pytest mock class from different module

Question

I am trying to mock a class which is instantiated in the constructor of the class I am trying to test. If I define the class I am trying to mock in the same module as the one I am trying to test, everything works fine but when they are in separate modules, I get errors.

Here's my example, taken from here (Note that in my real example, the test class is in a "tests" submodule and the other two files are in "app.src.code..." module.

What am I missing?

helper.py:

import os

class Helper:
    def __init__(self, path):
        self.path = path

    def get_path(self):
        base_path = os.getcwd()
        return os.path.join(base_path, self.path)

worker.py:

from helper import Helper

class Worker:    
    def __init__(self):
        self.helper = Helper('db')

    def work(self):
        path = self.helper.get_path()
        print(f'Working on {path}')
        return path

test_worker.py:

import unittest
from unittest.mock import patch
from worker import Worker


class WorkerTest(unittest.TestCase):
    def test_patching_class(self):
        with patch('helper.Helper') as MockHelper:
            MockHelper.return_value.get_path.return_value = 'testing'
            worker = Worker()
            MockHelper.assert_called_once_with('db')
            self.assertEqual(worker.work(), 'testing')

You need to patch the Helper object in worker.py. So try to change your with statement to with patch(worker.Helper) as MockHelper: — HitLuca
– HitLuca, Commented Apr 3, 2020 at 8:46
Read docs.python.org/3/library/unittest.mock.html#where-to-patch — jonrsharpe
– jonrsharpe, Commented Apr 3, 2020 at 8:47

Lin Du · Accepted Answer · 2020-04-27 06:47:06Z

You need to use the patch decorators to create the mock for the Helper class of the helper.py module.

E.g.

helper.py:

import os


class Helper:
    def __init__(self, path):
        self.path = path

    def get_path(self):
        base_path = os.getcwd()
        return os.path.join(base_path, self.path)

worker.py:

from helper import Helper


class Worker:
    def __init__(self):
        self.helper = Helper('db')

    def work(self):
        path = self.helper.get_path()
        print(f'Working on {path}')
        return path

test_worker.py:

import unittest
from unittest.mock import patch
from worker import Worker


class TestWorker(unittest.TestCase):
    def test_work(self):
        with patch('worker.Helper') as mock_Helper:
            mock_helper_instance = mock_Helper.return_value
            mock_helper_instance.get_path.return_value = 'testing'
            worker = Worker()
            mock_Helper.assert_called_once_with('db')
            self.assertEqual(worker.work(), 'testing')


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

unit test results with coverage report:

Working on testing
.
----------------------------------------------------------------------
Ran 1 test in 0.001s

OK
Name                                        Stmts   Miss  Cover   Missing
-------------------------------------------------------------------------
src/stackoverflow/61008064/helper.py            7      3    57%   6, 9-10
src/stackoverflow/61008064/test_worker.py      13      0   100%
src/stackoverflow/61008064/worker.py            8      0   100%
-------------------------------------------------------------------------
TOTAL                                          28      3    89%

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