✨ feat(env): add virtualenv_spec for per-env version pinning#3794
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gaborbernat merged 1 commit intotox-dev:mainfrom Feb 20, 2026
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✨ feat(env): add virtualenv_spec for per-env version pinning#3794gaborbernat merged 1 commit intotox-dev:mainfrom
gaborbernat merged 1 commit intotox-dev:mainfrom
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No single virtualenv version supports both very old (3.6) and very new (3.15+) Python versions. When the installed virtualenv cannot handle a target Python, the entire environment fails with no workaround. The new `virtualenv_spec` config key (e.g. `virtualenv<20.22.0`) lets users pin a different virtualenv version per environment. When set, tox bootstraps that version into a cached venv under `.virtualenv-bootstrap` and drives it via subprocess instead of the imported library. This keeps the default zero-overhead path for normal usage while enabling mixed old/new Python testing in a single tox config. Closes tox-dev#3656
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No single virtualenv release covers the full range of Python versions that projects need to test against — older versions like
virtualenv<20.22.0are required for Python 3.6, while the latest virtualenv is needed for Python 3.15+. Since tox imports virtualenv as a library, environments targeting unsupported Python versions simply fail with no workaround. ✨ This becomes a real blocker for projects that must maintain compatibility across a wide Python version range in a singletox.toml.The new
virtualenv_specper-environment config key (e.g.virtualenv_spec = "virtualenv<20.22.0") solves this by bootstrapping the specified virtualenv version into a cached venv under.tox/.virtualenv-bootstrap/and driving it via subprocess instead of the imported library. The bootstrap is content-addressed by spec hash, protected by file locks for concurrent safety, and reused across runs. Whenvirtualenv_specis empty (the default), tox continues using the imported virtualenv with zero overhead — the subprocess path only activates when explicitly configured.The
VirtualEnvclass now operates in dual mode: itssessionproperty routes to either the importedsession_via_clior a newSubprocessSessionthat mimics the sameSession/Creator/Describeinterface. Path accessors (bin_dir,purelib,exe, etc.) dispatch throughisinstancechecks so both modes integrate transparently with the rest of the environment lifecycle. Thepython_cache()dict includes the spec string when set, ensuring environments are automatically recreated when the pinned version changes.Closes #3656