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How to pass private parameters to multiple instances of node?

asked 2019-11-15 10:56:15 -0600

LukeAI gravatar image

updated 2019-11-19 06:18:42 -0600

gvdhoorn gravatar image

I have created a package containing a rospy node that I wish to be able to launch from arbitrary other packages with custom arguments with each launch

eg. myex.py

#!/usr/bin/env python
import rospy
foo = rospy.get_param("foo")          
print str(foo)

In a launch file somewhere else:

<launch>
<node pkg="min_ex" name="node_name" type="myex.py" output="screen">
    <param name="foo" type="int" value="5"/>
</node>
</launch>

This always returns KeyError: 'foo'

Adding "~" to the beginning of the param name in both or either of the launch or .py does not make a difference.

rosparam list shows me that foo is being published on mylib1/foo

So How can I get the node to access the parameter without having to know about the namespace in which it has been launched? I want to be able to create many instantiations of the same node using different parameters.

Code example to show my problem:

#!/usr/bin/env python
import rospy
foo = rospy.get_param("~foo")
print str(foo)

launch file:

<launch>
<node pkg="min_ex" name="node_name" type="myex.py" output="screen">
    <param name="~foo" type="int" value="5"/>
</node>
</launch>
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Comments

re: MWE in 'mini workspace': we don't need a workspace. The two files are sufficient.

gvdhoorn gravatar imagegvdhoorn ( 2019-11-19 06:20:08 -0600 )edit

2 Answers

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answered 2019-11-19 06:27:43 -0600

LukeAI gravatar image

The thing I was missing was that it is necessary to init the node in order to access private params. eg.

#!/usr/bin/env python
import rospy
rospy.init_node("some_name")
foo = rospy.get_param("~foo")
print str(foo)
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answered 2019-11-15 11:10:09 -0600

In rospy , private parameters use the ~ symbol.

http://wiki.ros.org/rospy/Overview/Pa...

Ei.

  foo = rospy.get_param("~foo")
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Comments

Thanks for response, I have read that page, unfortunately it doesn't work. I have produced a minimal working example that illustrates the point

LukeAI gravatar imageLukeAI ( 2019-11-19 05:36:49 -0600 )edit

Adding "~" to the beginning of the param name in both or either of the launch or .py does not make a difference.

you are only supposed to prefix ~ to the name of the parameter in your rospy.get_param(..) call. Not to the name in the .launch file.

The fact that you make the param element a child of the node element already makes it a private parameter.

gvdhoorn gravatar imagegvdhoorn ( 2019-11-19 06:17:10 -0600 )edit
1

You show this in your MWE:

#!/usr/bin/env python
import rospy
foo = rospy.get_param("~foo")
print str(foo)

this is not a node (as you don't call rospy.init_node(..)), so rospy.get_param(..) will also not be relative to a node. Hence the problem accessing the parameter.

gvdhoorn gravatar imagegvdhoorn ( 2019-11-19 06:19:39 -0600 )edit

yeah that got it!

LukeAI gravatar imageLukeAI ( 2019-11-19 06:26:43 -0600 )edit

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Asked: 2019-11-15 10:56:15 -0600

Seen: 26 times

Last updated: Nov 19