Python GTK 3 example using Glade and Gtk.Builder

example.py

#!/usr/bin/env python3

import gi

gi.require_version("Gtk", "3.0")

from gi.repository import Gtk

UI_FILE = "example.ui"


class Example:
    def __init__(self):
        self.builder = Gtk.Builder()
        self.builder.add_from_file(UI_FILE)
        self.builder.connect_signals(self)

        self.window = self.builder.get_object("window")
        self.window_label = self.builder.get_object("window_label")
        self.window.show_all()

    def button_clicked(self, button):
        self.window_label.set_text("You clicked!")

    def about_clicked(self, button):
        self.window_label.set_text("This is just an example.")

    def quit_clicked(self, button):
        self.on_window_destroy(self.window)

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


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


if __name__ == "__main__":
    main()

example.ui

<?xml version="1.0" encoding="UTF-8"?>
<!-- Generated with glade 3.36.0 -->
<interface>
  <requires lib="gtk+" version="3.20"/>
  <object class="GtkPopover" id="menu_popover">
    <property name="can_focus">False</property>
    <child>
      <object class="GtkBox" id="menu_box">
        <property name="visible">True</property>
        <property name="can_focus">False</property>
        <property name="border_width">6</property>
        <property name="orientation">vertical</property>
        <child>
          <object class="GtkModelButton" id="menu_about">
            <property name="visible">True</property>
            <property name="can_focus">True</property>
            <property name="receives_default">True</property>
            <property name="text" translatable="yes">About</property>
            <signal name="clicked" handler="about_clicked" swapped="no"/>
          </object>
          <packing>
            <property name="expand">False</property>
            <property name="fill">True</property>
            <property name="position">0</property>
          </packing>
        </child>
        <child>
          <object class="GtkSeparator" id="separator">
            <property name="visible">True</property>
            <property name="can_focus">False</property>
          </object>
          <packing>
            <property name="expand">False</property>
            <property name="fill">True</property>
            <property name="position">1</property>
          </packing>
        </child>
        <child>
          <object class="GtkModelButton" id="menu_quit">
            <property name="visible">True</property>
            <property name="can_focus">True</property>
            <property name="receives_default">True</property>
            <property name="text" translatable="yes">Quit</property>
            <signal name="clicked" handler="quit_clicked" swapped="no"/>
          </object>
          <packing>
            <property name="expand">False</property>
            <property name="fill">True</property>
            <property name="position">2</property>
          </packing>
        </child>
      </object>
    </child>
  </object>
  <object class="GtkWindow" id="window">
    <property name="can_focus">False</property>
    <property name="default_width">440</property>
    <property name="default_height">250</property>
    <signal name="destroy" handler="on_window_destroy" swapped="no"/>
    <child>
      <object class="GtkLabel" id="window_label">
        <property name="visible">True</property>
        <property name="can_focus">False</property>
      </object>
    </child>
    <child type="titlebar">
      <object class="GtkHeaderBar" id="header">
        <property name="visible">True</property>
        <property name="can_focus">False</property>
        <property name="title" translatable="yes">Example</property>
        <property name="subtitle" translatable="yes">Python GObject Glade</property>
        <property name="show_close_button">True</property>
        <child>
          <object class="GtkButton" id="button">
            <property name="label" translatable="yes">Click</property>
            <property name="visible">True</property>
            <property name="can_focus">True</property>
            <property name="receives_default">True</property>
            <signal name="clicked" handler="button_clicked" swapped="no"/>
          </object>
        </child>
        <child>
          <object class="GtkMenuButton" id="menu">
            <property name="visible">True</property>
            <property name="can_focus">True</property>
            <property name="focus_on_click">False</property>
            <property name="receives_default">True</property>
            <property name="popover">menu_popover</property>
            <child>
              <object class="GtkImage" id="menu_image">
                <property name="visible">True</property>
                <property name="can_focus">False</property>
                <property name="icon_name">open-menu-symbolic</property>
              </object>
            </child>
          </object>
          <packing>
            <property name="pack_type">end</property>
            <property name="position">1</property>
          </packing>
        </child>
      </object>
    </child>
  </object>
</interface>