widget javascript not installed message appears often

[jasongrout]

The python error message "Widget Javascript not detected. It may not be installed or enabled properly." appears a lot, and has been pretty fickle about appearing for a long time. Let's figure out why and fix the version check.

(Moved from jupyter/notebook#2087)

CC @SylvainCorlay

[jasongrout]

Okay, here's a possible sequence. I haven't tested this.

In notebook 4.4.1 or 5.0:

1. The notebook launches
2. The user quickly presses shift-enter to execute a cell that has widget code.
3. The cell is queued up for execution
4. The widget javascript is loaded. When the widget javascript is loaded, it immediately sends a comm message to the kernel with the version information, etc. This is comm message is queued behind the cell execution. (TODO: Check that the comm open message to jupyter.widget.version is queued behind the cells)
5. The kernel starts up.
6. The cell is executed. Since the kernel widgets has not seen the comm message from the javascript, it assumes there is no widget javascript and so prints an error message: "Widget Javascript not detected. It may not be installed or enabled properly."

Basically, it's a race condition between the widget javascript running before the user executes a cell, and an assumption from the kernel side that the ipywidgets will be able to execute a comm open before any user code runs.

Before jupyter/notebook#1002, here's what I am thinking was happening in the situations where the message seemed to randomly pop up.

1. The notebook launches
2. The user quickly presses shift-enter to execute a cell that has widget code.
3. The execution is silently ignored since the kernel is not up yet.
4. The widget javascript is loaded. When the widget javascript is loaded, it immediately sends a comm message to the kernel with the version information, etc. This is also silently ignored!
5. The kernel starts up.
6. Another cell is executed. Since the kernel widgets has not seen the comm message from the javascript, it assumes there is no widget javascript and so prints an error message: "Widget Javascript not detected. It may not be installed or enabled properly."

It always seemed fickle when the 'js not installed' error came up.

I think the real problem is that in ipywidgets, we are assuming that the widget javascript will have priority access to a kernel, before user execution. I think we probably need a more robust method for doing the version check.

And clearly this ipywidget check is faulty if we have multiple clients talking to the same kernel. The first client will okay its version of javascript, so the error won't show if the second client does not have the javascript installed.

[ialxn]

I am getting these warnings quite regularly too when using hyperspy in an Ipython console of spyder.

In [199]: P = s.find_peaks1D_ohaver(maxpeakn=1)[0]
Widget Javascript not detected. It may not be installed or enabled properly.

The same in a Jupyter Notebook:

jupyter nbextension enable --py --sys-prefix widgetsnbextension
Enabling notebook extension jupyter-js-widgets/extension...
- Validating: OK

And then in the Notebook itself:

In[1] :
import hyperspy.api as hs
s = hs.datasets.example_signals.EDS_TEM_Spectrum()
P = s.find_peaks1D_ohaver()[0]

Widget Javascript not detected. It may not be installed or enabled properly.

I am using a Conda installation on an Ubuntu 16.04 system and installed hyperspy from conda-forge.

uname -a
Linux PC12348 4.4.0-66-generic #87-Ubuntu SMP x86_64 x86_64 x86_64 GNU/Linux

conda list (excerpt):
hyperspy 1.2 py36_0 conda-forge
ipykernel 4.5.2 py36_0
ipython 5.3.0 py36_0
ipython_genutils 0.1.0 py36_0
ipywidgets 6.0.0 py36_0

Does this concern this issue (should be fixed in a next release or am I reading this wrong) or is this a Conda/Anaconda issue?

[jasongrout]

I've removed that message from the next version of ipywidgets. It is a bit concerning that it was appearing in spyder - that indicates that there still might be a problem with hyperspy. Do you expect any widgets to be used in the commands that you did above?

[aishenri]

I have it in k3d-jupyter
plot.display()
could not create model:
Model name K3DModel
Model module k3d
Model module version ^2.0.0
Script error for "k3d" http://requirejs.org/docs/errors.html#scripterror

Widget Javascript not detected. It may not be installed or enabled properly.