Can't listen to print function in __main__.py with unittest.mock

I am trying to listen to the print function from __main__.py. I use the patch function from unittest.mock.

def main_tester(command):
    fake_command()
    capturedoutput = io.StringIO()
    sys.stdout = capturedoutput
    with patch('sys.argv', command.split(" ")):
        from SIESTAstepper import __main__ as rtmain
    sys.stdout = sys.__stdout__
    return capturedoutput.getvalue()

The problem is, it returns an empty string "" instead of the captured output.

The code is from here if you are willing to see the full project.