Receiving binary buffers from frontend fails

[vidartf]

If receiving buffers from the front end, the following line fails: https://github.com/jupyter-widgets/ipywidgets/blob/master/ipywidgets/widgets/widget.py#L601

The error is TypeError: <memory at 0x000001E2CA4F8708> is not JSON serializable.

An outline of the trace is:

• Comm message with buffer received. Calls set_state.
• set_state:
  • Locks the properties.
  • Deserializes the data (to numpy array in this case).
  • Calls set_trait.
• notify_change calls _should_send_property.
• to_json (my serializer) gives back a dict with a memoryview.
• jsondumps chokes on the memoryview.

[vidartf]

A possible solution is to do the comparison in the the serialized state after calls to _separate_buffers. The buffers can be compared by instance (a is b) or possibly by numpy (that should maybe be left as an extension point for custom comparison).

[jasongrout]

Ah yes, good catch. That is from #1523. I like your solution.

We've tried to have a lot better testing around the syncing framework on the javascript side. We should have a unit test for this on the python side too - that would have caught this when the PR was merged.

[vidartf]

I'll PR a fix later. Gonna try to add a test as well!

[jasongrout]

Awesome, thanks!