How to write my own joint state

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Hello all i have a real robot. using ROS. i have a problem with (Joint State Publisher)

i understand that i should write my own (joint state publisher node). i read the related tutorials but i do not get how to convert encoders' messages to joint state message. i need steps or tutorial for doing that.

thanks in advance.

edit

i test the code in

my code becomes as follows

#!/usr/bin/env python
# Lucas Walter
# make a joint exactly what the command wants it to be- this only works
# for position control.

import rospy

from sensor_msgs.msg import JointState
from std_msgs.msg import Int64


class CommandToJointState:
    def __init__(self):
        self.joint_name = rospy.get_param("lwheel",True)
        self.joint_state = JointState()
        self.joint_state.name.append(self.joint_name)
        self.joint_state.position.append(0.0)
        self.joint_state.velocity.append(0.0)
        self.joint_pub = rospy.Publisher("joint_states", JointState, queue_size=1)
        self.command_sub = rospy.Subscriber("lwheel", Int64,
                                            self.command_callback, queue_size=1)


    def command_callback(self, msg):
        self.joint_state.position[0] = msg.data
        self.joint_state.header.stamp = rospy.Time.now()
        self.joint_pub.publish(self.joint_state)

if __name__ == '__main__':
    rospy.init_node('diffdrive_controller')
    diffdrive_controller = CommandToJointState()
rospy.spin()

but i get error as in the image attached

i am not familiar with python. all my codes are in C++