No mutations when running with --use-coverage

Steps to reproduce:

1. In an empty directory, create the file example_test.py with the following content:

def test_stuff():
    assert True

1. Run pytest --cov to generate .coverage file.
2. Run mutmut run --paths-to-mutate=example_test.py --use-coverage.

Expected result: 1 killed mutant (I tried running mutmut without --use-coverage and there was indeed 1 mutant; and coverage html shows that that line is covered).

Actual result: no mutants whatsoever.

1. Running tests without mutations
⠇ Running... Done
Using coverage data from .coverage file

2. Checking mutants
⠏ 0/0  🎉 0  ⏰ 0  🤔 0  🙁 0

---

Version info:

Python 3.7.2 (tags/v3.7.2:9a3ffc0492, Dec 23 2018, 23:09:28) [MSC v.1916 64 bit (AMD64)] on win32
pytest==4.1.1
pytest-cov==2.6.1
mutmut==1.2.0

[Vlad-Shcherbina]

Thank you, I confirm it works with mutmut 1.3.0.

I know it’s pathological, I was going for the simplest way to reproduce.

[boxed]

That would be great!