# Following NOTE 1 & 2 makes it work.

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

testlabel = """
<?xml version="1.0" encoding="UTF-8"?>
<interface>
  <template class="TestLabel" parent="GtkLabel">
    <property name="visible">True</property>
  </template>
</interface>
"""
@Gtk.Template(string=testlabel)
class TestLabel(Gtk.Label):

    __gtype_name__ = 'TestLabel'

    neem = GObject.Property(type=bool, default=False)

    def __init__(self):
        super().__init__()

        self.props.label = 'TestLabel'


testbox = """
<?xml version="1.0" encoding="UTF-8"?>
<interface>
  <template class="TestBox" parent="GtkBox">
    <property name="visible">True</property>
    <child>
      <object class="TestLabel" id="_testlabel"/>
    </child>
  </template>
</interface>
"""
@Gtk.Template(string=testbox)
class TestBox(Gtk.Box):

    __gtype_name__ = 'TestBox'

    # NOTE1: Disable this for it to work
    _testlabel = Gtk.Template.Child()

    noom = GObject.Property(type=bool, default=False)

    def __init__(self):
        super().__init__()

        # NOTE2: Enable this for it to work
        # self._testlabel = TestLabel()
        self._testlabel.props.neem = True

        print(self._testlabel.props.neem)

window = """
<?xml version="1.0" encoding="UTF-8"?>
<interface>
  <template class="MyWindow" parent="GtkWindow">
    <property name="title">"Hellow World"</property>
    <property name="visible">True</property>
    <child>
      <object class="TestBox" id="_testbox">
        <child>
          <object class="TestLabel" id="_testlabel"/>
        </child>
      </object>
    </child>
  </template>
</interface>
"""
@Gtk.Template(string=window)
class MyWindow(Gtk.Window):

    __gtype_name__ = 'MyWindow'

    _testbox = Gtk.Template.Child()
    _testlabel = Gtk.Template.Child()

    def __init__(self):
        super().__init__()

        self._testbox.props.noom = True
        self._testlabel.props.neem = True
        print("noom", self._testbox.props.noom)
        print("neem", self._testlabel.props.neem)


win = MyWindow()
win.connect("destroy", Gtk.main_quit)
win.show_all()
Gtk.main()