#!/usr/bin/python3

from gi.repository import Gtk

class TestWindow(Gtk.Window):
    __gtype_name__ = "TestWindow"

builder = Gtk.Builder()
builder.add_from_string('''
<interface>
  <object class="GtkWindow" id="mywindow">
  </object>
  <object class="TestWindow" id="test_window">
  </object>
</interface>
''')

objs = builder.get_objects()
print(objs)
