def export_graph(checkpoint_path, output_nodes):
    """
    Export a graph stored in a checkpoint as a *.pb file.
    :param checkpoint_path: The checkpoint path which should be frozen.
    :param output_nodes: The output nodes you care about as a list of strings (their names).
    :return:
    """
    if not tf.gfile.Exists(checkpoint_path):
        raise AssertionError(
            "Export directory doesn't exists. Please specify an export "
            "directory: %s" % checkpoint_path)

    if not output_nodes:
        print("You need to supply the name of a node to --output_node_names.")
        return -1

        # We retrieve our checkpoint fullpath
    checkpoint = tf.train.get_checkpoint_state(checkpoint_path)
    input_checkpoint = checkpoint.model_checkpoint_path

    # We precise the file fullname of our freezed graph
    output_graph = checkpoint_path + "/frozen_model.pb"

    # We clear devices to allow TensorFlow to control on which device it will load operations
    clear_devices = True

    # We start a session using a temporary fresh Graph
    with tf.Session(graph=tf.Graph()) as sess:
        # We import the meta graph in the current default Graph
        saver = tf.train.import_meta_graph(input_checkpoint + '.meta', clear_devices=clear_devices)

        # We restore the weights
        saver.restore(sess, input_checkpoint)

        # We use a built-in TF helper to export variables to constants
        output_graph_def = tf.graph_util.convert_variables_to_constants(
            sess,  # The session is used to retrieve the weights
            tf.get_default_graph().as_graph_def(),  # The graph_def is used to retrieve the nodes
            output_nodes  # The output node names are used to select the useful nodes
        )

        # Finally we serialize and dump the output graph to the filesystem
        with tf.gfile.GFile(output_graph, "wb") as f:
            f.write(output_graph_def.SerializeToString())
        print("%d ops in the final graph." % len(output_graph_def.node))

    return output_graph_def