An example of *fragile* unit tests due to bad mocks.

test_example_object.py

from unittest.mock import patch, Mock
from pytest import fixture
from src.example import SomeObjectToTest


class TestSomeObjectToTest:

    @fixture
    def expected_result(self):
        return 400

    def test_do_something(self, expected_result):
        object_under_test = SomeObjectToTest()

        mocked_calculator = Mock()
        mocked_calculator.add.return_value = expected_result

        result = object_under_test.do_something(mocked_calculator)

        assert result == expected_result
        mocked_calculator.add.assert_called_once_with(5, 10)

    @patch("src.example.SomeClassOutOfOurControl")
    def test_do_something_else(self, mocked_class, expected_result):
        mocked_class.return_value.add.return_value = expected_result
        object_under_test = SomeObjectToTest()

        result = object_under_test.do_something_else()

        assert result == expected_result
        object_under_test.calculator.add.assert_called_once_with(5, 10)