[Solved] Error when subscribing to multiple topics in python

Question

Hello, I'm new to ROS and I'm having a problem when I try to move my turtle to a goal that I'm publishing. I created a node that subscribes to 2 topics: the position of the turtle and the goal_position. The goal_position topic is being published in the terminal. The node also publishes the velocity of the turtle. The purpose of this node is that when the turtle arrives at the goal position, it will then go to the following goal. I tried to this code in python without using class in the following code, however I received the error:

NameError: global name 'goal_pose' is not defined
NameError: global name 'pose' is not defined

What I don't understand is that I've defined them both.

#!/usr/bin/env python

import rospy
from geometry_msgs.msg import Twist
from turtlesim.msg import Pose
from math import po`enter code here`w, atan2, sqrt
import angles, math

goal = False

def update_pose(data):
    global pose 
    pose = data
    pose.x = round(pose.x, 4)
    pose.y = round(pose.y, 4)

def update_goal(data1):
    global goal_pose
    goal_pose = data1
    goal_pose.x = round(goal_pose.x, 4)
    goal_pose.y = round(goal_pose.y, 4)

def turtleWalk():
    global pose,pub,vel_msg,goal_pose,goal
    linear_gain = 4
    angular_gain = 12
    distance_tolerance = 0.1
    vel_msg = Twist()
    
    while not goal:
        distance  = round(sqrt(pow((goal_pose.x - pose.x), 2) + pow((goal_pose.y - pose.y), 2)), 3)
        linear_vel = linear_gain*distance   
        steering_angle = round(atan2(goal_pose.y - pose.y, goal_pose.x - pose.x),4) 
        angular_vel = angular_gain*(angles.shortest_angular_distance(pose.theta,steering_angle))
        while distance >= distance_tolerance:
            
            vel_msg.linear.x = linear_vel
            vel_msg.angular.z = angular_vel
            
            pub.publish(vel_msg)
        goal = True 

def turtleController(): 
    global pub,vel_msg,goal_pose,pose

    rospy.init_node('turtle_controller',anonymous=True)
    pub = rospy.Publisher('/turtle1/cmd_vel',Twist,queue_size=10)
    rospy.Subscriber('/turtle1/pose',Pose,update_pose)
    rospy.Subscriber('/goal',Pose,update_goal)
    rate = rospy.Rate(10)

    while not rospy.is_shutdown():
        turtleWalk()
        rate.sleep()
    vel_msg.linear.x = 0        
    vel_msg.angular.z = 0
    pub.publish(vel_msg)

if __name__ == '__main__':
    try:
        turtleController()
    except rospy.ROSInterruptException:
        pass

The error happens in line 31 which is :

  distance  = round(sqrt(pow((goal_pose.x - pose.x), 2) + pow((goal_pose.y - pose.y), 2)), 3)

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Originally posted by Gi99 on ROS Answers with karma: 5 on 2019-05-21

Post score: 0

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Original comments

Comment by gvdhoorn on 2019-05-21:
Don't paraphrase error messages. Copy-paste the exact and complete error message into your question.

Line nrs are important here, and the backtrace you get from Python would show us those.

Comment by Gi99 on 2019-05-21:
Thanks!! I put rospy.sleep(10.) and it worked!

Comment by jayess on 2019-05-21:
@Gi99 perhaps you can put your solution as an answer and accept it?

Answer 1

You initialized goal, but not goal_pose or pose at the top. If you call turtleWalk() before any of your subscribers have had a chance to run, then those variables will still be undefined. I'd say you should either define them with a default value, or ensure your callbacks have run before calling turtleWalk().

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Originally posted by jarvisschultz with karma: 9031 on 2019-05-21

This answer was ACCEPTED on the original site

Post score: 0