Build requirements and editable install

[mancellin]

Hi,

Thank you for this package, I'm a happy user of meson-python and I have a quick question to better understand how it works (which actually might be a PEP 517 question as much as a meson-python question).

When listing the package build requirements in pyproject.toml, Meson-python's documentation suggests

[build-system]
build-backend = 'mesonpy'
requires = ['meson-python', 'cython']

without including meson itself or ninja. And indeed for my project I never had to list meson or ninja there.

On the other hand, when doing an editable install, the documentation suggests

python -m pip install meson-python meson ninja

And omitting ninja indeed leads to a failure of the installation (capytaine/capytaine#355 (comment))

Is there something different happening when requiring meson-python for the isolated build environment and pip-installing it myself? Why do I need to list explicitly ninja in one case and not the other?

[rgommers]

Hi @mancellin, thanks for the question, and glad you're happy so far:)

The reason is that editable installs use --no-build-isolation, which means that any necessary build dependencies must already be installed in the target environment, and pip + meson-python will not attempt to install them when you call pip install -e . --no-build-isolation. I hope that explains it.

Other note: ninja isn't really a Python package, it just happens to be installable from PyPI but it's easier/nicer to install it with your system package manager once and have it always available.

[mancellin]

Thank you for your answer.

By reading a bit more (e.g. #485), I think I found what was bugging me:
There is a mechanism with which the build backend (e.g. meson-python) can add to the build environment packages that where not specified in pyproject.toml.
Hence the list of packages that I have to install manually for the editable install might not match the one of the build-system.requires entry of pyproject.toml.

[rgommers]

Yes, that is right - and indeed a little annoying. It's an unfortunate downside from PyPI being (mostly) Python-packages-only and needing a system package manager for non-Python dependencies like compilers and C/C++ libraries and tools. ninja should normally come from the system package manager, so that's checked for first - and then it's inserted if and only if there's no system install of it detected.