#! /usr/bin/env python
from gi.repository import Gtk, GObject

test_dialog = Gtk.Dialog(title='Whatever',
                         buttons=(Gtk.STOCK_OK, Gtk.ResponseType.ACCEPT))

liststore = Gtk.ListStore(GObject.TYPE_STRING)
for ii in range(500):
    liststore.append([str(ii)])

combo = Gtk.ComboBox.new_with_model_and_entry(liststore)
combo.set_entry_text_column(0)
test_dialog.get_content_area().add(combo)
combo.show_all()

response = test_dialog.run()
