ipywidgets: implement buffers from client to server

[williamstein]

For example, I think the FileUpload widget uses this:

import ipywidgets as widgets

w = widgets.FileUpload(
    accept='',  # Accepted file extension e.g. '.txt', '.pdf', 'image/*', 'image/*,.pdf'
    multiple=False  # True to accept multiple files upload else False
)

---

w

---

w.metadata

All the client stuff works, but the file isn't actually uploaded and created in the project. Vidar says this is probably because I haven't implemented Buffer's for sending data from the client to the server (only the other way around). See also

#6011

NOTE: Interestingly the ipywidgets 8.x release will change the FileUpload API... but that shouldn't impact this: https://github.com/jupyter-widgets/ipywidgets/pull/3501/files#diff-96c1cb3714b9750f89a64c05634b5559ad20428416dc64aab026ce4b6212341d

[williamstein]

Binary buffers from client to server are now supported, even with compute servers. That said, for FileUpload right now make sure you upgrade to the 8.x version of ipywidgets:

pip install ipywidgets==8.1.3

since that what we are using on the frontend and the protocol changed (so 7.x fileupload won't work). And obviously restart your project if it has been running forever, and refresh your browser. Then:

import ipywidgets as widgets
print(widgets.__version__)
w = widgets.FileUpload(accept='.png', multiple=True); w

print(w.value)
widgets.Image(value=w.value[0].content.tobytes())