How to use the sister.download.get_cache_root function in sister

def test_set_cache_root(self):
        orig_root = download.get_cache_root()
        new_root = '/tmp/cache'
        try:
            download.set_cache_root(new_root)
            self.assertEqual(download.get_cache_root(), new_root)
        finally:
            download.set_cache_root(orig_root)

def test_get_cache_directory(self):
        root = download.get_cache_root()
        path = download.get_cache_directory('test', False)
        self.assertEqual(path, os.path.join(root, 'test'))

def setUp(self):
        self.default_cache_root = download.get_cache_root()
        self.temp_dir = tempfile.mkdtemp()
        download.set_cache_root(self.temp_dir)

def test_set_cache_root(self):
        orig_root = download.get_cache_root()
        new_root = '/tmp/cache'
        try:
            download.set_cache_root(new_root)
            self.assertEqual(download.get_cache_root(), new_root)
        finally:
            download.set_cache_root(orig_root)