import gi
gi.require_version('Gtk', '3.0')
from gi.repository import Gtk

class Handler:
    def onDeleteWindow(self, *args):
        Gtk.main_quit(*args)

    def on_button1_clicked(self, button):
        dialog.set_title('Select source folder')
        dialog.set_default_size(800, 400)

        response = dialog.run()
        if response == Gtk.ResponseType.OK:
            print("Folder selected: " + dialog.get_filename())
        elif response == Gtk.ResponseType.CANCEL:
            print("Cancel clicked")

        dialog.destroy()

builder = Gtk.Builder()
builder.add_from_file("test.glade")
builder.connect_signals(Handler())

dialog = builder.get_object("filechooserdialog1")
window = builder.get_object("window1")
window.connect('delete-event', Gtk.main_quit)
window.show_all()

Gtk.main()