Incompatible QoS on topic /battery_state

[chrisbuerginrogers]

So I am trying to write a very simple subscriber - but get the error [WARN] [1643999034.968400508] [minimal_subscriber]: New publisher discovered on topic '/battery_state', offering incompatible QoS. No messages will be received from it. Last incompatible policy: RELIABILITY
when I try to "spin it up" - do I have to set the QoS level in the creation of the subscription?

import sys,rclpy
from rclpy.node import Node
from sensor_msgs.msg import BatteryState
from irobot_create_msgs.msg import Button

class MinimalSubscriber(Node):

def __init__(self):
    super().__init__('minimal_subscriber')
    self.subscription = self.create_subscription(
        BatteryState,'/battery_state',self.listener_callback,10)
    self.subscription  # prevent unused variable warning

def listener_callback(self, msg):
    self.get_logger().info('I heard: "%s"' % msg.data)

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

minimal_subscriber = MinimalSubscriber()
try: 
    rclpy.spin(minimal_subscriber)
except KeyboardInterrupt:
    print('Caught keyboard interrupt')
except BaseException:
    print('Exception:', file=sys.stderr)
finally:
    minimal_subscriber.destroy_node()
    rclpy.shutdown()

[justinIRBT]

An answer to this question is explained in more detail here:
https://answers.ros.org/question/332207/cant-receive-data-in-python-node/

The short answer is instead of using 10 in the create_subscription call, use:

from rclpy.qos import qos_profile_sensor_data

self.subscription = self.create_subscription(
        BatteryState,'/battery_state',self.listener_callback,qos_profile_sensor_data)

[chrisbuerginrogers]

Thanks - that definitely got me farther - but it still does not receive anything - instead I get an Exception (with nothing returned from sys.stderr). Any idea why? thanks

[justinIRBT]

The problem is in your listener_callback logger printout:
self.get_logger().info('I heard: "%s"' % msg.data)
BatteryState doesn't have a data field. If you just print the msg, it works for me
self.get_logger().info('I heard: "%s"' % msg)

[chrisbuerginrogers]

Oh shoot - right - that worked great - sorry - too focussed on other parts of the code...