Using variable from rostopic in another python file

Question

I am new at using ROS and I want to use an int that is received by a topic and use it in another python file. The listener.py receives the correct int but when i use it in another file it won't update and just print None. I don't know what I'm doing wrong.

listener.py

#!/usr/bin/env python

import rospy
from std_msgs.msg import Int32

class listenerPy:

    def __init__(self):
        self.received_integer = None

    def callback(self, data):      

        self.received_integer = data.data
        print(self.received_integer)     
        
    def listener(self):
        rospy.init_node('listener', anonymous=True)        
        rospy.Subscriber('chatter', Int32, self.callback)
        rospy.spin()

if __name__ == '__main__':
    try:
        listener = listenerPy()  # Instantiate the listenerPy class
        listener.listener()  # Call the listener method on the instance
    except rospy.ROSInterruptException:
        pass

other file

#!/usr/bin/env python

from listener import listenerPy
from time import sleep

def main():
    
    local_var = listenerPy() 
    while True:
        value = local_var.received_integer
        print(value)
        sleep(1)
            



if __name__ == '__main__':
    try:
        main()
    except KeyboardInterrupt:
        pass 

Answer 1

If you want to run both files standalone, you need some sort of shared memory. However, in your example, you need to actually start the listener in your other file.

def main():
    local_var = listenerPy()
    local_var.listener() 
    while True:
        value = local_var.received_integer
        print(value)
        sleep(1)

However, you call rospy.spin() in your ros node, which will block. So you should remove that call from your node.