Replace `pyvenv.cfg`'s `home` key with `base-prefix` and `base-exec-prefix`

[FFY00]

Feature or enhancement

Proposal:

The home key in pyvenv.cfg is used to specify the directory where the base Python executable is. During the Path Initialization, this directory, and consecutively its parents, is searched for stdlib landmarks to determine sys.base_prefix and sys.base_exec_prefix. Naturally, this isn't ideal, it would be better if pyvenv.cfg contained these values directly.

Proposed plan:

• Update venv to start setting the base-prefix and base-exec-prefix keys in pyvenv.cfg when creating/updating environments
• In getpath, use the base-prefix and base-exec-prefix keys to set the prefixes
• Deprecate specifying home, in pyvenv.cfg

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

No response

[FFY00]

Optionally, we can also maybe add base-executable, to initialize sys._base_executable.

[pelson]

I also independently advocated for this yesterday in #106045 (comment). I then realised that these heuristics are also being used for non-venv based prefix determination, and now appreciate that when running a plain CPython interpreter, all you have is the executable path (plus a sysconfig full of paths for where Python was built) and heuristics are seemingly still necessary.

If we could remove the need for the core interpreter heuristics (e.g. by only using the build config), then I think we should also remove the heuristics from venv exec/prefix determination.

On the other hand, if we don't manage to remove the heuristics at the core, then I would advocate for removing home and replacing it with base-executable, since ultimately that is what is important from a venv path determination perspective.

All of this sounds like it needs a change to PEP-405 though?

[charliermarsh]

FWIW, I believe I agree with this, based on my experience computing home in uv.

[mayeut]

Optionally, we can also maybe add base-executable

I wonder if it wouldn't be required for venv launchers anyway (if not required, it would certainly simplify various paths computations in those launchers).
If this gets exposed, it should be made public.

[brettcannon]

I'm not sure if @vsajip has an opinion on this. I'm personally fine with the proposal (including adding base-executable).

[vsajip]

I have no problems with it either.

[pelson]

I believe there are two proposals on the table:

• (Original proposal): setting the base-prefix and base-exec-prefix keys in pyvenv.cfg. Additionally setting base-executable. Deprecate home.
• (My adapted proposal): setting base-executable, not setting base-exec-prefix and base-prefix and, as as argued above, use the same logic that is used for the normal startup sequence to determine them. Deprecate home.

My proposal avoids the redundancy of setting both base-exec-prefix and base-executable, and does not seek to introduce the ability to control the prefix independently of the exec-prefix (which is not possible with PEP-405, but may be one of the motivations of the original proposal? ).

Either approach is an improvement on the existing home mechanism IMO (since we no longer need to guess the executable so that we can proceed to guess the prefix and exec-prefix), and I'd be happy to take on the work, especially if there is somebody who would be willing to provide some mentorship (this would be my first CPython change which deprecates something specified in a PEP).

I note that PEP-405 states:

If a home key is found, this signifies that the Python binary belongs to a virtual environment, and the value of the home key is the directory containing the Python executable used to create this virtual environment.

I think this would need to be changed according to the proposals above (since I don't think this would take precedence over base-executable, if found).

[brettcannon]

I believe there are two proposals on the table

My comment was in regards to the proposal from @FFY00 . In the end, though, probably either is fine and it will be up to the core dev who does the work to choose what to go with. I'm here just trying to keep this moving so it can get closed out somehow.

I note that PEP-405 states

Non-active PEPs, once accepted, are historical documents, so it doesn't matter what it says at this point.