def test_create_random_keyfile(self, mock_os_urandom): secret = b'secret' mock_os_urandom.return_value = secret with patch('builtins.open', create=True) as mock_open: mock_open.return_value = MagicMock(spec=io.IOBase) file_handle = mock_open.return_value.__enter__.return_value LuksDevice.create_random_keyfile('some-file') file_handle.write.assert_called_once_with(secret)