Why am I getting an rospy error in getitem raise KeyError(key) ?

rospy
param

asked 2014-04-02 22:46:38 -0500

ct2034
862 ●21 ●75 ●44 https://twitter.com/ct2034

updated 2014-04-09 05:54:50 -0500

I am trying to launch a python script and hand over a parameter from the launchfile: script:

#!/usr/bin/env python
import roslib
roslib.load_manifest( 'package_name' )

import rospy
import datetime, sys, yaml
from gazebo_msgs.msg import ModelStates

# some functions

if __name__ == '__main__':
    sim = rospy.get_param( '~sim' ) # <- edit

    rospy.init_node('package_name', anonymous=True)
    #something else ...

and I am trying to start it like that:

<?xml version="1.0"?>
<launch>
    <arg name="sim" default="true"/>

    <node name="node_name" pkg="package_name" type="script.py" output="screen">

        <param name="sim" value="$( arg sim )" />
    </node>
</launch>

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Comments

import roslib and roslib.load_manifest should not be used if you're using ROS Groovy or later and catkin. Not sure if the inclusion of those lines may be causing your issue.

Brendan Andrade ( 2014-04-03 16:39:26 -0500 )edit

I ran into this and had simply forgotten that usually I provide a default parameter:

foo = rospy.get_param( '~foo', 5.0)

lucasw ( 2018-07-26 09:27:49 -0500 )edit

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1

answered 2014-04-09 02:10:15 -0500

ct2034
862 ●21 ●75 ●44 https://twitter.com/ct2034

I know where the problem was now! I was missing the rospy.init_node() So my main should be:

if __name__ == '__main__':
    rospy.init_node('package_name', anonymous=True)
    sim = rospy.get_param( '~sim' ) # <- edit

    #something else ...

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can't mark it as answered :( but it is ..

ct2034 ( 2014-04-09 02:24:16 -0500 )edit

It's probably a bug that trying to get params without calling init_node doesn't raise an error. @William, @Dirk Thomas?

Dan Lazewatsky ( 2014-04-09 03:44:11 -0500 )edit

I'm surprised that doesn't raise an exception.

William ( 2014-04-09 07:28:24 -0500 )edit

The only thing I can imagine is that someone else called `rospy.init_node` in another part of the code and there for the `~` got expanded to something other than `package_name`.

William ( 2014-04-09 07:29:31 -0500 )edit

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3

answered 2014-04-03 04:06:31 -0500

Dan Lazewatsky
9115 ●79 ●103 ●160 http://dan.lazewatsky.com/

updated 2014-04-03 04:08:22 -0500

To get a param in the node's namespace, which is what your launchfile will create, use:

sim = rospy.get_param( '~sim' )

See getting parameters in python.

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Comments

Thanks for your answer. I have actually tried this before. Then I get this exception: 20, in <module> sim = rospy.get_param( '~sim' ) File "/opt/ros/groovy/lib/python2.7/dist-packages/rospy/client.py", line 452, in get_param return _param_server[param_name] #MasterProxy does all the magic for us File "/opt/ros/groovy/lib/python2.7/dist-packages/rospy/msproxy.py", line 117, in __getitem__ raise KeyError(key) KeyError: '~sim'

ct2034 ( 2014-04-03 04:11:48 -0500 )edit

if you do rosparam list, does the paramater show up?

Dan Lazewatsky ( 2014-04-03 05:15:14 -0500 )edit

how can i do a rosparam in this namespace? As I understand it, this param only exists in the namespace of the launchfile.

ct2034 ( 2014-04-08 21:56:44 -0500 )edit

And like I said, I edited my question code with the '~' as suggested above. -> same result

ct2034 ( 2014-04-08 21:58:20 -0500 )edit

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1

answered 2017-10-02 12:07:26 -0500

biglotusturtle
165 ●3 ●10 ●10

I too had this issue but my problem was not because of rospy init.

Instead I assumed that get_param would just quietly fail if a parameter did not exist (like it does in cpp). However in python for some reason this method raises an error and kills the node if the parameter does not exist (This is a poor design IMO).

So if you're like me and have this issue, use the following:

if rospy.has_param('~sim'):
    sim = rospy.get_param('~sim')

(I realize this KEY_ERROR is documented on the rospy parameter server page, however coming from roscpp to rospy one assumes the methods behave similarly)

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1

The API docs for rospy.get_param() say:

Raises: ...
KeyError - if value not set and default is not given

It doesn't "kill the node" itself, that's just default Python behaviour with uncaught exceptions.

gvdhoorn ( 2017-10-02 12:22:17 -0500 )edit

Wrapping everything in a try: .. catch would be a way to deal with that.

re: rospy vs roscpp behaviour: yes, that is a known problem with ROS1, and is one of the reasons why in ROS2 all (official) client libraries will wrap the C client library, so we get consistent behaviour.

gvdhoorn ( 2017-10-02 12:24:02 -0500 )edit

Agreed a try: .. catch block would achieve the same functionality but the has_param method is simpler IMO. I'm interested in this ROS2 implementation. Any info on timeline for that?