Jupyer Lab prints to the log

[SimeonEhrig]

Problem

When I try to run the Button Widget section of the example in Jupyter Lab, the output of foo() is printed to the log.

(Kernel C++14)

Environment

I have installed xeus-cling and xwidgets via conda:

conda create -n xwidgets python=3.7
conda activate xwidgets
conda install xeus-cling xwidgets -c conda-forge
conda install -c conda-forge jupyterlab
conda install -c conda-forge nodejs
jupyter labextension install @jupyter-widgets/jupyterlab-manager

conda list

# Name                    Version                   Build  Channel
_libgcc_mutex             0.1                        main  
attrs                     19.3.0                     py_0    conda-forge
backcall                  0.1.0                      py_0    conda-forge
binutils_impl_linux-64    2.33.1               he1b5a44_7    conda-forge
binutils_linux-64         2.33.1              h9595d00_17    conda-forge
bleach                    3.1.3              pyh8c360ce_0    conda-forge
ca-certificates           2019.11.28           hecc5488_0    conda-forge
certifi                   2019.11.28       py37hc8dfbb8_1    conda-forge
clang_variant             1.0                  cling_v0.6    conda-forge
clangdev                  5.0.0             hb222dda_1006    conda-forge
cling                     0.6                  he513fc3_2    conda-forge
cppzmq                    4.3.0             hc9558a2_1001    conda-forge
decorator                 4.4.2                      py_0    conda-forge
defusedxml                0.6.0                      py_0    conda-forge
entrypoints               0.3             py37hc8dfbb8_1001    conda-forge
gcc_impl_linux-64         7.3.0                habb00fd_1  
gcc_linux-64              7.3.0               h553295d_17    conda-forge
gxx_impl_linux-64         7.3.0                hdf63c60_1  
gxx_linux-64              7.3.0               h553295d_17    conda-forge
icu                       64.2                 he1b5a44_1    conda-forge
importlib-metadata        1.5.2            py37hc8dfbb8_0    conda-forge
importlib_metadata        1.5.2                         0    conda-forge
ipykernel                 5.2.0            py37h43977f1_0    conda-forge
ipython                   7.13.0           py37hc8dfbb8_2    conda-forge
ipython_genutils          0.2.0                      py_1    conda-forge
jedi                      0.16.0           py37hc8dfbb8_1    conda-forge
jinja2                    2.11.1                     py_0    conda-forge
json5                     0.9.0                      py_0    conda-forge
jsonschema                3.2.0            py37hc8dfbb8_1    conda-forge
jupyter_client            6.1.1                      py_0    conda-forge
jupyter_core              4.6.3            py37hc8dfbb8_1    conda-forge
jupyterlab                2.0.1                      py_0    conda-forge
jupyterlab_server         1.0.7                      py_0    conda-forge
ld_impl_linux-64          2.33.1               h53a641e_7  
libedit                   3.1.20181209         hc058e9b_0  
libffi                    3.2.1                hd88cf55_4  
libgcc-ng                 9.1.0                hdf63c60_0  
libsodium                 1.0.17               h516909a_0    conda-forge
libstdcxx-ng              9.1.0                hdf63c60_0  
libuuid                   2.32.1            h14c3975_1000    conda-forge
libuv                     1.34.0               h516909a_0    conda-forge
markupsafe                1.1.1            py37h8f50634_1    conda-forge
mistune                   0.8.4           py37h516909a_1000    conda-forge
nbconvert                 5.6.1                    py37_0    conda-forge
nbformat                  5.0.4                      py_0    conda-forge
ncurses                   6.2                  he6710b0_0  
nlohmann_json             3.7.3                he1b5a44_1    conda-forge
nodejs                    13.10.1              hf5d1a2b_0    conda-forge
notebook                  6.0.3                    py37_0    conda-forge
openssl                   1.1.1e               h516909a_0    conda-forge
pandoc                    2.9.2                         0    conda-forge
pandocfilters             1.4.2                      py_1    conda-forge
parso                     0.6.2                      py_0    conda-forge
pexpect                   4.8.0            py37hc8dfbb8_1    conda-forge
pickleshare               0.7.5           py37hc8dfbb8_1001    conda-forge
pip                       20.0.2                   py37_1  
prometheus_client         0.7.1                      py_0    conda-forge
prompt-toolkit            3.0.4                      py_0    conda-forge
ptyprocess                0.6.0                   py_1001    conda-forge
pugixml                   1.10                 he1b5a44_1    conda-forge
pygments                  2.6.1                      py_0    conda-forge
pyrsistent                0.16.0           py37h8f50634_0    conda-forge
python                    3.7.7           h191fe78_0_cpython  
python-dateutil           2.8.1                      py_0    conda-forge
python_abi                3.7                     1_cp37m    conda-forge
pyzmq                     19.0.0           py37hac76be4_1    conda-forge
readline                  7.0                  h7b6447c_5  
send2trash                1.5.0                      py_0    conda-forge
setuptools                46.1.1                   py37_0  
six                       1.14.0                     py_1    conda-forge
sqlite                    3.31.1               h7b6447c_0  
terminado                 0.8.3            py37hc8dfbb8_1    conda-forge
testpath                  0.4.4                      py_0    conda-forge
tk                        8.6.8                hbc83047_0  
tornado                   6.0.4            py37h8f50634_1    conda-forge
traitlets                 4.3.3            py37hc8dfbb8_1    conda-forge
wcwidth                   0.1.8              pyh9f0ad1d_1    conda-forge
webencodings              0.5.1                      py_1    conda-forge
wheel                     0.34.2                   py37_0  
xeus                      0.23.5               h4d8c418_0    conda-forge
xeus-cling                0.8.1                he513fc3_2    conda-forge
xproperty                 0.10.1               hc9558a2_0    conda-forge
xtl                       0.6.13               hc9558a2_0    conda-forge
xwidgets                  0.20.0               hc9558a2_0    conda-forge
xz                        5.2.4                h14c3975_4  
zeromq                    4.3.2                he1b5a44_2    conda-forge
zipp                      3.1.0                      py_0    conda-forge
zlib                      1.2.11               h7b6447c_3  

[SylvainCorlay]

Thanks @SimeonEhrig this is in fact expected behavior (same as with ipywidgets in lab) and we should change the notebook to use an output widget.

[SimeonEhrig]

Thanks for the quick answer. Is the output widget already implemented?

[SylvainCorlay]

Yes it is! You can see it in the widgets list notebook I think. It is used as a scope guard.

[SylvainCorlay]

If you are interested in pushing on xwidgets, I should connect sometimes. We have some ideas (that are rather low-hanging) on how to make it as usable as ipywdgets.

[SimeonEhrig]

I got it:

and also with pictures:

I'm not sure if you should change the example notebook, because the Jupyter Notebook way is much easier than the Lab way. But you should at least provide a Jupyter Lab example, because I think the behaviour of Jupyter Lab is confusing for somebody who is not familiar with ipython widgets.

That's a good idea. We try to do interactive simulation and analysis in Jupyter Notebook with C++. Therefore we also need a simple way to quickly create a user interface for the analyses.

For the current Nvida GTC, @ax3l and I also offer a presentation that explains our idea: https://doi.org/10.5281/zenodo.3713753

[SylvainCorlay]

Thanks! I think we should change the notebook, although I would display the output widget before the button is created so that there is something printed right away.