debugging a ros Python program

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This will work, but rosrun (and roslaunch) does a little more than just cd $pkg && ./node_script: it also provides the node with its name, location of the logfile, private parameters (optional) and (topic) remappings.

It is working for me with rosrun node_example pytalker.py by adding import pdb and pdb.set_trace(). But, the program doen't stop in the breakpoint. It continues to run the callback functions ?

Never tried that, but https://stackoverflow.com/questions/1... suggests to add a set_trace in each thread. In any case, this would be a generic pdb-threading problem, rather than a ROS-specific problem. You might find better help in a generic Python forum/site.

How about just rospy.loginfo('Some text or variable')?

It works for me when adding pdb.set_trace() at the beginning of each method I want to debug.