Making py.test, coverage and tox work together: __init__.py in tests folder?

I'm having a weird problem with `tox`, `py.test`, `coverage` and `pytest-cov`: when `py.test` with the `--cov` option is launched from `tox`, it seems to require an `__init__.py` file in the `tests` folder which is not immediately obvious. While writing this post, I have kind of solved the initial problem by adding the aforesaid `tests/__init__.py`, but to this moment I don't fully understand why exactly it works or doesn't work, so I'm still asking for help. Please see below for details. I've found a related question on SO but it only makes it more confusing because the answer seems to be opposite to what I've figured out so far: `py.test` and `__init__.py` files See also the official docs here: py.test - Good Integration Practices (the very bottom of the page). Simplified project structure: ``` setup.py tox.ini .coveragerc project/ __init__.py module1.py module2.py tests/ __init__.py (optional, an empty file) test_module1.py test_module2.py ``` Relevant part of `tox.ini`: ``` [testenv:check] commands = py.test --cov=project --cov-report=term deps = pytest coverage pytest-cov [pytest] python_files = test_*.py norecursedirs = .tox ``` Relevant part of `.coveragerc`: ``` [run] branch = True omit = project/tests/* ``` Now, the results: - `py.test --cov=project --cov-report=term` run from project root => correct coverage whether `tests/__init__.py` file is present or not. - `tox -e check` without `tests/__init__.py` => the tests are discovered and run, but I get a warning "Coverage.py warning: No data was collected." and the coverage is 0% for all modules - `tox -e check` with `tests/__init__.py` => correct coverage again. It's not immediately obvious to me why the `tests/__init__.py` file has to be there (adding this empty file solved the initial problem) for the `tox` run, but it doesn't matter when you run the tests/coverage manually. Any ideas?