Mock class attribute

Question

Let's say I have some class:

class Worker: _processor: Processor def set_processor(self, processor: Processor): self._processor = processor def start_work(self, data: bytes): self._processor.parse(data)

It's pretty simple to test set_processor method, smth like that:

def test_set_processor(): mock_processor = Mock() worker = Worker() worker.set_processor(mock_processor) assert worker._processor == mock_processor

We just pass mock into set_processor method and check if self._processor equals mock now.

But how can I test start_work method? I want to be sure that self._processor will be called with data? If I try to patch _processor attribute I'm getting error.

Why do you need to patch anything? You can test this whole thing in one go: pass the mock to the set method, call the other method and assert the mock got exercised as expected. — jonrsharpe
– jonrsharpe, Commented Dec 5, 2019 at 20:15
Yes, sounds good. But I think it's not bad idea to test separate every method. Now I solved with set _processor to mock directly: worker._processor = Mock(). And then call method and check if mock were called. — Mike
– Mike, Commented Dec 5, 2019 at 20:26
But then you're interfering with internal implementation details as part of the test. Test through the public API. — jonrsharpe
– jonrsharpe, Commented Dec 5, 2019 at 20:27

Lin Du · Accepted Answer · 2019-12-10 09:40:23Z

Here is the unit test solution:

worker.py:

from typing import Any Processor = Any class Worker: _processor: Processor def set_processor(self, processor: Processor): self._processor = processor def start_work(self, data: bytes): self._processor.parse(data)

test_worker.py:

import unittest from worker import Worker from unittest.mock import Mock class TestWorker(unittest.TestCase): def test_set_processor(self): mock_processor = Mock() worker = Worker() worker.set_processor(mock_processor) assert worker._processor == mock_processor worker.start_work(b'') mock_processor.parse.assert_called_once_with(b'') if __name__ == '__main__': unittest.main()

Unit test result with 100% coverage:

. ---------------------------------------------------------------------- Ran 1 test in 0.001s OK (venv) ☁ python-codelab [master] ⚡ coverage report -m Name Stmts Miss Cover Missing ------------------------------------------------------------------------- src/stackoverflow/59202648/test_worker.py 13 0 100% src/stackoverflow/59202648/worker.py 8 0 100% ------------------------------------------------------------------------- TOTAL 21 0 100%

Source code: https://github.com/mrdulin/python-codelab/tree/master/src/stackoverflow/59202648

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