def testSample(self): mockInput = MagicMock(return_value=StringIO(self.sampleInput)) output = StringIO() mockOutput = MagicMock(return_value=output) with patch("__builtin__.open", mockInput): inputFile = FileRecordStream("input_path") with patch("__builtin__.open", mockOutput): outputFile = FileRecordStream("output_path", fields=inputFile.getFields(), write=True) anomalyzer.sample(inputFile, outputFile, 1) result = StringIO(output.getvalue()) result.next() result.next() result.next() reader = csv.reader(result) _, value = reader.next() self.assertIn(int(value), (1, 2, 3, 4, 5, 6)) self.assertRaises(StopIteration, result.next)