My setup is a mixed C/C++/Python project. The C and C++ code builds independently of the Python code (using waf, but I think this barely matters -- the point is that the C/C++ build is triggered by a straightforward command and that it rebuilds correctly based on changed source code). The Python code depends on the C/C++ code via ctypes and cffi (which load a .so file produced by the C/C++ build), and there are no extension modules.

Python builds via [tool.hatch.build.targets.wheel.hooks.custom] in pyproject.toml and a hatch_build.py that invokes waf and force-includes the .so files into useful locations.

Use case 1: Development. I change something (C/C++ source, the waf configuration, etc) and then try to run Python code (via uv sync, uv run, or activating a venv with an editable install). Since there doesn't seem to be a way to have the build feed dependencies out to uv (this seems to be a deficiency in PEP 517/660), I either need to somehow statically generate cache-keys or resort to reinstall-package to get uv commands to notice when something changed. I can force the issue with uv pip install -e ., although apparently I can also force the issue with uv run/sync --reinstall-packages [distro name]. [0] So I guess uv pip is not actually needed here.

It would be very nice if there was an extension to PEP 660 that would allow the editable build to tell the front-end what its computed dependencies are.

Use case 2: Production

IMO uv sync and uv run have no place in production. I do not want my server to resolve dependencies or create environments at all, let alone by magic, when I am running a release of my software built for the purpose.

My code has, long before pyproject.toml or uv was a thing and even before virtual environments existed (!), had a script to build a production artifact. The resulting artifact makes its way to a server, and the code in it gets run. If I want to use dependencies as found by uv, or if I want to use entrypoints (a massive improvement over rolling my own way to actually invoke a Python program!), as far as I can tell I can either manually make and populate a venv using uv venv and uv pip or I can use UV_PROJECT_ENVIRONMENT with uv sync and abuse uv sync to imperatively create a venv.

Maybe some day uv will come up with a better way to produce production artifacts. (And maybe in the distant future, the libc world will come up with a decent way to make C/C++ virtual environments that don't rely on mount namespaces or chroot.)

[0] As far as I can tell, the accepted terminology is that the thing produced by a pyproject.toml is possibly a "project" or a "distribution" and that these are both very much distinct from a "package". I think it's a bit regrettable that uv's option here is spelled like it rebuilds a _package_ when the thing you feed it is not the name of a package and it does not rebuild a particular package. In uv's defense, PEP 517 itself seems rather confused as well.