Bidirectional Comm Communication

[Dominic Kuang]

I've been trying to figure out how to use comms to communicate from Python <-> JS. Using the following simple example:

# Cell 1

from IPython.kernel.comm import Comm

def handle_comm_message(message):

    print(message)

comm = Comm('comm-target-name')

comm.on_msg(handle_comm_message)

// Cell 2
%%javascript
 var IPython = window.IPython IPython.notebook.kernel.comm_manager.register_target('comm-target-name', function(comm, data) {
    comm.on_msg(function(msg) { console.log(msg) });
    comm.send({'data': 'sent from the client'})
});


# Cell 3
comm.send(data={'data': 'sent from python'})

I can reliably get Python -> JS to work. In this case, after executing Cell 3, I receive the message in the console containing "Sent from Python" as expected. However, I am unable to trigger the sending from client. I am not sure if I have something set up incorrectly or I am simply misunderstanding how comms work. Any help would be appreciated! Thanks!

[Dominic Kuang]

I've uploaded the notebook here for convenience. http://nbviewer.jupyter.org/gist/finaiized/ad8836b64a874518d32c40e7a4a03966

[Jason Grout]

Try switching your cell one and two.

(Sorry, on phone so can't test)

Thanks,

Jason

On Fri, May 5, 2017, 09:54 Dominic Kuang <fina...@gmail.com> wrote:

I've uploaded the notebook here for convenience. http://nbviewer.jupyter.org/gist/finaiized/ad8836b64a874518d32c40e7a4a03966

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[Dominic Kuang]

Thanks for the reply Jason. I tried switching the cells and randomly trying different execution orders but to no avail. If I'm not wrong, I should expect to see something printed in the output cell from the Python callback being triggered.

[Jason Grout]

Ah, right. In order to see output, you'll need to set the right javascript callbacks to handle the resulting display messages. I think if you make your python handler write to a file, you'll see the output in the file. Alternatively, you can use an Output ipywidget to capture the output:

from ipywidgets import Output

out = Output()

def handle_comm_message(message):

    with out:

        print(message)

and then display the output widget in another cell somewhere

Thanks,

Jason

[Dominic Kuang]

That works perfectly Jason, thanks! I also goofed up by not putting the comm.send inside the on_msg callback for some reason, which is what I had original wanted. Appreciate the help!