如何单元测试一个模块是否在另一个 function、Python 3 中实例化？

Question

I have a file my_work.py . The class MyModel is used inside of the class MyWork 's run method. Suppose the functionality of MyModel is well tested by its own dedicated unit-tests. My focus is to test if MyModel is used properly inside MyWork .

I need to test 2 things:

1. If MyModel is instantiated with given arguments.
2. If MyModel.run method is called. Because it is a long and expensive process, I have to mock it with fake returns.

Note that, both classes MyModel and MyWork are defined in the same file my_work.py . This is just an example for this post. In my real application, they are defined in different py-files. Not sure if this can make a difference or not.

I have tried myself to write unit-test for it, but I could not make it. Could you please help me? See my test file test_my_work.py . Thanks a lot in advance!

This is the file: my_work.py .

from typing import Tuple


class MyModel:
    def __init__(self, a: float, b: str):
        self._a = a
        self._b = b

    def run(self) -> Tuple[float, float]:
        # some long and expensive calculation.
        return self._a + 1, self._a + 2


class MyWork:
    def __init__(self, name: str):
        self._name = name
        self._res_1: float = 0
        self._res_2: float = 0

    def run(self) -> Tuple[float, float]:
        a = 2.5
        b = self._name
        model = MyModel(a=a, b=b)
        res_1, res_2 = model.run()
        return res_1, res_2

And this is the file: test_my_work.py .

import unittest
from unittest.mock import patch
from my_work import MyModel, MyWork


class TestMyWork(unittest.TestCase):

    @patch.object(MyModel, 'run', return_value=(3.5, 4.5))
    def test_if_my_model_run_is_called_in_my_work_run(self, mocked_run):
        self._my_work = MyWork(name='XX')
        self._my_work.run()
        mocked_run.assert_called_once()

    @patch.object(MyModel, '__init__')
    def test_if_my_model_is_initiated_in_my_work_run(self, mocked_init):
        self._my_work = MyWork(name='XX')
        self._my_work.run()
        mocked_init.assert_called_once_with(a=2.5, b='XX')


if __name__ == '__main__':
    unittest.main()

Answer 1

I would recommend to rather use the patch method instead of patch.object because MagicMock does not support mocking the __init__ method.

For the patch method, you'll need the "target" path to your class. In this case it will be 'my_work.MyModel' .

If you patch your model like this:

    @patch('my_work.MyModel')
    def test_if_my_model_is_initiated_in_my_work_run(self, my_model_mock):
        ...

Then my_model_mock will be a mock of the class MyModel and will be replaced with MagicMock .

my_model.return_value will be called when a new instance of MyModel is called. The run method can then be mocked like this:

        my_model_mock.return_value.run.return_value = (3.5, 4.5)