how to access the msg in the subscriber callback in ros2 rclpy

edit

Hello,

I am trying to run the following ros2 node. however, it seems that I cannot access the msg inside the callback function.

import rclpy
from rclpy.node import Node

import time
from std_msgs.msg import String


class MinimalSubscriber(Node):

    def __init__(self):
        super().__init__('minimal_subscriber')
        self.subscription = self.create_subscription(
            String,
            'topic',
            self.listener_callback,
            10)
        self.subscription  # prevent unused variable warning
        self.topub = 0

    def listener_callback(self, msg):
        self.topub = msg.data

    def printmsg (self):
        while True:
            self.get_logger().info('I heard: "%s"' % self.topub)
            time.sleep(0.5)

def main(args=None):
    rclpy.init(args=args)

    minimal_subscriber = MinimalSubscriber()
    minimal_subscriber.printmsg()

    rclpy.spin(minimal_subscriber)


if __name__ == '__main__':

    main()

if anyone has a clue, please help.

Thank you