Differences in coverage percentage between Python 3.7 and Python 3.8

Describe the bug The coverage percentage appears to be sensible to the Python version under which coverage run and coverage combine + coverage report are executed. Specifically, the coverage percentage varies depending on the combinations of Python versions those commands are run with (while using the same version of coverage.py).

I have experienced the following patterns:

coverage run	coverage combine + coverage report	coverage percentage
Python 3.7	Python 3.7	100%
Python 3.8	Python 3.8	100%
Python 3.7	Python 3.8	92%
Python 3.8	Python 3.7	100%

To Reproduce

• Clone this repository: https://github.com/sanjioh/tox-interpreters.
• Run tox -r -e py38-tox314,coverage-report. This should report a 100% coverage.
• Run tox -r -e py37-tox314,coverage-report. This should report a 92% coverage.
• If basepython for the coverage-report tox env is changed to python3.7, coverage consistently reports 100%, as per the above table.

coverage.py version: 4.5.4

Expected behavior I would expect to get 100% coverage for the combination of coverage run run with Python 3.7, coverage combine + coverage report run with Python 3.8.

Thanks for your support, please let me know if you need any further information.

[nedbat]

I’ve updated the FAQ in 8ed8cbec7581d942a8aaafd1341fe20fb33aef96

[ArturKlauser]

Just FYI (I ran into the same issue): I think this problem is bound to become more prevalent now that python 3.8 is starting to become the ‘default’ python version in places, e.g. CI environments. E.g. I was running coverage over a test matrix of python versions and then combining+reporting with the default (i.e. newest stable) python version, which brings out this decorator problem. Maybe add to FAQ?