topic syntax is ambiguous "Invalid topic spec"

[dirk-thomas]

From @sloretz on October 20, 2016 1:30

rqt_plot uses slashes to specify the value in a published message that should be plotted. This syntax leads to the error "PlotWidget.update_plot(): error in rosplot: Invalid topic spec [...]" in cases like the following

To reproduce

1. save the following as simple_pub.py and make it executable with chmod

   #!/usr/bin/env python
   
   import rospy
   import std_msgs
   import sys
   import random
   rospy.init_node("simple_pub", anonymous=True)
   pub = rospy.Publisher(sys.argv[1], std_msgs.msg.Float32, queue_size=10)
   r = rospy.Rate(10)
   while not rospy.is_shutdown():
       r.sleep()
       msg = std_msgs.msg.Float32()
       msg.data = random.random()
       pub.publish(msg)
   

2. open 4 terminals, one with each command

   • roscore
   • ./simple_pub.py imu/data
   • ./simple_pub.py imu/data/processed
   • rqt_plot

3. Type the following into the topic box and press the green + to add them to the plot

   1. /imu/data/data
   2. /imu/data/processed/data

4. rqt_plot will correctly plot the first topic, /imu/data/data. The second topic will not be plotted, and the terminal from which rqt_plot was started will spam

   PlotWidget.update_plot(): error in rosplot: Invalid topic spec [/imu/data/processed/data]: 'Float32' object has no attribute 'processed'
   

This can be fixed by changing the character used to separate message fields. I think the ':' character is a good choice

• /imu/data:data
• /imu/data/processed:data

Copied from original issue: ros-visualization/rqt_common_plugins#402

[dirk-thomas]

The choice of using the same character as a namespace as well as field separator is certainly a bad one. But that decision was made at a very early time in ROS and is implemented in some of the core ROS packages. So I don't think this package can do much about it if it doesn't want to diverge from the "standard". I am not sure how feasible it will be to change this in the core.

[josephcoombe]

Would it be reasonable to enable the alternative syntax via an optional flag,
e.g. rosrun rqt_plot rqt_plot -a /imu/data/processed:data (-a for alternative)?

I would be willing to make these changes.

[dirk-thomas]

While this could be easily worked around locally in this package I think this needs to be globally agreed on and being "defined" as part of the ROS naming standard. Otherwise every package might end up doing its own thing.