Pass an object to Service handler callback() function

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Hello

is there a way to pass objects to a callback()? I just found this post: https://answers.ros.org/question/2314...

but if I pass the object like mentioned in the post I get this error message:

Traceback (most recent call last):
  File "/opt/ros/noetic/lib/python3/dist-packages/rospy/impl/tcpros_base.py", line 737, in receive_once
    self.stat_bytes += recv_buff(sock, b, p.buff_size)
  File "/opt/ros/noetic/lib/python3/dist-packages/rospy/impl/tcpros_base.py", line 104, in recv_buff
    d = sock.recv(buff_size)
TypeError: 'AiService' object cannot be interpreted as an integer

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib/python3.8/threading.py", line 932, in _bootstrap_inner
    self.run()
  File "/usr/lib/python3.8/threading.py", line 870, in run
    self._target(*self._args, **self._kwargs)
  File "/opt/ros/noetic/lib/python3/dist-packages/rospy/impl/tcpros_service.py", line 667, in handle
    requests = transport.receive_once()
  File "/opt/ros/noetic/lib/python3/dist-packages/rospy/impl/tcpros_base.py", line 758, in receive_once
    raise TransportException("receive_once[%s]: unexpected error %s"%(self.name, str(e)))
rospy.exceptions.TransportException: receive_once[/image_ack]: unexpected error 'AiService' object cannot be interpreted as an integer

My code (shortened):

def callback(request, arg):
    response = ImageAckResponse()
    ai_machine = arg[0]
    ...
    return response

def main():
    rospy.init_node('ai_service')
    ai_machine = AiMachine()
    ...
    rospy.Service('image_ack', ImageAck, callback, (ai_machine))
    rospy.spin()