Testing instance methods in Python using mock

Question

I have a class similarly structured to this:

class TestClass:
    def a(self):
        pass

    def b(self):
        self.a()

I want to test if running TestClass().b() successfully calls TestClass().a():

class TestTestClass(unittest.TestCase):
    def test_b(self):
        with patch('test_file.TestClass') as mock:
            instance = mock.return_value
            TestClass().b()
            instance.a.assert_called()

However, it fails because a isn't called. What am I doing wrong? I went through the mock documentation and various other guides to no avail.

Thanks!

You mock the whole class, so everything called for that class will just return a mock and not call the real method (like b in your case). You have to mock specifically a instead (e.g. patch("test_file.TestClass.a")). — MrBean Bremen
– MrBean Bremen, Commented Mar 19, 2021 at 19:48

Lin Du · Accepted Answer · 2021-03-22 02:47:40Z

You can use patch.object() to patch the a() method of TestClass.

E.g.

test_file.py:

class TestClass:
    def a(self):
        pass

    def b(self):
        self.a()

test_test_file.py:

import unittest
from unittest.mock import patch
from test_file import TestClass


class TestTestClass(unittest.TestCase):
    def test_b(self):
        with patch.object(TestClass, 'a') as mock_a:
            instance = TestClass()
            instance.b()
            instance.a.assert_called()
            mock_a.assert_called()


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

unit test result:

 ⚡  coverage run /Users/dulin/workspace/github.com/mrdulin/python-codelab/src/stackoverflow/66707071/test_test_file.py && coverage report -m --include='./src/**'
.
----------------------------------------------------------------------
Ran 1 test in 0.001s

OK
Name                                           Stmts   Miss  Cover   Missing
----------------------------------------------------------------------------
src/stackoverflow/66707071/test_file.py            5      1    80%   3
src/stackoverflow/66707071/test_test_file.py      12      0   100%
----------------------------------------------------------------------------
TOTAL                                             17      1    94%

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