Running pytest.main(...) does not collect tests

I've created a sub folder with test_*.py files and additional .py files which hold additional methods which are being called. In the root directory of the project I've created main_test.py in which I call pytest.main(['./subdfolder']). pytest is being triggered but I'm getting the blow output:

============================= test session starts =============================
platform win32 -- Python 2.7.14, pytest-3.6.1, py-1.5.3, pluggy-0.6.0
rootdir: C:\PycharmProjects\TestingFramework, inifile:
plugins: tap-2.2, report-0.2.1
collected 0 items

======================== no tests ran in 0.01 seconds =========================

Process finished with exit code 0