Mock class in Python with decorator patch

Question

I would like to patch a class in Python in unit testing. The main code is this (mymath.py):

class MyMath:
    def my_add(self, a, b):
        return a + b

def add_three_and_two():
    my_math = MyMath()
    return my_math.my_add(3, 2)

The test class is this:

import unittest
from unittest.mock import patch
import mymath

class TestMyMath(unittest.TestCase):
    @patch('mymath.MyMath')
    def test_add_three_and_two(self, mymath_mock):
        mymath_mock.my_add.return_value = 5

        result = mymath.add_three_and_two()

        mymath_mock.my_add.assert_called_once_with(3, 2)
        self.assertEqual(5, result)

unittest.main()

I am getting the following error:

AssertionError: Expected 'my_add' to be called once. Called 0 times.

The last assert would also fail:

AssertionError: 5 != <MagicMock name='MyMath().my_add()' id='3006283127328'>

I would expect that the above test passes. What I did wrong?

UPDATE: Restrictions:

• I would not change the tested part if possible. (I am curious if it is even possible, and this is the point of the question.)
• If not possible, then I want the least amount of change in the to be tested part. Especially I want to keep the my_add() function non-static.

Answer 1

Instead of patching the entire class, just patch the function.

class TestMyMath(unittest.TestCase):
    @patch.object(mymath.MyMath, 'my_add')
    def test_add_three_and_two(self, m):
        m.return_value = 5

        result = mymath.add_three_and_two()

        m.assert_called_once_with(3, 2)
        self.assertEqual(5, result)

I think the original problem is that my_math.my_add produces a new mock object every time it is used; you configured one Mock's return_value attribute, but then checked if another Mock instance was called. At the very least, using patch.object ensures you are disturbing your original code as little as possible.