ipywidgets expects Comm, not BaseComm

[matthewturk]

Hi! I've been trying to track down an issue with my usage of ipywidgets, described on discourse. The basic gist is:

• I'm attempting to create a widget from the client by using a model.widget_manager's method new_widget. In previous versions, this worked, but now, it doesn't.
• The issue seems to stem from the kernel attempting to create a new comm instance for a widget, but supplying to that widget a BaseComm rather than a Comm, which throws a validation error.

After digging through this, it looks to me like ipywidgets declares comm = Instance('ipykernel.comm.Comm', allow_none=True), but ipykernel.ipkernel.create_comm explicitly creates a BaseComm, so the traitlet doesn't validate, and the comm fails to properly initialize. This results in the widgets not being synced back to the kernel from the frontend.

I confess that while I've spent time going through the code, I'm not entirely sure if this could potentially be a version mismatch, or if this is an oversight for a use case that may not come up terribly frequently.

My versions:

• ipywidgets - 8.0.4
• ipykernel - 6.21.1
• comm - 0.1.2

I was able to address the problem I was having by changing the import and usage of BaseComm to Comm in ipkernel.py, and so if this is an acceptable solution I'd be happy to issue a pull request. (My hesitance is that there may be unexpected downstream effects of a change to something so deep in the kernel code, and so I wanted to open the issue as possible first step.)

[blink1073]

Hi @matthewturk, please do open a PR. We test most known downstream consumers in Jupyter and spyder, seeing if those downstream tests fail would be a good indicator.

[maartenbreddels]

I think this is an ipywidget issue.

@matthewturk In jupyter-widgets/ipywidgets#3602 (comment) I removed the comm being an Instance of this class. I think this will fix it, but we never merged it.

[matthewturk]

@maartenbreddels That PR is pretty big though -- does it need to be a part of that, or would a smaller PR that simply loosened restrictions work, so that we don't have to pin all our dependencies to a lower version?

[maartenbreddels]

I think if we just cherry-pick, jupyter-widgets/ipywidgets@c1c34f9 we should be able to get this in sooner.

An even saver option, it to change it to traitlets.Any. I don't think we need the type safety for the comm.

[martinRenou]

I will work on finishing jupyter-widgets/ipywidgets#3533 this morning. This PR uses traitlets.Any and we should backport it to ipywidgets 7

[maartenbreddels]

I think we should backport this indeed