def launch_modes(launch_mode_conf):
    """Set of launch modes in which the tests will actually be run.

    The value of this fixture depends on the value of `launch_mode_conf`:
    - 'inprocess'  -> {'inprocess'}
    - 'subprocess' -> {'subprocess'}
    - 'both'       -> {'inprocess', 'subprocess'}
    - None         -> {'inprocess'}
    """
    if launch_mode_conf == 'both':
        return {'inprocess', 'subprocess'}
    if launch_mode_conf is not None:
        return {launch_mode_conf}
    return {'inprocess'}