import gtk

def select_none_cb (button):
    global chooser_button
    chooser_button.unselect_all ()

window = gtk.Window ()

box = gtk.VBox ()
window.add (box)

chooser_button = gtk.FileChooserButton ("select a file")
box.pack_start (chooser_button, False, False, 0)

button = gtk.Button ("select none")
button.connect ("clicked", select_none_cb)
box.pack_start (button, False, False, 0)

window.show_all ()
gtk.main ()
