import gi

gi.require_version("Gtk", "3.0")
gi.require_version("Handy", "1")

from gi.repository import Gtk, Handy

Handy.init()

window = Handy.Window()
headerbar = Handy.HeaderBar()
button = Gtk.Button(label="example")
headerbar.pack_start(button)
squeezer = Handy.Squeezer()
squeezer.add(headerbar)
box = Gtk.Box(orientation=Gtk.Orientation.VERTICAL)
box.pack_start(squeezer, False, False, 0)
window.add(box)

window.show_all()

Gtk.main()