pytest: pytest.main(...) hangs but works if run with subprocess.check_output

In my code, I call pytest.main(arguments) to run Pytest.

        arguments = ['-p', 'no:warnings', '--no-print-logs', '--junit-xml', log_path]
        arguments.extend(test_paths)
        if parallel:
            arguments += ['-n', 'auto']
        if pytest_args:
            arguments += pytest_args
        logger.info('Running: pytest {}'.format(' '.join(arguments)))
        return pytest.main(arguments)

This code hangs when run on OSX (10.14.1) but works fine on Windows.

If I change the last line to instead essentially call pytest through subprocess.check_output it executes fine. It also works if I simply copy-paste the command to my terminal. This behavior is observed whether I specify -n=auto or not, which is why I’m posting this here instead of in pytest-xdist. Is there something I’m doing wrong here?

Pip List

pytest (4.0.0)
pytest-forked (0.2)
pytest-xdist (1.24.1)

OS Info

Python (Darwin) 2.7.10 (default, Aug 17 2018, 17:41:52) 
[GCC 4.2.1 Compatible Apple LLVM 10.0.0 (clang-1000.0.42)]