`ModuleNotFoundError` when importing internal libraries with `py_venv` in `aspect_rules_py` 1.6.3

[MohitBhar]

Summary

Importing internal libraries in py_venv virtual environments fails with ModuleNotFoundError in aspect_rules_py 1.6.3. The issue is caused by the missing {target_name}.venv.pth file (e.g., venv → venv.venv.pth) that was generated in version 1.4.0 to add internal library paths to sys.path.

Affected Versions

• Issue observed: aspect_rules_py 1.6.3
• Working as expected: aspect_rules_py 1.4.0

Error Observed

• ModuleNotFoundError when importing internal libraries in an activated py_venv virtual environment:
• Virtual environment name is not displayed in the terminal prompt (should show (.venv.venv) ➜)

Root Cause

The ModuleNotFoundError occurs because the {target_name}.venv.pth file does not appear to be generated in version 1.6.3. Based on the behavior in version 1.4.0, this file seems to be necessary to add internal library paths to sys.path when the virtual environment is activated.

Steps to Reproduce

A minimal reproduction case is available at: https://github.com/MohitBhar/py-venv-pth-issue/tree/main

git clone https://github.com/MohitBhar/py-venv-pth-issue/tree/main
cd py-venv-pth-issue
bazel run //app:venv
source .app+venv/bin/activate 
python app/main.py  # Fails with ModuleNotFoundError

Expected: App runs successfully; prompt shows (.venv.venv) ➜
Actual: ModuleNotFoundError: No module named 'calculator'

Regression Test

The repository includes a regression test:

bazel test //app:venv_test --test_output=all

• Test PASSES when imports work (no regression)
• Test FAILS when ModuleNotFoundError occurs (regression detected)

Verification (1.4.0 works)

Change MODULE.bazel:

bazel_dep(name = "aspect_rules_py", version = "1.4.0")

Then run:

rm -rf .app+venv
bazel run //app:venv
source .app+venv/bin/activate
python app/main.py  # ✅ Works!

Environment

• Bazel version: 8.4.2
• Python version: 3.9
• OS: macOS

[bu3]

Hi @arrdem any news about this?

[arrdem]

Sketch of a workaround here --

1. Add a new string_flag which allows users to control the virutalenv strategy (py/BUILD.bazel)
2. Update the macros (py/defs.bzl) to generate an alias+select() indirection depending on
3. Need to review the linking code; think this is just py/private/py_venv/link.py; maybe add a new attribute that controls the name of the generated symlink
4. Document that the user can set --@aspect_rules_py//py:link_venv_mode=[dynamic|static] or something to switch back and forth between the strategies
5. Fix [Bug]: pip install no longer works in 1.6.0 venvs #595 by switching the default venv linking mode to dynamic
6. As part of the 2.0 flip the flag