Support relative home in pyvenv.cfg (host-relocatable venv)

Feature or enhancement

Proposal:

The central idea is to allow a venv to be relocatable between different hosts by using a relative path in pyvenv.cfg for the home value. Relative paths allow avoiding use of an absolute path that is likely host-specific. Concretely, I propose making a relative home value relative to the pyvenv.cfg file.

Today, the home key is officially documented to be an absolute path. If a relative path is used, then official behavior is undefined. In practice, it is treated as relative to the current working directory (so, in effect, could be anywhere). (By omitting the home key entirely (https://github.com/python/cpython/issues/135773), it gets computed based on the bin/python3 symlink, but this is undesirable behavior).

I think there's a variety of things this enables or otherwise makes much simpler. The main one that comes to mind is that multiple venvs can efficiently share a runtime (while still being host-relocatable). Not needing a system Python itself to create the venv is another benefit. The explicitly looser coupling between the venv and system state (i.e where the desired Python is) is also appealing.

For the runtime itself, making relative home paths work is simple and about 2 lines. See https://github.com/python/cpython/compare/main...rickeylev:cpython:feat.relative.pyvenv.home

As the python-ideas discussion points out, other parts of venv creation and dependency installation would also need to be aware of this.

I expect a PEP will be requested, so I'll start on that and put more details there rather than here.

Parties that should probably be part of the discussion:

• rules_python: @rickeylev @aignas @groodt
• rules_py: @alexeagle @arrdem
• uv owners
• python venv module owners
• pip owners

Related

• PEP 711 (self contained application zip files)
• uv venv --relocatable. From a cursory look, the basic thing it does is generate "relative aware" scripts in bin/ and put a relocatable=true setting in pyvenv.cfg.
• python-build-standalone relocatable python installation
• relative pyvenv.cfg PEP draft: https://github.com/python/peps/pull/4476

Has this already been discussed elsewhere?

I have already discussed this feature proposal on Discourse

Links to previous discussion of this feature:

https://discuss.python.org/t/making-venvs-relocatable-friendly/96177

https://discuss.python.org/t/q-what-stops-a-venv-from-being-relocatable/57166

Start with the linked relative pyvenv.cfg PEP draft and the venv module behavior described in this issue, then review the linked Discourse discussions. Determine the full scope for relative home paths, including venv creation and dependency installation. Done requires an agreed design and implementation plan that addresses host relocation beyond the runtime-only change.