python optional-dependencies build-time support

[arcnmx]

Some python applications use optional dependencies / extras_requires to represent support for non-essential features.

Being able to set extraDependencies = [ "feat1" "feat2" ] or something in a derivation would be nice, similar to how buildFeatures = [] works in buildRustPackage.

Technical details

The setuptools build hook calls pip install -e . directly and does not allow any environment variables to customize the invocation. Installing optional dependencies seems to require running pip install -e '.[feat1,feat2,etc]' instead.

[fabaff]

optional-dependencies was introduced to deal with optional requirements of Python module (see 17.27.2.3.2. Optional extra dependencies in the manual for details).

[arcnmx]

Note that I am specifically referring to derivations that end users will interact with (applications mainly), not modules used by other python projects/derivations. The difference is that while another python module/application/etc can enable extras by depending on dask[complete], a derivation building a final package that isn't used as a python dependency cannot.

So this is mainly useful for buildPythonApplication (and hybrid toPythonApplication modules probably too, as well as modules that act as dynamic plugins rather than dependencies). For example, a python application might optionally support sqlite as a database backend, and mark it as an optional feature. Even though you can just shove the dependencies into propagatedBuildInputs anyway and it will be in the closure/PYTHONPATH, you lose various checks and safety that can result in runtime failures (but successful builds).

Thanks for pointing that out though, it is relevant and helpful!