I believe the docs as per pytest-from-python-code are incorrect. Their text is as follows:

pytest.main(['-x', 'mytestdir'])
or pass in a string:
pytest.main("-x mytestdir")

The string argument option flags a deprecation message, whereas I believe the array args option should be
pytest.main([ 'mytestdir', '-x'])

platform darwin -- Python 3.5.2, pytest-3.0.2, py-1.4.31, pluggy-0.3.1