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Rosbag Python API: why do messages from a rosbag have mangled __class__?

asked 2018-02-25 21:17:39 -0500

Rick Armstrong gravatar image

I find that messages loaded from a rosbag via the Python API, have a weird __class__ attribute:

import rosbag
from sensor_msgs.msg import PointCloud2

# Round-trip a PointCloud2 message to a rosbag and back
bag = rosbag.Bag('test.bag', 'w')
try:
    scan = PointCloud2()
    print("__class__should be: \n{}".format(scan.__class__))
    bag.write('scan', msg)
finally:
    bag.close()

bag = rosbag.Bag('test.bag')
for topic, msg, t in bag.read_messages(topics=['scan']):
    print("But it comes out as: \n{}".format(msg.__class__))
bag.close()

The output:

__class__should be: 
<class 'sensor_msgs.msg._PointCloud2.PointCloud2'>
But it comes out as: 
<class 'tmpf1u1_e._sensor_msgs__PointCloud2'>

Is this a bug, or a feature?

ROS Indigo, Ubuntu 14, Python 2.7

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answered 2018-02-26 01:49:57 -0500

ahendrix gravatar image

updated 2018-02-26 01:50:15 -0500

This is a feature, not a bug.

Instead of using or relying on the local message definitions, which may not match the message definitions stored in the bag file (or may not exist at all!), the python API for rosbag generates class definitions on the fly for each message type stored in the bag file. Unfortunately, these auto-generated classes don't have the same names as the original classes.

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Asked: 2018-02-25 21:17:39 -0500

Seen: 121 times

Last updated: Feb 26 '18