title: ROS topic is not published yet
body: <p>I have this code which receives data from node (subscribe) and then inside this node I do some
  mathematical operation, then I want to publish the data to a certain topic.</p>
  <pre><code>#!/usr/bin/env python
  
  import rospy
  from std_msgs.msg import String, Float32
  from math import cos,sin
  
  import sys
  import os
  import logging
  from types import *
  import time
  
  class exact_pose():
  
      def callback_car_yaw(self, data):
          self.car_yaw = data
  
      def callback_line_pose(self,data):
          self.line_pose = data
  
          
      def init(self):
        
   
          self.car_yaw = None
          self.line_pose = None
        
  
      def subscriber(self):
          
   
              
          self.car_yaw_sub = rospy.Subscriber('~car/yaw',Float32, self.callback_car_yaw)
              
          self.line_pose_sub = rospy.Subscriber('~line_pose',Float32, self.callback_line_detect_xL15)
  
      def publisher(self):
  
          self.exact_pose_pub = rospy.Publisher('exact_pose',Float32, queue_size=10)
          
  
          rospy.Rate(1)
          rospy.spin()
        
        
          #sloving for exact pose 
  
      def calculation(self,):
          while not rospy.is_shutdown():
  
              self.exact_pose = self.line_pose_sub * cos(self.car_yaw_sub) + self.line_pose_sub * sin(self.car_yaw_sub)
              info1 = Float32()
              info1.data = self.exact_pose
              self.exact_pose_pub.publish(info1)
  
  
              rospy.Rate.sleep()
              rospy.spin()
  
  
  if __name__ == '__main__':
      rospy.init_node('exact_pose', anonymous=True)
      try:
        ne = exact_pose()
  
      except rospy.ROSInterruptException: pass
  
  </code></pre>
  <p>so what the code does, subscribe to a topic from a previous node, then do some mathematical operation for these data inside the topics, then publish the last result on separate Node</p>