Failed to load plugins when create a virtualenv with --system-site-packages

[msclock]

• a detailed description of the bug or problem you are having
• output of pip list from the virtual environment you are using
• pytest and operating system versions
• minimal example if possible

When a virtual environment is created with --system-site-packages, pytest fails to load plugins.

minimal reproducible steps:

virtualenv .venv --system-site-packages
source .venv/bin/activate
pip install pytest pytest-benchmark
pytest --fixtures | grep benchmark || echo $?
created virtual environment CPython3.10.12.final.0-64 in 1918ms
  creator CPython3Posix(dest=/home/fengli/test/.venv, clear=False, no_vcs_ignore=False, global=True)
  seeder FromAppData(download=False, pip=bundle, setuptools=bundle, via=copy, app_data_dir=/home/fengli/.local/share/virtualenv)
    added seed packages: pip==25.1.1, setuptools==80.9.0
  activators BashActivator,CShellActivator,FishActivator,NushellActivator,PowerShellActivator,PythonActivator
Looking in indexes: https://nexus.infra.agibot.com/repository/pypi-proxy/simple, https://nexus.infra.agibot.com/repository/pypi/simple
Requirement already satisfied: pytest in /home/fengli/.local/lib/python3.10/site-packages (8.1.1)
Collecting pytest-benchmark
  Downloading https://nexus.infra.agibot.com/repository/pypi-proxy/packages/pytest-benchmark/5.1.0/pytest_benchmark-5.1.0-py3-none-any.whl (44 kB)
Requirement already satisfied: iniconfig in /usr/local/lib/python3.10/dist-packages (from pytest) (2.0.0)
Requirement already satisfied: packaging in /usr/local/lib/python3.10/dist-packages (from pytest) (23.2)
Requirement already satisfied: pluggy<2.0,>=1.4 in /usr/local/lib/python3.10/dist-packages (from pytest) (1.5.0)
Requirement already satisfied: exceptiongroup>=1.0.0rc8 in /usr/local/lib/python3.10/dist-packages (from pytest) (1.2.2)
Requirement already satisfied: tomli>=1 in /home/fengli/.local/lib/python3.10/site-packages (from pytest) (2.0.2)
Requirement already satisfied: py-cpuinfo in /usr/local/lib/python3.10/dist-packages (from pytest-benchmark) (9.0.0)
Installing collected packages: pytest-benchmark
Successfully installed pytest-benchmark-5.1.0

[notice] A new release of pip is available: 25.1.1 -> 25.2
[notice] To update, run: pip install --upgrade pip
1

It reports no benchmark marks in the output.

Pytest seems to use python path to load plugins only. It works fine with --system-site-packages when plugins are pre-installed in systems.

[The-Compiler]

Reproduced:

$ pytest --version --version       
This is pytest version 8.4.1, imported from /usr/lib/python3.13/site-packages/pytest/__init__.py
registered third-party plugins:
  pytest-asyncio-0.26.0 at /usr/lib/python3.13/site-packages/pytest_asyncio/plugin.py
  anyio-4.10.0 at /usr/lib/python3.13/site-packages/anyio/pytest_plugin.py
  pytest-cov-6.1.1 at /usr/lib/python3.13/site-packages/pytest_cov/plugin.py

though when running with python -m pytest, it works fine:

$ python -m pytest --version --version
This is pytest version 8.4.1, imported from /usr/lib/python3.13/site-packages/pytest/__init__.py
registered third-party plugins:
  pytest-benchmark-5.1.0 at /home/florian/tmp/13637/.venv/lib/python3.13/site-packages/pytest_benchmark/plugin.py
  pytest-asyncio-0.26.0 at /usr/lib/python3.13/site-packages/pytest_asyncio/plugin.py
  anyio-4.10.0 at /usr/lib/python3.13/site-packages/anyio/pytest_plugin.py
  pytest-cov-6.1.1 at /usr/lib/python3.13/site-packages/pytest_cov/plugin.py

it's unclear to me whether this is really an issue with pytest, or if this is something about how pip/virtualenv/importlib interact.

[msclock]

@The-Compiler Thanks for inspiraion. I figured out the problem. The reason why it did not work is I created a venv with --system-site-packages to reference the system site-packages which contained a satisfied versioned pytest installed.

It shows:

msclock@msclock:~/repos/test$ cat ~/.local/bin/pytest
#!/usr/bin/python3
# -*- coding: utf-8 -*-
import re
import sys
from pytest import console_main
if __name__ == '__main__':
    sys.argv[0] = re.sub(r'(-script\.pyw|\.exe)?$', '', sys.argv[0])
    sys.exit(console_main())

So it prefers to parse the system python path for loading the system site-package plugins. But this can be addressed with python -m pytest in the created venv to enforce to use the venv python path to load those plugins.

[RonnyPfannschmidt]

This is one of the reasons why fully venv isolation is recommend

Depending on system packages some scripts may not install

And pytest from the system path will be unaware of the venv

[msclock]

I'll close this because this is no a bug.