I recently switched from nosetests to pytest and pytest-cov for one of my Python project (http://easydev-python.readthedocs.io).
Locally, everything seemed to work fine and once on Travis, all tests passed as expected.
Yet, before the coverage report, I noticed this warning:
Coverage.py warning: No data was collected"
and more importantly, the final coverage reported was 0% !!
I later realised that one __init__.py file in the test/ directory was present locally but not in the github source code. After a commit, travis found the missing file and the coverage came back to normal.
So, this __init__.py seems important for pytest-cov