No test selected
This is considered a failure by every tool, including Jenkins and other CI systems.
On the other hand you won't see any failed test reported. After all if no tests are run, then none of them fails. This can be confusing.
$ pytest -k long test_by_marker.py
============================= test session starts ============================== platform linux -- Python 3.8.6, pytest-6.1.2, py-1.9.0, pluggy-0.13.1 rootdir: /home/gabor/work/slides/python/examples/pytest, configfile: pytest.ini plugins: flake8-1.0.6, dash-1.17.0 collected 5 items / 5 deselected ============================ 5 deselected in 0.00s =============================
$ echo $? 5