error pickling_unable to pickle : attribution look up failed

edit

This a node for a new experiment that I'm doing and it collects the data from tf topic while running a turtlebot3_gazebo_world.launch and stores it's value as a txt/pkl file in form of serialized list of objects.

import rospy
import pickle
from teleop_record.msg import TB3Pose

msg = """

Record Your TurtleBot3 Path!
----------------------------
r - Record current point as step

m - set current point as next milestone path

s - stop and save file

CTRL-C to save & quit

"""

e = """

Communications Failed

"""


tb3_pose = TB3Pose()
path_object_list = []
steps = 0
milestones = 0
file_ext = ".txt"

class path(object):
       def __init__(self, step, pose, milestone):
            self.step = step
            tb3pose = TB3Pose()    
            tb3pose = pose    
            self.pose_x = tb3pose.x    
            self.pose_y = tb3pose.y    
            self.pose_w = tb3pose.theta    
            self.milestone = milestone   


def updatepose(data):    
          global tb3_pose    
          tb3_pose.x = data.x    
          tb3_pose.y = data.y    
          tb3_pose.theta = data.theta    
          #print("i'm here")    
          #rospy.loginfo(tb3_pose)    


def incr_mile():    
  global milestones    
  milestones = milestones + 1    

def record_pose():    
            global steps    
            global milestones    
            global path_object_list    
            steps = steps + 1    
            pose = TB3Pose()    
            pose = tb3_pose    
            x = path(steps,pose,milestones)    
            path_object_list.append(x)    
            print(path_object_list)


def save_path():    
          global path_object_list    
          file_name = input("Define name for the Path file : ")    
          file_path = "/home/paul_pavish/catkin_ws/src/teleop_record/nodes/"#
          file_path_name = file_path + file_name + file_ext    
          print("Creating File..." + file_path_name)    
          open_file = open(file_path_name, "wb")    
          print("Writing Path to file...")    
          pickle.dump(path_object_list, open_file)    
          open_file.close()    
          print("Path file saved as " + file_path_name)    


def print_list():    
  x = 0    
  global steps    
  #while x < steps:    
  #  print(path_object_list[x])    
  #  x = x + 1

  for obj in path_object_list:    
        print(obj)


def subs():    
  print(msg)    
  rospy.init_node('turtlebot3_recorder')    
  sub = rospy.Subscriber('TB3Poser', TB3Pose, updatepose)   

  while(1):
        key = input('')    
        if key == 'r':
          record_pose()    
          print("recording point")

        elif key == 'm':
          print("Recording milestone")
          incr_mile()
          record_pose()

        elif key == 's':
          print("Exiting record loop..Press ctrl+s to save and exit.")    
          save_path()    
          break

        else:    
          print("INVALID COMMAND")

  rospy.spin()


if __name__=="__main__":

   try:    
            subs()    
            #print_list()

   except rospy.ROSInterruptException:
            pass

The program runs well collecting values and storing them as a list(even printing the list of objects), but it enters the save_path() function and reaches pickle.dump() it shows this error :

Record Your TurtleBot3 Path!
----------------------------

r - Record current point as step
m - set current point as next milestone path
s - stop and save file


CTRL-C to save & quit

r
[<__main__.path object at 0x7ff1323c3e20>]
recording point
r
[<__main__.path object at 0x7ff1323c3e20>, <__main__.path object at 0x7ff1323f92e0>]
recording point
r
[<__main__.path object at 0x7ff1323c3e20>, <__main__.path object at 0x7ff1323f92e0>, <__main__.path object at 0x7ff1323f9070>]
recording point
m
Recording milestone
[<__main__.path object at 0x7ff1323c3e20>, <__main__.path object at 0x7ff1323f92e0>, <__main__.path object at 0x7ff1323f9070>, <__main__.path object at 0x7ff1323f9820>]
s
Exiting record loop..Press ctrl+s to save and exit.
Define name for the Path file : sd
Creating File.../home/paul_pavish/catkin_ws/src/teleop_record/nodes/sd.txt
Writing Path to file...
Traceback (most recent call last):
  File "/home/paul_pavish/catkin_ws/devel/lib/teleop_record/teleop_record_node.py", line 15, in <module>
    exec(compile(fh.read(), python_script, 'exec'), context)
  File "/home/paul_pavish/catkin_ws/src/teleop_record/nodes/teleop_record_node.py ...