import gtk

def on_clicked(w):
        global entry
        entry.select_region(0,-1)
        entry.grab_focus()

window = gtk.Window()
window.set_title("Hello")
window.connect("destroy", lambda w: gtk.main_quit() )
box = gtk.VBox()
entry = gtk.Entry()
button = gtk.Button("Select")
button.connect("clicked", on_clicked)
entry.set_text("Hello world")
box.add(entry)
box.add(button)
window.add(box)
window.show_all()
gtk.main()