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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