[Bug]: uv based venv generator creates .venv not usable with uv

[ha1dfo]

What happened?

I've got an environment that's mocked here.
I had previous ticket on venv generation here.

I am generating the .venv so that my IDE (vscode) can pick it up.
Vscode uses uv pip list --python=<path> to parse the .venv.

However, when generating a .venv with aspect_rules_py, it creates one that's not parseable by uv.

When looking at it with strace, I can see that it prints that we've got python 3.13 and nothing more.

$ strace -ff -o /tmp/uvpip uv pip list --python .<...>+orchestrator.venv/bin/python
Using Python 3.13.12 environment at: /home/<USER>/.cache/bazel/_bazel_<USER>/09e1a13f6e7dc94649c8d65787235f07/execroot/_main/bazel-out/k8-fastbuild-ST-f38e6014d46d/bin/<...>/orchestrator.venv.runfiles/_main/<...>/.orchestrator.venv

We do have a pyvenv.cfg:

unix_users  359 Mar 26 06:44 pyvenv.cfg
$ cat /home/<USER>/repos/<repo>/.<...>+orchestrator.venv/pyvenv.cfg
home = ./bin/
implementation = CPython
version_info = 3.13.0
include-system-site-packages = false
aspect-include-user-site-packages = false
relocatable = true
# Non-standard extension keys used by the Aspect shim
aspect-runfiles-interpreter = aspect_rules_py++python_interpreters+python_3_13_x86_64_unknown_linux_gnu/bin/python3
aspect-runfiles-repo = _main

But uv pip list fails:

$ uv pip list --python /home/<USER>/repos/<REPO>/.venv/bin/python
Using Python 3.13.12 environment at: /home/<USER>/.cache/bazel/_bazel_<USER>/09e1a13f6e7dc94649c8d65787235f07/execroot/_main/bazel-out/k8-fastbuild-ST-f38e6014d46d/bin/<...>/venv.runfiles/_main/<...>/.venv

Strace:

$ strace -ff -o /tmp/uvpip uv pip list --python .<...>+orchestrator.venv/bin/python
Using Python 3.13.12 environment at: /home/<USER>/.cache/bazel/_bazel_<USER>/09e1a13f6e7dc94649c8d65787235f07/execroot/_main/bazel-out/k8-fastbuild-ST-f38e6014d46d/bin/<...>/orchestrator.venv.runfiles/_main/<...>/.orchestrator.venv

It finds the proper directory: lib/python3.13/site-packages, but then the process exits:

statx(AT_FDCWD, "/home/<USER>/.local/share/uv", AT_STATX_SYNC_AS_STAT, STATX_ALL, {stx_mask=STATX_ALL|STATX_MNT_ID, stx_attributes=0, stx_mode=S_IFDIR|0775, stx_size=4096, ...}) = 0
getcwd("/home/<USER>/repos/<REPO>", 512) = 26
ioctl(2, TCGETS, {B38400 opost isig icanon echo ...}) = 0
ioctl(2, TCGETS, {B38400 opost isig icanon echo ...}) = 0
write(2, "\33[2m", 4)                   = 4
ioctl(2, TCGETS, {B38400 opost isig icanon echo ...}) = 0
ioctl(2, TCGETS, {B38400 opost isig icanon echo ...}) = 0
write(2, "Using Python 3.13.12 environment"..., 285) = 285
ioctl(2, TCGETS, {B38400 opost isig icanon echo ...}) = 0
ioctl(2, TCGETS, {B38400 opost isig icanon echo ...}) = 0
write(2, "\33[0m", 4)                   = 4
ioctl(2, TCGETS, {B38400 opost isig icanon echo ...}) = 0
ioctl(2, TCGETS, {B38400 opost isig icanon echo ...}) = 0
write(2, "\n", 1)                       = 1
openat(AT_FDCWD, "/home/<USER>/.cache/bazel/_bazel_<USER>/09e1a13f6e7dc94649c8d65787235f07/execroot/_main/bazel-out/k8-fastbuild-ST-f38e6014d46d/bin/<...>/orchestrator.venv.runfiles/_main/<...>/.orchestrator.venv/lib/python3.13/site-packages", O_RDONLY|O_NONBLOCK|O_CLOEXEC|O_DIRECTORY) = 10
newfstatat(10, "", {st_mode=S_IFDIR|0555, st_size=4096, ...}, AT_EMPTY_PATH) = 0
getdents64(10, 0x7f8038000f40 /* 74 entries */, 32768) = 3096
getdents64(10, 0x7f8038000f40 /* 0 entries */, 32768) = 0
close(10)                               = 0

Normally I would chalk this up as an uv failure, however, the main purpose of the uv based venv generation is to create one that's usable with uv, so I'm playing my luck and ask for help here.

To make things more interesting, when I'm trying to repro this with the mock repo, I am not even getting that far, not even the python3.13 is found:

$ strace -ff -o /tmp/uvpip uv pip list --python /home/<USER>/repos/repro-py-proto/.application+apple.venv

The strace details say we're trying to run a mock script (i guess it must be like what ./configure used to be before bazel), it fails on loading encodings.

