Stack Switcher

import gi
gi.require_version('Gtk', '3.0')
from gi.repository import Gtk, GdkPixbuf, Gdk
import os, sys

class GUI:
    def __init__(self):

        self.stack  = Gtk.Stack()
        switcher = Gtk.StackSwitcher()
        switcher.set_stack(self.stack)
        label1 = Gtk.Label("label 1")
        label2 = Gtk.Label("label 2")
        self.stack.add_titled (label1, "1", "Page 1")
        self.stack.add_titled (label2, "2", "Page 2")
        box = Gtk.Box()
        box.pack_start(switcher, True, False, 0)
        box.pack_start(self.stack, True, True, 0)
        box.set_orientation(Gtk.Orientation.VERTICAL)
        window = Gtk.Window()
        window.add(box)

        window.show_all()
        window.connect("key-press-event", self.key_press)

    def key_press (self, window, event):
        keyname = Gdk.keyval_name(event.keyval)
        if not Gdk.ModifierType.CONTROL_MASK:
            return
        #forward scroll

        previous_child_active = False
        if keyname == "r":
            for child in self.stack.get_children():
                if previous_child_active == True:
                    self.stack.set_visible_child(child)
                    return
                previous_child_active = self.stack.get_visible_child() == child

        #reverse scroll
        previous_child = None
        if keyname == "s":
            for child in self.stack.get_children():
                if self.stack.get_visible_child() == child and previous_child != None:
                    self.stack.set_visible_child(previous_child)
                    return
                previous_child = child

    def on_window_destroy(self, window):
        Gtk.main_quit()

def main():
    app = GUI()
    Gtk.main()

if __name__ == "__main__":
    sys.exit(main())