Anything to do or cleanup after pip venv isolation

[LecrisUT]

I remember I had a very ugly approach to detect if we are in the pip fake venv, but I cannot remember the context or if it was only in a PR and did not survive in the end.

Context pypa/pip#13222 has been resolved with --use-feature=venv-isolation

[henryiii]

🤖 AI text below 🤖

The code you're thinking of survived — it's in main today. Builder.configure() in builder/builder.py:

# Add site-packages to the prefix path for CMake
site_packages = Path(sysconfig.get_path("purelib"))
if self.settings.search.site_packages:
    self.config.prefix_dirs.append(site_packages)
    logger.debug("SITE_PACKAGES: {}", site_packages)
    if site_packages != DIR.parent.parent:          # <- the "fake venv" detector
        self.config.prefix_dirs.append(DIR.parent.parent)
        logger.debug("Extra SITE_PACKAGES: {}", DIR.parent.parent)
        logger.debug("PATH: {}", sys.path)

DIR.parent.parent is the site-packages scikit-build-core itself is installed into. Under pip build isolation, sysconfig.get_path("purelib") reports the target venv, but we run out of pip's separate build-env overlay, so the two differ — that inequality is the heuristic for "we're in the pip fake venv," and it adds both dirs to CMAKE_PREFIX_PATH so CMake can still find build deps (pybind11, nanobind, ...) installed into the overlay.

Introduced in #24, gated behind search.site-packages in #880. It's the only such detection — nothing analogous in get_requires.py/sysconfig.py/wheel.py.

Re cleanup: with --use-feature=venv-isolation the overlay becomes a real nested venv layered on the target, so DIR.parent.parent and purelib should land in the same tree and the != branch becomes a no-op — good, but not removable yet. It still needs to fire for pip without the flag and for other front-ends. It can only go once a pip floor guarantees venv-isolation, which is a long way off.

[henryiii]

(The really ugly workaround would have been in cmake, but it never shipped, didn't work on Windows. Though I might be able to fix it on Windows these days, didn't have patience to work it out by hand. I'd still probably need to boot up a a Windows machine, so would rather just wait and point people at the new flag.)