While I agree that it is better to give such a message, I think that we should be careful with the directive. Basically, we could install with brew, debian, pip, conda, etc. We could be a bit less assertive and refer to the PIL documentation:

Refer to https://pillow.readthedocs.io/en/stable/installation.html for installing PIL.

WDYT?

By the way, the CI failure is a new one. Never saw it before.

We always use pip install ... in the contributing docs, I don't see why we should do otherwise here. The first thing the Pillow link recommends is also pip install ...

yeah I agree with @NicolasHug. We could otherwise refer to the installation docs of pytest, cython, etc.

This looks good to me, thanks!

We always use pip install ... in the contributing docs, I don't see why we should do otherwise here.

Because people will have to read the documentation to understand what it implies to pip install pillow and it might not be what they want.

We might want to change our contribution guide, starting with an early note mentioning that we will use pip and that one might need to change slightly the install instruction of the dependencies if you want to use system or another package manager.

I think that @amueller did a similar comment on one of @thomasjpfan PR regarding the installation of pandas or matplotlib (I could not find which PR thought)

The first thing the Pillow link recommends is also pip install ...

Yes but it also mentioned brew or apt-get. Then is up to the user to make the right choice.

I don't mind. Feel free to push your changes

We always use pip install ... in the contributing docs [...] We could otherwise refer to the installation docs of pytest, cython, etc.

We do have https://scikit-learn.org/stable/developers/advanced_installation.html though. The problem is that indicating pip install something in an error message can be bad advice if the user is using the system python, gets a permission error, starts to run sudo pip install .. and messes up his system all by following instructions without thinking. So in this case letting them search for a solution is not so bad IMO. (Though the error message do need improvement).

For contributing instructions, I think pip install can be fine, as potential contributors are a small fraction of users that should be more aware of these things (or at least figuring these things out it is part of the "contribution experience").

why would we need pil? Shouldn't we use imageio?

Shouldn't we use imageio?

imageio depends on pillow so that would be 1 extra dependency in addition to pillow.

really? but why? ok, just ignore me...