'Should return empty set if no file options.'

def test_returns_empty_set_if_no_files(self):

    instance = self.test_instance
    for name in ['files_preserve', 'stdin', 'stdout', 'stderr']:
        setattr(instance, name, None)
    expect_result = set()
    result = instance._get_exclude_file_descriptors()
    self.failUnlessEqual(expect_result, result)
