Roslaunch combine dictionary lookups from list

test_list_to_dict_lookup.launch

<?xml version="1.0"?>
<launch>
  <!-- List of things to lookup in dict, in our case sensors --> 
  <arg name="sensors" value="s1,s2" />
  <!-- Actual python dictionary. In our case this maps sensors to topics -->
  <arg name="sensor_topics" value="{
			   's1':'t1',
			   's2':'t2',
			   's3':'t3'
			   }" />
  <!-- Make a space seperated string of the lookups (topics) from the sensor list -->
  <!-- This should set tout to 't1 t2' -->
  <!-- NOTE: map() is used instead of list comprehensions because roslaunch fails to parse them -->
  <!-- NOTE: the sensors and sensor_topics args are mapped in automagically so args('sensors') is not neeeded -->
  <param name="tout" value="$(eval ' '.join(map(eval(sensor_topics).get, sensors.split(','))))" />
</launch>