CODEWITHSUNDEEP
pythoncallbackros
maven
maven

Reputation: 29

Store value of a callback function in Python

I am trying to store the content of a callback function, in order to access and manipulate the data within the script. As far as I am concerned, the given function .subscribe() in my code below does not return anything (None). My function is only passed as a reference to the function as an argument. Is there a way to return the data from the function that calls my function?

My code is a simple example with roslibpy (a library for Python that interacts with the open-source robotics framework ROS through Websockets). It is mentioned, that the data is published as a stream via a Websocket each time a message is published into the topic /turtle1/pose. My goal here is to return the data that is being published into the topic. The print command provides a nice visualization of the data, which just works fine.

import roslibpy
  
client = roslibpy.Ros(host='localhost', port=9090)
client.run()

def myfunc(msg):
    print(msg)
      
listener = roslibpy.Topic(client, '/turtle1/pose', 'turtlesim/Pose')

#my function is passed as an argument
listener.subscribe(myfunc)

try:
    while True:
         pass
except KeyboardInterrupt:
    client.terminate()

The subscribe() method in the roslibpy library is defined as follows:

def subscribe(self, callback):
    """Register a subscription to the topic.

    Every time a message is published for the given topic,
    the callback will be called with the message object.

    Args:
        callback: Function to be called when messages of this topic are published.
    """
    # Avoid duplicate subscription
    if self._subscribe_id:
        return

    self._subscribe_id = 'subscribe:%s:%d' % (
        self.name, self.ros.id_counter)

    self.ros.on(self.name, callback)
    self._connect_topic(Message({
        'op': 'subscribe',
        'id': self._subscribe_id,
        'type': self.message_type,
        'topic': self.name,
        'compression': self.compression,
        'throttle_rate': self.throttle_rate,
        'queue_length': self.queue_length
    }))

Is there common way to deal with such problems? Does it make more sense to store the output as an external source (e.g. .txt) and then access the source trough the script?

Upvotes: 1

Views: 2832

Answers (1)

Fricative Melon
Fricative Melon

Reputation: 389

You can define a Python class that acts like a function, that can modify its own state when called, by defining the magic __call__ method. When obj(whatever) is done on a non-function obj, Python will run obj.__call__(whatever). subscribe only needs its input to be callable; whether it is an actual function or an object with a __call__ method does not matter to subscribe.

Here's an example of what you could do:

class MessageRecorder():
    def __init__(self):
        self.messages = []

    # Magic python 'dunder' method
    # Whenever a MessageRecorder is called as a function
    # This function defined here will be called on it
    # In this case, adds the message to a list of received messages
    def __call__(self, msg):
        self.messages.append(msg)

recorder = MessageRecorder()

# recorder can be called like a function
recorder("Hello")

listener.subscribe(recorder)

try:
    while True:
         pass
except KeyboardInterrupt:
    client.terminate()

"""Now you can do whatever you'd like with recorder.messages,
which contains all messages received before termination,
in order of reception. If you wanted to print all of them, do:"""
for m in recorder.messages:
    print(m)

Upvotes: 1

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