✨ feat(env): control the isolated build environment location#1083
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henryiii
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build always created its isolated environment in a random temporary directory and deleted it on exit. That left three needs unmet, all raised in pypa#655: builds on read-only containers that only expose a small writable path, inspecting the environment of a failed build, and giving compilation caches like ccache and sccache a stable path (a build-environment path that changes every run shows up as a new file and misses the cache). Until now the only workarounds were nudging TMPDIR, which moves the parent directory but keeps the random name and still deletes it, or hand-rolling a venv with --no-isolation. Add an env_dir argument to DefaultIsolatedEnv, surfaced as the --env-dir option and the BUILD_ENV_DIR environment variable. The location must be empty so build never clobbers existing data; build removes it after a successful build and keeps it after a failure for inspection. Removing on success keeps the location reusable across the default sdist-then-wheel run and across repeated builds, which is what the caching use case needs.
henryiii
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Today
buildputs every isolated build environment in a random temporary directory and removes it the moment the build finishes. People who need a say in that location have no good option. Some setTMPDIR, which moves the parent folder but keeps the random name and still wipes it; others give up on isolation and assemble a virtual environment by hand. #655 collects the three situations where this hurts.🛠️ Teams building compiled projects want their compilation cache to pay off. ccache and sccache decide what to reuse from the paths in each compiler command, so a build-environment path that changes on every run looks brand new and the cache stays cold. A stable location turns those misses into hits.
📦 People building inside containers often have a read-only filesystem with one small writable corner. They need the environment to land in that corner.
🔍 Anyone debugging a failed build wants to open the environment afterward and see what went in. A directory that disappears on exit takes the evidence with it.
This change lets you choose the location with
--env-dir(or theBUILD_ENV_DIRenvironment variable), and exposes the samepathsetting on the API. You point it at an empty folder,buildfills it, clears it after a clean build so the next run can reuse the spot, and leaves it in place after a failure so you can look inside. The documentation also corrects an older claim that the temporary directory survives a failed build.Closes #655.
Workarounds people use today
The stopgaps people reach for in the issue thread and across the toolchain until this lands:
TMPDIR/TEMP/TMPto steer the env into a chosen (or writable) parent folder.TMPDIRhad no effect, which is exactly why a dedicated location flag is needed.base_dir/ sccacheSCCACHE_BASEDIR— strip the volatile path before hashing (a partial mitigation, not a fix for the container or debugging cases).python -m build --no-isolationto get a controllable, persistent environment.Seen in the wild on GitHub
Projects steer the build environment by hand — relocating the temp dir or skipping isolation — to keep a compiler cache warm:
macos-build.sh(L26-L47) — install sccache shims onPATH, thenpython -m build --wheel --no-isolationso the cache sees a stable environment.python_wheel_macos_build.sh(L170) —python -m build --sdist --wheel . --no-isolationwith-DARROW_USE_CCACHE=ON(L124).TMPDIRat a writable$GITHUB_WORKSPACE/.tmp(created at L40) sopython -m build(L204) lands in the one writable spot, with sccache stats at L217.mingw-w64-mesonPKGBUILD (L67) —python -m build --wheel --skip-dependency-check --no-isolationso the wheel builds in the prepared env where ccache/sccache (L26-L27) live.