Widget doesn't load if c.LabServerApp.notebook_starts_kernel = False

[lstngr]

Describe the issue

I am unsure if this should be reported in https://github.com/jupyterlab/jupyterlab or here.
Please feel free to let me know and close the issue should this be the case.

Widgets do not load if JupyterLab is started with the following configuration,

# cat ~/.jupyter/jupyter_lab_config.py
c = get_config()  #noqa
c.LabServerApp.notebook_starts_kernel = False

In particular, running the documentation's example,

%matplotlib ipympl
import matplotlib.pyplot as plt
import numpy as np

fig, ax = plt.subplots()


x = np.linspace(0, 2*np.pi, 100)
y = np.sin(3*x)
ax.plot(x, y)

produces the following output (please note that the kernel disconnected because I shut it down, it otherwise remains running),

and results in the following error in the console,

06:59:36.073 Exception opening new comm 4 jlab_core.a4c5e1f5bac9ba5dc7f6.js:1:1401914
    _handleCommOpen http://localhost:8888/static/lab/jlab_core.a4c5e1f5bac9ba5dc7f6.js?v=a4c5e1f5bac9ba5dc7f6:1

I could reproduce the issue in Firefox 136.0.2 (on Arch Linux), Firefox ESR 128.8.0 (on OpenSUSE Leap 15.6), Chromium 134.0.6998.117 (on OpenSUSE Leap 15.6).

If the jupyterlab configuration is modified such that c.LabServerApp.notebook_starts_kernel = True, the widgets display correctly.

Versions

# python -c "import sys; print('\n',sys.version); import ipympl; print('ipympl version:', ipympl.__version__)" && jupyter --version && jupyter labextension list

 3.13.2 | packaged by conda-forge | (main, Feb 17 2025, 14:10:22) [GCC 13.3.0]
ipympl version: 0.9.7
Selected Jupyter core packages...
IPython          : 8.34.0
ipykernel        : 6.29.5
ipywidgets       : 8.1.5
jupyter_client   : 8.6.3
jupyter_core     : 5.7.2
jupyter_server   : 2.15.0
jupyterlab       : 4.3.6
nbclient         : 0.10.2
nbconvert        : 7.16.6
nbformat         : 5.10.4
notebook         : not installed
qtconsole        : not installed
traitlets        : 5.14.3
JupyterLab v4.3.6
/home/me/miniconda3/envs/phys206/share/jupyter/labextensions
        jupyterlab_pygments v0.3.0 enabled OK (python, jupyterlab_pygments)
        jupyter-matplotlib v0.11.7 enabled OK
        @jupyter-widgets/jupyterlab-manager v5.0.13 enabled OK (python, jupyterlab_widgets)

which has been installed from conda with: conda install -c conda-forge jupyterlab ipympl.

[ianhi]

Hi @lstngr is this true for all ipywidgets, or just for ipympl? i.e. can you check with an ipywidgets.IntSlider()?

[lstngr]

Thanks for the swift reply, the issue also occurs for ipywidgets with the same console error.

from IPython.display import display
import ipywidgets as widgets

w = widgets.IntSlider()
display(w)

I assume this means it's rather a JupyterLab issue?

[ianhi]

Yes, It''s either a jupyterlab or an ipywidgets issue. So not somethign that can be fixed here unfortunately. I would suggest opening an issue on the ipywidgets repo

[lstngr]

Thank you very much for your help! 🙏