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!

• MrBean Bremen: 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")).

Answer 1

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%