builder.add_from_string(
    """
<interface>
  <object class="GtkWindow" id="window1">
    <child>
      <object class="GtkComboBox" id="combobox1">
        <property name="model">liststore1</property>
      </object>
    </child>
  </object>
</interface>"""
    )
How to reuse a widget tree from a glade file with GtkBuilder?