(...)
stat("/install/lib/python313.zip", 0x7ffcd1f596d8) = -1 ENOENT (No such file or directory)
stat("/install/lib", 0x7ffcd1f59708)    = -1 ENOENT (No such file or directory)
stat("/install", 0x7ffcd1f59708)        = -1 ENOENT (No such file or directory)
stat("", 0x7ffcd1f59708)                = -1 ENOENT (No such file or directory)
stat("/install/lib/python313.zip", 0x7ffcd1f59a08) = -1 ENOENT (No such file or directory)
stat("/install/lib/python3.13", 0x7ffcd1f59708) = -1 ENOENT (No such file or directory)
stat("/install/lib", 0x7ffcd1f59708)    = -1 ENOENT (No such file or directory)
stat("/install", 0x7ffcd1f59708)        = -1 ENOENT (No such file or directory)
stat("", 0x7ffcd1f59708)                = -1 ENOENT (No such file or directory)
stat("/install/lib/python3.13", 0x7ffcd1f59a08) = -1 ENOENT (No such file or directory)
stat("/install/lib/python3.13/lib-dynload", 0x7ffcd1f59708) = -1 ENOENT (No such file or directory)
stat("/install/lib/python3.13", 0x7ffcd1f59708) = -1 ENOENT (No such file or directory)
stat("/install/lib", 0x7ffcd1f59708)    = -1 ENOENT (No such file or directory)
stat("/install", 0x7ffcd1f59708)        = -1 ENOENT (No such file or directory)
stat("", 0x7ffcd1f59708)                = -1 ENOENT (No such file or directory)
stat("/install/lib/python3.13/lib-dynload", 0x7ffcd1f59a08) = -1 ENOENT (No such file or directory)
write(2, "Fatal Python error: ", 20)    = 20
write(2, "Failed to import encodings modul"..., 33) = 33
write(2, "\n", 1)                       = 1
write(2, "Python runtime state: ", 22)  = 22
write(2, "core initialized", 16)        = 16
write(2, "\n", 1)                       = 1
write(2, "ModuleNotFoundError", 19)     = 19
write(2, ": ", 2)                       = 2
write(2, "No module named 'encodings'", 27) = 27
write(2, "\n", 1)                       = 1
write(2, "\n", 1)                       = 1
write(2, "Current thread 0x", 17)       = 17
write(2, "00007f1721ee9740", 16)        = 16
write(2, " (most recent call first):\n", 27) = 27
write(2, "  <no Python frame>\n", 20)   = 20
exit_group(1)                           = ?
+++ exited with 1 +++

Interestingly, when I'm activating the .venv and try load the .venv with python, it works all fine:

$ . .application+apple.venv/bin/activate
$ which python
.application+apple.venv/bin/python
$ python
Python 3.13.11 (main, Dec  9 2025, 19:04:10) [Clang 21.1.4 ] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import encodings
>>>
$

At this point again could be a uv issue, but still; I was hoping to generate a venv with aspect_rules_py that is usable by vscode, and something is just not clicking.

I am happy to collect more data to work this out.

Version

Development (host) and target OS/architectures:

Output of bazel --version: 9.0.0

Version of the Aspect rules, or other relevant rules from your
WORKSPACE or MODULE.bazel file: bazel_dep(name = "aspect_rules_py", version = "1.10.0")

Language(s) and/or frameworks involved: Python

How to reproduce

Please advise, see above.

Any other information?

Happy to gather.

[ha1dfo]

update: if I install pip in bazel deps, I can do pip list, which lists the installed packages, while uv pip list still returns empty, while the repro repo has this issue:

$ . .application+apple.venv/bin/activate

$ uv pip list
error: Failed to inspect Python interpreter from active virtual environment at `.application+apple.venv/bin/python3`
  Caused by: Broken Python trampoline at `.application+apple.venv/bin/python3`, was the underlying Python interpreter removed?

hint: Consider recreating the environment (e.g., with `uv venv`)

$ .application+apple.venv/bin/python3
Python 3.13.11 (main, Dec  9 2025, 20:26:22) [Clang 21.1.4 ] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import encodings
>>>

[ha1dfo]

Update on the prod issue, since I was running out of options, I've asked AI what it thinks, and found that uv is not following symlinks when scanning site-packages for .dist-info directories.
This is rather weird why am I the first one seeing this, but since symlinks are so essential for bazel, this would need to be a question on uv.

[ha1dfo]

Ultimately what we want is vscode to be able to find everything that's used (either code in bazel, or by "@pypi//") in a bazel built project.
Using .venvs generated by python_binary rule and having "Python Environments" to discover those seemed like a good idea.
However, "Python Environments" use "uv" under the hood to digest the .venv that's generated by bazel; which is not working (you can follow vscode output under the "Python Environments" flavor, it will tell you how it's issuing uv pip list and that it finds nothing).

[arrdem]

Based on the error you're relating above it seems like uv pip list is attempting to execute the venv's interpreter, and doing so probably using -I or other hermeticity flags which disable PYTHONHOME, which is unfortunately load bearing due to the architecture of our relocatable virtualenvs. This causes the interpreter to fail to discover its own C-extensions, resulting in the encodings import error.

The fix is either to work with @geofft for revisions to the interpreter's behavior so that we can make relocatable venvs with our shim work better, or something like #705 as part of the shim.