You can use this sandbox to reproduce the error: https://gist.github.com/sirex/1bf7dda1cc3ba14ce7ebb6f964bf0d39

Sandbox contains module_tests.py, but if I run following command this test is not discovered:

> py.test -o "python_files=*_tests.py"
==================================================================== test session starts =====================================================================
platform linux2 -- Python 2.7.12, pytest-3.0.6, py-1.4.32, pluggy-0.4.0
rootdir: /tmp/pytest/pytest, inifile: 
collected 0 items 

After some debugging I found, that initially --override-ini options are ignored and python_files are cached with default values, then ofter some time --override-ini becomes available, but python_files already have cached values and ignores --override-ini.

But if I add pytest.ini:

> cat pytest.ini 
[pytest]
python_files=*_tests.py

Then this test file is collected.