame().size
    screenx = frame_size.width
    screeny = frame_size.height
else:
    # For mobile devices, use full screen
    screenx,screeny = 800,600  # return something

#print "screenx,screeny = ",repr((screenx,screeny))

from kivy.config import Config   # config should appear  before any other kivy stuff
Config.set('graphics','position','custom')

from kivy.app import App
from kivy.uix.screenmanager import Screen, ScreenManager
from kivy.core.window import Window

class Screen_Manager(ScreenManager):
    pass

def center_window(sizex,sizey):
    Window.size = (sizex, sizey)
    Window.left = (screenx - sizex)/2
    Window.top = (screeny - sizey)/2

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

    def on_pre_enter(self):
        center_window(900,500)

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

    def on_pre_enter(self):
        center_window(400, 300)        

class MultiScreenApp(App):
    def build(self):
        return Screen_Manager()

MultiScreenApp().run()