Querying Requested Extras in build_ext

Hi,

I am building an external CMake extension with

from setuptools import setup, Extension
from setuptools.command.build_ext import build_ext


class CMakeExtension(Extension):
    def __init__(self, name, sourcedir=''):
        Extension.__init__(self, name, sources=[])
        self.sourcedir = os.path.abspath(sourcedir)


class CMakeBuild(build_ext):
    def run(self):
        # ...

        for ext in self.extensions:
            self.build_extension(ext)

    def build_extension(self, ext):
        # ...

# ...
setup(
    # ...
    ext_modules=[CMakeExtension('openpmd_api')],
    extras_require={
        'mpi': ['mpi4py'],
    },
    # ...
)

and would like to query the user-requested extras during the build_extension step, in order to find out if the extra 'mpi' was requested (via pip install .[mpi]) or not. Is there a way to query this?

Full example: setup.py

[jaraco]

Here’s my (late) response as posted in the Gitter inquiry (I haven’t read the thread above).

That’s not possible as far as I know. If the current project depends on it to decide how to build itself, then it’s not really an extra (as intended). Extras are intended for install-time dependency resolution, not build-time. You’ll want to structure your project so it can support running with and without the extras at run-time… maybe that means building with and without the libs the extras might require, or maybe it means pushing that functionality into the extras themselves.

[pganssle]

@ax3l I do not think that “extras” are intended to control additional functionality in the build, they are intended to allow you to specify dependencies for optional runtime features (see the documentation). I could certainly be wrong because I don’t know much about the history of the “extras” feature, but it’s really about declaring metadata for the front-end.

At least the way the build tools currently work, the idea is that at build time you would build everything you support, and at install time those features would only actually be usable if you had specified the extra options. This means that either you make a hard build-time requirement of mpi, regardless of the extra options, but make it optional at install time, or you separate out the parts that require mpi into a separate package that itself has a hard requirement of mpi.