coverage.py does not cover script if py.test executes it from another directory

This turned out to be a problem of relative paths confusing coverage when the measured script is run from another directory. Coverage result files ended up in that directory, instead of the root directory of the project.

To solve this, I stopped using pytest-cov, and used pure coverage instead. I used full paths instead of relative paths wherever relevant.

So, e.g. define the environment variable necessary to enable subprocess coverage via export COVERAGE_PROCESS_START=/full/path/to/.coveragerc. In the .coveragerc, the coverage result file is specified via

     [run]
     data_file = /full/path/to/.coverage

and any --source and --include options should use full paths, too. Then it was possible to get correct coverage measurement.

I encountered the same issue when calling "py.test --cov ..." from tox. I found a hint on this page: http://blog.ionelmc.ro/2014/05/25/python-packaging/ even though it does not mention this explicitly. Using "--develop" for tox will make sure that coverage data gathering is called from the same directory as coverage analysis. This section in tox.ini made it work for me to have a test environment for coverage:

[tox]
envlist = ...,py34,cov

[testenv:cov]
# necessary to make cov find the .coverage file
# see http://blog.ionelmc.ro/2014/05/25/python-packaging/
usedevelop = true
commands = py.test --cov=<MODULE_NAME>
deps = pytest pytest-cov