How to capture image of a gtk ApplicationWindow in

I need to capture an image of an ApplicationWindow in Python, in linux.

This would be my hypothetical test app taken from here:

from gi.repository import Gtk
import sys


class MyWindow(Gtk.ApplicationWindow):
    # constructor for a Gtk.ApplicationWindow

    def __init__(self, app):
        Gtk.Window.__init__(self, title="Welcome to GNOME", application=app)
        self.set_default_size(200, 100)

        # create a label
        label = Gtk.Label()
        # set the text of the label
        label.set_text("Hello GNOME!")
        # add the label to the window
        self.add(label)


class MyApplication(Gtk.Application):

    def __init__(self):
        Gtk.Application.__init__(self)

    def do_activate(self):
        win = MyWindow(self)
        win.show_all()

    def do_startup(self):
        Gtk.Application.do_startup(self)

app = MyApplication()
exit_status = app.run(sys.argv)
sys.exit(exit_status)

I need the contents of the window, the application bar excluded, to be saved in an image file.

on_save_image method, saves the current window to a png file.

You can also apply cairo tranformations to the cairo_context before saving.

from gi.repository import Gtk, Gdk
import cairo
import sys


class MyWindow(Gtk.ApplicationWindow):
    # constructor for a Gtk.ApplicationWindow

    def __init__(self, app, image_file="/tmp/window.png"):
        Gtk.Window.__init__(self, title="Welcome to GNOME", application=app)
        self.set_default_size(200, 100)
        self.image_file = image_file

        vbox = Gtk.VBox()
        self.add(vbox)

        # create a label
        label = Gtk.Label()
        # set the text of the label
        label.set_text("Hello GNOME!")
        # add the label to the window
        vbox.pack_start(label, True, True, 0)

        button = Gtk.Button("ScreenshotMe!")
        button.connect("clicked", self.on_save_image)
        vbox.pack_start(button, False, False, 0)

    def on_save_image(self, button):
        """
        Get the surface of the current window and save it as a png
        """
        window = self.get_window()
        width, height = window.get_width(), window.get_height()
        surface = Gdk.Window.create_similar_surface(window,
                                                    cairo.CONTENT_COLOR,
                                                    width, height)
        cairo_context = cairo.Context(surface)
        Gdk.cairo_set_source_window(cairo_context, window, 0, 0)
        cairo_context.paint()

        surface.write_to_png(self.image_file)


class MyApplication(Gtk.Application):

    def __init__(self):
        Gtk.Application.__init__(self)

    def do_activate(self):
        win = MyWindow(self)
        win.show_all()

    def do_startup(self):
        Gtk.Application.do_startup(self)

app = MyApplication()
exit_status = app.run(sys.argv)
sys.exit(exit_status)