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Is there a way to subscribe to a topic without setting the type?

asked 2012-06-19 18:13:21 -0600

updated 2014-01-28 17:12:44 -0600

ngrennan gravatar image

I try to create some UI to allow users to select a topic and visualize the data published on that topic. To get started I wanted to have only one field for the topic name and connect to it with python. In theory it should be possible to receive callbacks without knowing the message type since the messages are introspectable in python using:


I tried to use the Subscriber class but that one enforces the use of a data class when you subscribe to a topic.

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answered 2012-06-22 11:13:59 -0600

You can try and use the internal rostopic API directly to list topic names and detect the topic type (see the code API page for rostopic here)

The only drawback of using the API directly is that code breaking changes may be made to it in the future. If you are worried about this, you could potentially try and parse the output of the rostopic command, as this command line tool is stable.

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answered 2012-06-23 08:41:31 -0600

SL Remy gravatar image

piyushk is correct, but according to the wiki, "rostopic is a stable command-line tool within the ROS core toolchain. The underlying code may undergo refactoring for easier library use, but the external API is expected to be fairly stable."

Using functions like:


data_type = rostopic.get_topic_type('my_topic', blocking=False)[0]
if data_type:
      data_class = roslib.message.get_message_class(data_type)
      #... do fun things here

Now, according to REP100 roslib.message is supposed to be moved to rosgraph but I can't find it there, even in fuerte.

I hope that this was really a reference to something else.. because there is sooo much code that is still using roslib.message...

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Thank you! To get the data class you can use "rostopic.get_topic_class('topic')" directly.

Felix Kaser gravatar imageFelix Kaser ( 2012-06-24 13:54:13 -0600 )edit

Thank you!To get the data class you can use "rostopic.get_topic_class('topic')" directly.

Felix Kaser gravatar imageFelix Kaser ( 2012-06-24 13:54:13 -0600 )edit

answered 2019-06-05 07:32:15 -0600

Davide Faconti gravatar image

For your information, there is a package in C++ that does exactly this:

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answered 2019-05-22 04:28:45 -0600

KindDragon gravatar image

import rospy import sys

from importlib import import_module

class Listener(object):
    def __init__(self):
        self._binary_sub = rospy.Subscriber(
            'some_topic', rospy.AnyMsg, self.topic_callback)

    def topic_callback(self, data):
        assert sys.version_info >= (2,7) #import_module's syntax needs 2.7
        connection_header =  data._connection_header['type'].split('/')
        ros_pkg = connection_header[0] + '.msg'
        msg_type = connection_header[1]
        print 'Message type detected as ' + msg_type
        msg_class = getattr(import_module(ros_pkg), msg_type)
        msg = msg_class().deserialize(self._buff)

        print data.known_field

Based on article

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Asked: 2012-06-19 18:13:21 -0600

Seen: 1,438 times

Last updated: Jun 05