What breaks if you omit --no-build-isolation with editable installs?

[inklesspen]

The doc page on Editable Installs says that "packages installed in editable mode are rebuilt on import", and that failure to use --no-build-isolation can cause dependencies to be missing at import time.

However, in my experience running pip install -e ., the build directory persists even without this flag. Importing the package works just fine, and in fact I can't see any difference between running it with that flag or without it (except of course that I have to manually install the build dependencies when using that flag).

Here's my sample project: https://github.com/inklesspen/meson-python-cffi-example

Perhaps the breakage occurs with features I've not yet used? Or perhaps this is intended for versions of pip older than 21.3?

[eli-schwartz]

The issue is that you have to manually install build dependencies because the ones originally installed during the build got uninstalled.

Also: they may be different versions which means that you may need to reconfigure the build anyway. Worse dependencies may have been originally detected and cached in a location that doesn't exist and doesn't work anymore, resulting in the build failing.

[inklesspen]

But that doesn't happen…

1. I create a virtual environment and activate it. (I use direnv for that, but I could probably do it manually too.)
2. I run pip install -e .
3. pip installs cffi and meson-python in the activated virtualenv, then builds and installs my package
4. pip does not, in fact, delete the activated virtualenv, nor the build directory
5. I run python and import the package and everything works fine.

[rgommers]

pip 24.0 definitely does this for me, and it always has AFAIK. Note that the logging won't actually say it creates a venv, even when you add -v - but it does. Can you please make sure with an empty venv without direnv? If you have a reproducer from scratch where build dependencies stay available, I'll try it.

nor the build directory

Yes, the build dir is left in place.

[inklesspen]

All right, I've sorted it out; click that link for Dockerfiles to make a reproducible example.

It turns out that when meson and ninja are installed at the system level (as they are in my regular work machine) you don't see the breakage from an editable install with build isolation; if meson and ninja are not installed system-wide, you do see the breakage (since pip installs them in a temporary directory, as you said).

[rgommers]

Thank you, that is indeed reproducible and all makes sense. The one issue in my explanation above is that cffi didn't actually go missing, but that is because you also declared it as a runtime dependency with dependencies = ['cffi'] so it does end up in myvenv. You can check that declared build-only deps don't actually get preserved in your example.working with:

$ docker run --rm -it example:working bash
$ /home/builder/myvenv/bin/python3 -c "import cffi"  # works, because also a runtime dep
$ /home/builder/myvenv/bin/python3 -c "import mesonpy"  # build-only dep, so went missing
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ModuleNotFoundError: No module named 'mesonpy'

[inklesspen]

I'm pretty sure it is a runtime dependency, so yeah. Anyway, much is explained now. Thanks.