ipywidgets 6 + jupyter-js-widgets 7 incompatibility

I ran into an issue when installing ipywidgets to a fresh conda environment and jupyter notebook loading jupyter-js-widgets/extension from /usr/local/share/jupyter vs. /Users/grant/anaconda/envs/mycondaenv/share/jupyter. ipywidgets in the conda environment was 6.0.0 and jupyter-js-widgets/extension was linked to my dev install of ipywidgets (7.0.0).

The issue looked like:

The root of the issue is a breaking change introduced by https://github.com/jupyter-widgets/ipywidgets/pull/1194, specifically https://github.com/jupyter-widgets/ipywidgets/commit/9d896f76d01b23b3b9fb60a2948141f77da769ef#diff-30b37447269c75d7eff843470858ea28, which looks for widget state in msg.content.data.state vs msg.content.data.

1. I remember in previous versions, ipywidgets would return an error message if there as a version mismatch between JS and Python? That would’ve made this issue much easier to debug.
2. Given that this breaks backwards-compatibility, is there a legitimate need to move widget state from msg.content.data to msg.content.data.state? This shouldn’t affect anyone unless they version mismatch between JS and Python, so if we solve #1 then we solve this.
3. Is this the way that jupyter paths are supposed to work? I assumed that an nbextension installed to a conda environment would take precedence over an nbextension installed elesewhere?

[jasongrout]

I’m working on this by implementing a version check for the manager protocol version when a comm is opened. The protocol version will be sent in the metadata of the comm open message.

[jasongrout]

Fixed in #1470 by giving a much clearer message early on that there is a version incompatibility.