Class attributes not getting called outside the class in ROS

Question

I was doing in a project where i have to take values from two subscribers then fulfill some conditions and then publish data. Im using classes to values for the two subscribers. However, whenever In the attribute move_bot i call self.ranges and self.linear_pose which should work as they have been set when the code subscribed to them. However, i have to call the move_bot attribute within the laser_callback to make it work but now it only recognizes the self.ranges and not the self.linear_pose. If i do place the move_bot in the odom_callback the same thing happens but with self.linear_pose. Why are the callbacks not working?

import rospy
import math
from sensor_msgs.msg import LaserScan
from geometry_msgs.msg import Twist
from nav_msgs.msg import Odometry

class Move(object):
  def __init__(self):
      rospy.init_node('robot_maze')
      rospy.Subscriber('/kobuki/laser/scan',LaserScan,self.laser_callback)
      rospy.Subscriber('/odom',Odometry,self.odom_callback)
      self.pub = rospy.Publisher('/cmd_vel', Twist, queue_size=1)
      self.twist_angle = Twist()

  def laser_callback(self,msg):
      self.ranges = msg.ranges

  def odom_callback(self,msg):
      self.linear_pose = msg 

  def move_bot(self):
      pi = math.pi
      kp = -0.027

      val =[]

      if self.linear_pose.twist.twist.linear.x < 0.001:
         self.twist_angle.linear.x = -0.2
         for i in self.ranges:
             if i <= 0.6:
                  if self.ranges.index(i) > 360:
                      self.twist_angle.angular.z = 1
             else:
                  self.twist_angle.angular.z = -1
             break
      elif self.ranges[360] > 1.2:
         self.twist_angle.angular.z = 0
         self.twist_angle.linear.x = 0.5
      else:
         for i in self.ranges:
            if i > 1.2:
               val.append(i)    
         angle = self.ranges.index(max(val))
         print(max(val))


         if self.ranges[angle] > self.ranges[360]:
            self.twist_angle.angular.z = kp * (360 - angle)

      self.pub.publish(self.twist_angle)




if  __name__== '__main__':

  try:
      move = Move()
      move.move_bot()
      rate = rospy.Rate(10)
  except rospy.ROSInterruptException:  pass

  while not rospy.is_shutdown():
     rate.sleep()

I get the following error message

  Traceback (most recent call last):
    File "/home/user/catkin_ws/src/robot_maze/src/robot_maze.py", line 100, in <module>
      move.move_bot()
    File "/home/user/catkin_ws/src/robot_maze/src/robot_maze.py", line 45, in move_bot
      if self.ranges[360] > 1.2:
  AttributeError: 'Move' object has no attribute 'ranges'
  [robot_maze-1] process has died [pid 16344, exit code 1, cmd 
  /home/user/catkin_ws/src/robot_maze/src/robot_maze.py 
  __name:=robot_maze __log:=/home/user/.ros/log/e4216cee-9f0e-11ea-9fd8-06b6df32753c/robot_maze- 
  1.log].
  log file: /home/user/.ros/log/e4216cee-9f0e-11ea-9fd8-06b6df32753c/robot_maze-1*.log

Seems like it takes some time for the callback to be invoked. So either you can supply a sensible default for `ranges` or you wait until is has been called for the first time. — a_guest, May 27 '20 at 07:00
It should be sufficient to wait for the first measurement, before taking the first action. — a_guest, May 28 '20 at 14:04

score 0 · Answer 1 · answered May 29 '20 at 14:50

To point out what @a_guest said in the comments:

class Move(object):
  def __init__(self):
      ...
      self.ranges = None
      self.linear_pose = None
  ...
  def move_bot(self):
      if self.ranges is not None and self.linear_pose is not None:
          if self.linear_pose.twist.twist.linear.x < 0.001:
              ...
          elif self.ranges[360] > 1.2:
              ...
          else:
              ...
          self.pub.publish(self.twist_angle) # Don't publish until received initial values

Yeah i tried initalizing it before but did not add the condition for them being not none. Thanks ill try and report — Abhinash Bharat, May 30 '20 at 05:18

Class attributes not getting called outside the class in ROS

1 Answers1