ROS param KeyError - ros

Question

I started learn ROS. I try to read data from launch file with param. but it gave me an error which I did not understand. Please help me. Here is the constructor:
def__init__(self,nodeName,goalx,goaly):
rospy.init_node("node", anonymous="True")
self.goalX = rospy.get_param("x1")
self.goalY = rospy.get_param("y1")
self.pub = rospy.Publisher(nodeName + '/cmd_vel',Twist,queue_size=10)
self.sub = rospy.Subscriber(nodeName+"/odom",Odometry,callback=self.update_pose)
self.odometry = Odometry()
self.rate = rospy.Rate(10)
if __name__ == "__main__":
nodeId = sys.argv[1]
try:
nodeName = "robot_"+nodeId
x = Turtlebot(nodeName,0,0)
x.move2goal()
rospy.spin()
...
Launch File:
<param name="x1" type="int" value="4" />
<param name="y1" type="int" value="0" />
<param name="x2" type="int" value="5" />
<param name="y2" type="int" value="0" />
<node pkg="stage_ros" name="stageros" type="stageros" args="$(find beginner_tutorials)/world/worldfile.world" output="screen"/>
<!-- <node pkg="beginner_tutorials" name="rotate1" args="0" type="projecttask2.py"/> -->
<node pkg="beginner_tutorials" name="rotate2" args="1" type="projecttask2.py"
Error: KeyError('x1')

Answer 1

You're running into this issue because of parameter namespacing. When using rospy.get_param("x1") you're checking pulling the param with respect to the relative namespace. This means you're actually checking for a param at <node_namespace>/x1. However in your node you're not declaring the param under your node; this means it's setup under the global namespace. You have two options to fix this. First, you can simply declare the params within your node's namespace like so:
<node pkg="stage_ros" name="stageros" type="stageros" args="$(find beginner_tutorials)/world/worldfile.world" output="screen">
<param name="x1" type="int" value="4" />
<param name="y1" type="int" value="0" />
<param name="x2" type="int" value="5" />
<param name="y2" type="int" value="0" />
<node/>
Secondly, you could fix the namespacing inside your node so that it pulls params from the global namespace like this:
def__init__(self,nodeName,goalx,goaly):
rospy.init_node("node", anonymous="True")
self.goalX = rospy.get_param("/x1")
self.goalY = rospy.get_param("/y1")
For more information regarding namespaces you can check the [ros wiki page].1