Question

Hello,

I am currently trying for my thesis Project to create topics with ROS where I publish and subscribe a list of lists with python. I have created two messages: intlist1d.msg : int32[] data and another message
intlist2d.msg : intlist1d[] data. I use the message for the list of lists (intlist2d) but, despite the fact that the catkin_make is completed without errors when I execute the roslaunch file for the two nodes I receive the following answer: AttributeError: 'list' object has no attribute 'data'. Can you please help me? I am stuck. Thank you

The code comprises of two files:
pub.py

import rospy
from iot_humans_track.msg import int_array2d
if __name__ == "__main__":
    rospy.init_node('pub', anonymous=True)
    msg_list = int_array2d()
    msg_list.data = [[1,2,3], [1,2]]

    puber = rospy.Publisher('topic', int_array2d, queue_size=10)
    while not rospy.is_shutdown():
        puber.publish(msg_list)

and the sub.py

import rospy
from iot_humans_track.msg import int_array2d

def callback(msg):
    print msg.data

if __name__ == "__main__":
    rospy.init_node('sub', anonymous=True)
    sub_node = rospy.Subscriber('topic', int_array2d, callback, queue_size=10) 
    rospy.spin()

Asked by pskampas on 2019-02-19 07:01:37 UTC

Answer 1

ROS can not publish 2D arrays. Alternatively you can publish a 1D array (use message int32[] data)

import rospy
import numpy as np

from iot_humans_track.msg import int_array1d

if __name__ == "__main__":
rospy.init_node('pub', anonymous=True)

data = np.array([[1,2,3], [4,5,6]])

#Convert to 1d array
data_1d = np.reshape(data,6) 

puber = rospy.Publisher('topic', int_array1d, queue_size=10)
while not rospy.is_shutdown():
    puber.publish(data_1d)

and convert it later as follows:

import rospy
import numpy as np
from iot_humans_track.msg import int_array1d

def callback(msg):

    data_2d = np.reshape(msg.data,(-1,3))
    print data_2d

if __name__ == "__main__":
    rospy.init_node('sub', anonymous=True)
    sub_node = rospy.Subscriber('topic', int_array1d, callback, queue_size=10) 
    rospy.spin()

Asked by Rao on 2019-02-19 08:25:45 UTC

Comments

This is a rather bad idea. ROS has MultiArray message type (e.g. http://docs.ros.org/api/std_msgs/html/msg/ByteMultiArray.html) that could be used for this task. If you just reshape the message, you destroy the information about the individual array sizes.

Asked by NEngelhard on 2019-02-20 04:08:03 UTC

Answer 2

There is an inconsistency in your question. You first say that your messages are int_list_1d.msg and int_list_2d.msg but then you create an int_array2d-message.

msg_list = int_list_2d()
msg_lists.data <- this is a list of int_list_1d-messages and not a list of int32-lists.

So you'd rather have to go for

msg_list = int_list_2d()
msg_list.data.append(int_list_1d(data=[1,2,3])
msg_list.data.append(int_list_1d(data=[1,2])

"despite the fact that the catkin_make is completed without errors"

Well, python is a language without strict data types, so it's no big surprise that you find type errors at runtime. If you want to find problems like this earlier, use something like C++

Asked by NEngelhard on 2019-02-19 08:35:03 UTC

Comments

Thank you! It works!

Asked by pskampas on 2019-02-19 10:15:43 UTC

glad to hear. then please accept the question so that's clear that it's solved

Asked by NEngelhard on 2019-02-19 10:45:12 UTC

publish list of lists with Python

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