problem with coverage stats when using multiprocessing

[franklingu]

I am using multiprocessing.Process in some file. When I run test just for that file, seems the coverage data is OK.

py.test --cov=data_utils --cov-report=html -vv -s  tests/test_utils_multiprocessing.py

And data:

Coverage for data_utils/utils/_multiprocessing.py : 95% Show keyboard shortcuts 85 statements 81 run 4 missing 0 excluded

But when I run test for whole tests directory the coverage for _multiprocessing.py and tests run after that dropped a lot -- compared to when being run in single file.

py.test --cov=data_utils --cov-report=html -vv -s

Coverage for data_utils/utils/_multiprocessing.py : 47% Show keyboard shortcuts 85 statements 40 run 45 missing 0 excluded

I am using Python 3.6.1 and pytest==3.4.2, pytest-cov==2.5.1,pytest-forked==0.2,pytest-mock==1.7.1 on ubuntu 16.04. I looked at #139 and I think this is not the same issue. The coverage is OK if run single test file but is too low if run the whole directory