venv using symlinks and empty pyvenv.cfg isn't recognized as venv / able to find python home

[rickeylev]

Bug report

Bug description:

Previously, it was possible to have a functional (enough) venv by simply doing two things:

1. Creating an empty pyvenv.cfg file
2. Creating a bin/python3 which symlinked to the actual interpreter

The net effect was you had a sufficiently relocatable venv (at the interpreter level) to make it into a fully relocatable venv (at the application level). This is because Python would follow the symlink to locate its home, which let it get through initial bootstrapping, enough to pick a venv-relative site-packages directory.

The net effect was Python followed the symlink to find home, which let it bootstrap itself enough to find the venv's site-packages. Once at that point, application code is able to hook in and do some fixups.

This bug seems to be introduced by #126987

I'm still looking at the changes to figure out why, exactly. I'm pretty confident it's that PR, though -- in my reproducer, things work before it, but break after it.

This behavior of automatically find Python home is something Bazel's rules_python has come to rely on. Insofar as venvs are concerned, the two particular constraints Bazel imposes are:

1. Absolute paths can't be used. This is because they may be machine specific. In Bazel, the machine that generates pyvenv.cfg may be different than who uses it at runtime (e.g a remote build worker generates it, and a user runs it locally).
2. The file system must be assumed to be read-only after building. This prevents creating/modifying a pyvenv.cfg at runtime in the venv's directory to work around (1).

(As an aside, how the env var PYTHONEXECUTABLE gets handled might have also been affected by this; I haven't looked deeply yet, but one of my tricks was to set that to the venv bin/python3 path and invoke the real interpreter, but that seems to no longer work, either)

I'll post a more self contained reproducer in a bit. In the meantime, something like this can demonstrate the issue:

mkdir myvenv
cd myvenv
touch pyvenv.cfg
mkdir bin
ln -s <path to python> bin/python3
mkdir -p lib/python3.14/site-packages
bin/python3 -c 'import sys; print(sys.prefix); print(sys.base_prefix); print(sys.path)'

After the change, it'll print warnings and e.g. /usr/local as the prefixes, which is incorrect. sys.path won't have the venv's site-packages directory, which is incorrect.

Before the change, it'll print the venv directory, underlying interpreter directory, and the venv's site-packages, which is correct.

cc @FFY00 (author of above PR)

CPython versions tested on:

3.14

Operating systems tested on:

Linux

[rickeylev]

After digging in a bit more, I think the fix might be simple: don't null out venv_prefix if a home key wasn't found. i.e. remove the venv_prefix=None line around L418 in getpath.py

I'm also more convinced this should be considered a regression because the logic that was ported from site.py to getpath.py doesn't result in the same behavior as before.

I think what was happening previously was:

(1) getpath.py would calculate venv_prefix, and ultimately set it to None. This meant nothing, though, because the venv_prefix variable was entirely unused (all references to it are on commented out lines).

(2) site.py comes around and does its own pyvenv.cfg logic. This does two things: sets sys.prefix to the pyvenv.cfg location and adds the venv's (sys.prefix) site-packages directory to sys.path

The net effect is sys.prefix vs sys.base_prefix looks correct, and the venv's site packages is also correct. Yay, functional venv.

After the change:

getpath.py now computes and sets sys.prefix. However, it doesn't preserve the "treat pyvenv.cfg location as sys.prefix" logic from (2) -- the "else" clause of the "find home key" loop sets venv_prefix=None. So, it ignores the pyvenv.cfg location and acts like there's no venv. Now I understand what the comment about "home key is required" means.

The net effect is boo, the venv is ignored and we don't have a functional venv.

I realize that "else" clause to set venv_prefix=None has been there awhile, however:

1. It doesn't comport with the logic that used to be in site.py
2. The venv_prefix variable in getpath.py existing at all was weird -- it was never actually used? It looks like it has always been commented out, even in the commits that introduced it? I get the sense that the logic in site.py was supposed to have lived in getpath.py all along, but circumstances ended up with it not working at the time, so things were commented out in getpath.py and hacks put into site.py (and then some later hacks to handle site.py changing sys.prefix after the fact).