pytest infra: tests/__init__.py collision + pytest_plugins location restriction #75

Summary

Two pytest infrastructure traps that hit every multi-module project:

1. tests/__init__.py files cause pytest plugin-name collisions under --import-mode=importlib. When the workspace root pyproject sets addopts = "--import-mode=importlib", having modules/<m>/tests/__init__.py files in two modules makes pytest try to register tests.conftest as a plugin twice — collision error. The fix is to not create tests/__init__.py; pytest discovers test files via path walk under importlib mode.

2. pytest_plugins = [...] in non-top-level conftest is forbidden by pytest 7.x. The framework's bundled host_conftest.py is meant to be loaded as a plugin, but if a module's conftest tries to register it via pytest_plugins, pytest raises:

   pytest_plugins variable is not allowed in non-rootdir conftest.py files
   

   Fix: register at the workspace-root conftest.py (one level above all modules) with pytest_plugins = ["host_conftest"]. Module conftests then inherit the plugin.

Reproduction

# Trap 1
echo > modules/lacowiki_legends/tests/__init__.py
echo > modules/lacowiki_datasets/tests/__init__.py
uv run pytest modules
# → "tests.conftest" plugin registered twice / import collision

# Trap 2
# In modules/lacowiki_legends/tests/conftest.py:
pytest_plugins = ["host_conftest"]
uv run pytest
# → ERROR: pytest_plugins variable is not allowed in non-rootdir conftest.py files

Workarounds

• Don't create tests/__init__.py. Test files import via path discovery.
• Workspace root has a single conftest.py with pytest_plugins = ["host_conftest"]. Module conftests are named conftest.py (no __init__.py parent) and just declare local fixtures.

Suggested fix

Update the framework docs / sm new scaffold to:

1. Not create tests/__init__.py in any module template.
2. Generate a workspace-root conftest.py with pytest_plugins = ["host_conftest"] when scaffolding a uv-workspace project.
3. Document the importlib-mode + plugin-registration pattern prominently.

Environment

• pytest>=8.0, pytest-asyncio>=0.24, importlib mode
• Discovered Phases 2–5 of laco_wiki_python rewrite.