One way to read a bagfile from python in ros2

read_bagfile_ros2_python.py

def open_bagfile(bagfile: Path):
    reader = rosbag2_py.SequentialReader()
    storage_options = rosbag2_py.StorageOptions(bagfile.as_posix())
    converter_options = rosbag2_py.ConverterOptions("cdr", "cdr")
    reader.open(storage_options, converter_options)

    topic_types = reader.get_all_topics_and_types()

    # Create a map for quicker lookup
    type_map = {
        topic_types[i].name: topic_types[i].type for i in range(len(topic_types))
    }
    return reader, type_map

def get_first_message_on_topic(reader, type_map, topic):
    storage_filter = rosbag2_py.StorageFilter(topics=[topic])
    reader.set_filter(storage_filter)

    while reader.has_next():
        (topic, data, _) = reader.read_next()
        msg_type = get_message(type_map[topic])
        msg = deserialize_message(data, msg_type)
        return msg