Coverage is not measured with Python 3.7.0

I’ve noticed that pytest-cov doesn’t report properly the coverage with Python 3.7.0. Here a simple repro:

Directory structure

|-- foo
|   |-- __init__.py
|   `-- bar.py
`-- tests
    |-- __init__.py
    `-- test_baz.py

Files content

All __init__.py files are empty.

bar.py

def baz():
    print('baz')

test_baz.py

from foo import bar


def test_foo():
    bar.baz()

pip freeze for both Python 3.6.5 and 3.7.0 environments

$ pip freeze
atomicwrites==1.1.5
attrs==18.1.0
coverage==4.5.1
more-itertools==4.2.0
pluggy==0.6.0
py==1.5.4
pytest==3.6.3
pytest-cov==2.5.1
six==1.11.0

Test results with Python 3.6.5 (correct result)

$ py.test -p no:logging --strict --cov-report=term-missing --cov=foo tests
=========================================== test session starts ============================================
platform darwin -- Python 3.6.5, pytest-3.6.3, py-1.5.4, pluggy-0.6.0
rootdir: /private/tmp/foo, inifile:
plugins: cov-2.5.1
collected 1 item

tests/test_baz.py .                                                                                  [100%]

---------- coverage: platform darwin, python 3.6.5-final-0 -----------
Name              Stmts   Miss  Cover   Missing
-----------------------------------------------
foo/__init__.py       0      0   100%
foo/bar.py            2      0   100%
-----------------------------------------------
TOTAL                 2      0   100%


========================================= 1 passed in 0.03 seconds =========================================

Test results with Python 3.7.0 (incorrect result)

$ py.test -p no:logging --strict --cov-report=term-missing --cov=foo tests
=========================================== test session starts ============================================
platform darwin -- Python 3.7.0, pytest-3.6.3, py-1.5.4, pluggy-0.6.0
rootdir: /private/tmp/foo, inifile:
plugins: cov-2.5.1
collected 1 item

tests/test_baz.py .                                                                                  [100%]

---------- coverage: platform darwin, python 3.7.0-final-0 -----------
Name              Stmts   Miss  Cover   Missing
-----------------------------------------------
foo/__init__.py       0      0   100%
foo/bar.py            2      2     0%   1-2
-----------------------------------------------
TOTAL                 2      2     0%


========================================= 1 passed in 0.03 seconds =========================================

[funkyfuture]

fwiw, i’m currently experiencing the same a significant drop of covered statements with Python 3.8 (compared to 3.7).

[volans-]

I cannot reproduce it either anymore, so either some new version of any of the dependency fixed it or there was something in my test environment that caused it.