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

Question

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?

Answer 1

Use --cov {envsitepackagesdir}/<your-package-name> in tox.ini.