Make ipykernel work without debugpy

[frenzymadness]

debugpy is an optional dependency because only frontends with
debugging support (Jupyter lab) can really use its features.

closes #712

There is also one new test in the CI to check that all tests are passing when debugpy is not installed.

[blink1073]

LGTM, thank you! I'll leave this open for a day in case another maintainer has input

[frenzymadness]

I'm testing this also in Fedora where 25 out of 28 packages built fine with the patched version without debugpy. The three packages fail to build for different reasons.

[minrk]

Thanks!

[fcollonval]

Should debugpy be removed from install requirements following this change?

In particular if debugpy is kept a mandatory dependency, it is considered as bad practice to have a set of packages breaking pip check.

[frenzymadness]

That's a good question for maintainers. I can imagine that it is an extra dependency and then jupyter notebook can depend on ipykernel without debugby and jupyterlab can depend on something like ipykernel[debug] because as far as I understand it, Jupyter lab is the only frontend with implemented debugging.

[martinRenou]

This approach might not work in the current state of things.

JupyterLab does not depend on ipykernel directly, it depends on nbclassic which itself depends on notebook which depends on ipykernel.
Also this kind of installation "flavors" has no counterpart in conda which seems to be the preferred way of installing the jupyter packages in the ecosystem.

I would also like to say that jupyterlab (and notebook) should ideally not depend on ipykernel, it's a language agnostic tool after all. It should also not depend on notebook, but that's a different problem.

[minrk]

Should debugpy be removed from install requirements following this change?

In particular if debugpy is kept a mandatory dependency, it is considered as bad practice to have a set of packages breaking pip check.

No, it shouldn't be removed in this repo. If a downstream packager wants to split this dependency, it may, but at the same time, it should also make sure to patch the package metadata when doing so to remove the dependency and satisfy pip check.

[frenzymadness]

Should debugpy be removed from install requirements following this change?

In particular if debugpy is kept a mandatory dependency, it is considered as bad practice to have a set of packages breaking pip check.

No, it shouldn't be removed in this repo. If a downstream packager wants to split this dependency, it may, but at the same time, it should also make sure to patch the package metadata when doing so to remove the dependency and satisfy pip check.

And that's exactly what we do in Fedora. Except pip check does not make sense for the system site-packages.