How to properly use coverage.py in Python?

Coverage looks for a .coverage file to read and generate that report for you. Py.test on its own does not create one. You need py.test plugin for coverage:

pip install pytest-cov

If you already have it, then you can run both at once like this:

py.test test.py --cov=sample.py

Which means run test module test.py and record/display coverage report on sample.py.

If you need to have multiple test runs and accumulate their recorded coverage and then display a final report, you can run it like this:

py.test test.py --cov=sample.py --cov-report=py.test test.py --cov=sample2.py --cov-report=py.test test.py --cov=sample3.py --cov-report=

Which means run test module test.py and record (only) coverage on sample.py - don't display a report.

Now you can run coverage command separately for a complete report:

coverage report -m

The command above simply displays a formatted coverage report based on the accumulated .coverage data file from previous test runs. -m means show lines missed i.e. lines not covered by tests:

Name        Stmts   Miss  Cover   Missing-----------------------------------------sample.py       6      0   100%  

Coverage supports more switches like --include and --omit to include/exclude files using path patterns. For more info check out their docs: https://coverage.readthedocs.io/en/coverage-4.5.1/cmd.html#reporting

It's a little hard to parse through your experiments, and you haven't included the command lines you used with each experiment. But: if you run the tests with:

python -m py.test test.py

then you can run them under coverage.py with:

coverage run -m py.test test.py

The below command worked for me:

coverage run --source=sample -m pytest test.pycoverage report -m