Updating a widget in the background as advertised in the doc fails

[jmandreoli]

Description

Updating a widget in the background, as advertised in the doc, fails.

Reproduce

Open a notebook and copy-paste the code (from the documentation):

import threading
from IPython.display import display
import ipywidgets as widgets
import time
progress = widgets.FloatProgress(value=0.0, min=0.0, max=1.0)

def work(progress):
    total = 100
    for i in range(total):
        time.sleep(0.2)
        progress.value = float(i+1)/total

thread = threading.Thread(target=work, args=(progress,))
display(progress)
thread.start()

Running it yields an error:

Exception in thread Thread-5 (work):
Traceback (most recent call last):
  File "[/storage/tools/anaconda/202501/lib/python3.12/threading.py", line 1075](https://localhost:8962/storage/tools/anaconda/202501/lib/python3.12/threading.py#line=1074), in _bootstrap_inner
    self.run()
  File "[/storage/tools/anaconda/202501/lib/python3.12/site-packages/ipykernel/ipkernel.py", line 788](https://localhost:8962/storage/tools/anaconda/202501/lib/python3.12/site-packages/ipykernel/ipkernel.py#line=787), in run_closure
    _threading_Thread_run(self)
  File "[/storage/tools/anaconda/202501/lib/python3.12/threading.py", line 1012](https://localhost:8962/storage/tools/anaconda/202501/lib/python3.12/threading.py#line=1011), in run
    self._target(*self._args, **self._kwargs)
  File "[/tmp/ipykernel_60690/3988194817.py", line 11](https://localhost:8962/tmp/ipykernel_60690/3988194817.py#line=10), in work
  File "[/storage/tools/anaconda/202501/lib/python3.12/site-packages/traitlets/traitlets.py", line 716](https://localhost:8962/storage/tools/anaconda/202501/lib/python3.12/site-packages/traitlets/traitlets.py#line=715), in __set__
    self.set(obj, value)
  File "[/storage/tools/anaconda/202501/lib/python3.12/site-packages/traitlets/traitlets.py", line 706](https://localhost:8962/storage/tools/anaconda/202501/lib/python3.12/site-packages/traitlets/traitlets.py#line=705), in set
    obj._notify_trait(self.name, old_value, new_value)
  File "[/storage/tools/anaconda/202501/lib/python3.12/site-packages/traitlets/traitlets.py", line 1513](https://localhost:8962/storage/tools/anaconda/202501/lib/python3.12/site-packages/traitlets/traitlets.py#line=1512), in _notify_trait
    self.notify_change(
  File "[/storage/tools/anaconda/202501/lib/python3.12/site-packages/ipywidgets/widgets/widget.py", line 700](https://localhost:8962/storage/tools/anaconda/202501/lib/python3.12/site-packages/ipywidgets/widgets/widget.py#line=699), in notify_change
    self.send_state(key=name)
  File "[/storage/tools/anaconda/202501/lib/python3.12/site-packages/ipywidgets/widgets/widget.py", line 586](https://localhost:8962/storage/tools/anaconda/202501/lib/python3.12/site-packages/ipywidgets/widgets/widget.py#line=585), in send_state
    self._send(msg, buffers=buffers)
  File "[/storage/tools/anaconda/202501/lib/python3.12/site-packages/ipywidgets/widgets/widget.py", line 825](https://localhost:8962/storage/tools/anaconda/202501/lib/python3.12/site-packages/ipywidgets/widgets/widget.py#line=824), in _send
    self.comm.send(data=msg, buffers=buffers)
  File "[/storage/tools/anaconda/202501/lib/python3.12/site-packages/comm/base_comm.py", line 144](https://localhost:8962/storage/tools/anaconda/202501/lib/python3.12/site-packages/comm/base_comm.py#line=143), in send
    self.publish_msg(
  File "[/storage/tools/anaconda/202501/lib/python3.12/site-packages/ipykernel/comm/comm.py", line 42](https://localhost:8962/storage/tools/anaconda/202501/lib/python3.12/site-packages/ipykernel/comm/comm.py#line=41), in publish_msg
    parent=self.kernel.get_parent(),
           ^^^^^^^^^^^^^^^^^^^^^^^^
  File "[/storage/tools/anaconda/202501/lib/python3.12/site-packages/ipykernel/kernelbase.py", line 797](https://localhost:8962/storage/tools/anaconda/202501/lib/python3.12/site-packages/ipykernel/kernelbase.py#line=796), in get_parent
    return self._shell_parent.get()
           ^^^^^^^^^^^^^^^^^^^^^^^^
LookupError: <ContextVar name='shell_parent' at 0x7fc4699136f0>

Expected behavior

What is described in the doc should happen: the progress bar gets updated at regular intervals. It used to work. Started to fail since a recent conda update --all.

Context

• ipywidgets version 8.1.7
• Operating System and version: Fedora 42
• Browser and version: Firefox 143 (same error on jupyterlab desktop)

If using JupyterLab

• JupyterLab version: JupyterLab v4.4.9

Installed Labextensions

        jupyterlab_pygments v0.3.0 enabled OK (python, jupyterlab_pygments)
        jupyter-leaflet v0.20.0 enabled OK
        jupyter-matplotlib v0.11.8 enabled OK
        @jupyter-widgets/jupyterlab-manager v5.0.15 enabled OK (python, jupyterlab_widgets)
        @jupyter-notebook/lab-extension v7.4.7 enabled OK
Other labextensions (built into JupyterLab)

[jmandreoli]

Sorry guys, the issue seems to be with ipykernel:7.0. I backtracked to ipykernel:6.31 and it worked fine. I leave this issue open so people who have the same problem can see it (and the fix) but feel free to close it.

[martinRenou]

cc. @ianthomas23

[ianthomas23]

Thanks for the bug report. This was a problem introduced in ipykernel 7.0.0 and I've confirmed that it is fixed by ipython/ipykernel#1451 and will be in the next release of ipykernel (7.1.0) due out very soon.