Unable to read messages: AttributeError: 'NoneType' object has no attribute 'seek'

[EricWiener]

Hi,

I'm trying to loop through all the messages in a bag and save them to JSON. The bag appears to be correctly formatted because I can do both:

$ rostopic echo -b raise-the-flag_imu.bag -p /imu/data/raw > imu.csv
$ rostopic echo -b raise-the-flag_imu.bag /imu/data/raw > imu.yaml

and get valid results.

However, when I try to use rosbag to read the messages, I get the error:

Traceback (most recent call last):
  File "IMU_pose.py", line 70, in <module>
    main(args["input_bag"], args["topic"], args["output"])
  File "IMU_pose.py", line 40, in main
    for topic, msg, timestamp in tqdm(input_bag.read_messages(topics=topic_specified)):
  File "/home/uma/.local/lib/python3.8/site-packages/tqdm/std.py", line 1193, in __iter__
    for obj in iterable:
  File "/opt/ros/noetic/lib/python3/dist-packages/rosbag/bag.py", line 2700, in read_messages
    yield self.seek_and_read_message_data_record((entry.chunk_pos, entry.offset), raw, return_connection_header)
  File "/opt/ros/noetic/lib/python3/dist-packages/rosbag/bag.py", line 2824, in seek_and_read_message_data_record
    self.bag._file.seek(chunk_header.data_pos)
AttributeError: 'NoneType' object has no attribute 'seek'

This error occurs when trying to read the message with field.header.seq = 32737, but when I inspect the bag, this message is valid:

header: 
  seq: 32737
  stamp: 
    secs: 1561043457
    nsecs: 548082597
  frame_id: "imu"
orientation: 
  x: 0.028720000758767128
  y: 0.009557000361382961
  z: -0.9885789752006531
  w: 0.14763100445270538
orientation_covariance: [0.00030461741978670857, 0.0, 0.0, 0.0, 0.00030461741978670857, 0.0, 0.0, 0.0, 0.00030461741978670857]
angular_velocity: 
  x: -2.286379308102937e-09
  y: -1.1013018053063206e-08
  z: -1.6179187767306757e-08
angular_velocity_covariance: [2.7415567780803768e-09, 0.0, 0.0, 0.0, 2.7415567780803768e-09, 0.0, 0.0, 0.0, 2.7415567780803768e-09]
linear_acceleration: 
  x: 0.10222382098436356
  y: 0.14998410642147064
  z: 9.50407600402832
linear_acceleration_covariance: [5.29e-10, 0.0, 0.0, 0.0, 5.29e-10, 0.0, 0.0, 0.0, 5.29e-10]

I am attaching both the script I am using as well as the ROS bag file. Additionally, I am use ROS Noetic, Python 3.8.5, and rosbag==1.15.9.

Thank you!

code_imu_bag.zip

[jinmenglei]

@EricWiener
It's not rosbag's problem.It's a logic bug.

1. Why do you generate a file for every message
2. After yield, you call input_bag.close(). I think you should put close() outside the for loop.

    # ============ Loop through ROS bag ===========
    for topic, msg, timestamp in tqdm(input_bag.read_messages(topics=topic_specified)):
        print(msg.header.seq)

        json_str = json_message_converter.convert_ros_message_to_json(msg)

        # Save
        # Why do you generate a file for every message
        file_name = str(timestamp)
        file_path = os.path.join(output_directory, file_name + ".json")
        with open(file_path, "w") as outfile:
            json.dump(json_str, outfile)

        # After yield, you call input_bag.close(). I think you should put close() outside the for loop
        input_bag.close()

I made a little change

   # ============ Loop through ROS bag ===========
    for topic, msg, timestamp in tqdm(input_bag.read_messages(topics=topic_specified)):
        print(msg.header.seq)

        json_str = json_message_converter.convert_ros_message_to_json(msg)

        # Save
        # Why do you generate a file for every message
        file_name = str(timestamp)
        file_path = os.path.join(output_directory, file_name + ".json")
        with open(file_path, "w") as outfile:
            json.dump(json_str, outfile)

        # After yield, you call input_bag.close(). I think you should put close() outside the for loop
    input_bag.close()

And then there were more than 3000 files.

[EricWiener]

@jinmenglei thanks so much! So sorry for not seeing that issue. I really appreciate your help