'Persists this training data to disk and returns necessary information to load it again.'

def persist(self, dir_name):

    data_file = os.path.join(dir_name, u'training_data.json')
    with io.open(data_file, u'w') as f:
        f.write(self.as_json(indent=2))
    return {u'training_data': u'training_data.json'}
