How does "stream" message, sent as reply to comm, bound to cell?

[Ilya Kazakevich]

Hello,

I have the following code

```

btn = widgets.Button(description='Medium')
display(btn)
def btn_eventhandler(obj):
    print('Hello from the {} button!'.format(obj.description))
btn.on_click(btn_eventhandler)

``` 

When I click on the button, it sends comm message to python side.

Python replies with "stream" message.

As I understand, button widget uses it's model to send comm message.

Model may have more than one view. 

So, how does jupyter know which cell should be used to output this stream?

I know that for regular code execution it registers callback and uses "parent" header of reply, and I am sure something similar takes place here, but I don't see any callback registered by view nor by model.

Thank you.

Ilya.

[Jason Grout]

The view is the one to actually send the comm message from js: https://github.com/jupyter-widgets/ipywidgets/blob/a311d753385f0867c34525eacad9496108e43cc1/packages/controls/src/widget_button.ts#L124

In the classic Notebook, the view registers a display callback that puts any output coming back from the kernel in the cell containing the view. The process for getting that callback right is a bit convoluted, involving asking the widget manager for the right callback, etc. The relevant code is at https://github.com/jupyter-widgets/ipywidgets/blob/a311d753385f0867c34525eacad9496108e43cc1/widgetsnbextension/src/manager.js#L311-L335

In JupyterLab, right now nothing is done with the output, so the output essentially disappears: . We are working on a logging extension (hopefully merged for jlab 1.2) that will log a message with any output returned from the kernel.

In the docs, we also encourage people to use the OutputWidget to explicitly capture output if that's what you want: https://ipywidgets.readthedocs.io/en/latest/examples/Output%20Widget.html#Debugging-errors-in-callbacks-with-the-output-widget

Thanks,

Jason

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[Ilya Kazakevich]

Thank you, Jason.

It is clear now.

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