Patching in helper function not appled to instance

Question

I am creating a test with unittest.mock.patch that looks something like this:

class TestService:
  def test_patched(self):
    service = Service()
    with patch.object(service, "_send_to_third_party") as patch_send:
      with patch.object(service, "_notify_listeners") as patch_notify:
        service.do_something()
        patch_send.assert_called_once()
        patch_notify.assert_called_once()

All that works fine, but I'd like to put those patches in a method so that I can reuse it across a bunch of tests. Like this:

class TestService:
  def _patched_service(self):
    service = Service()
    with patch.object(service, "_send_to_third_party") as patch_send:
      with patch.object(service, "_notify_listeners") as patch_notify:
        return service, patch_send, patch_notify

  def test_patched(self):
    service, patch_send, patch_notify = self._patched_service()
    service.do_something()
    patch_send.assert_called_once()
    patch_notify.assert_called_once()

This fails because it's calling the real methods not the patched one.

To simplify, in the following test the third option fails, and I'm trying to wrap my head around why and if there's a good way to do what I want?

from unittest.mock import patch

class ExampleClass:
    def __init__(self):
        self.value = 0

    def add(self, value):
        self.value += self._add(value)

    def subtract(self, value):
        self.value -= self._subtract(value)

    def what_is_my_value(self):
        return self.value

    def _add(self, value):
        return value

    def _subtract(self, value):
        return value

def patch_me(exa: ExampleClass):
    with patch.object(exa, '_add', return_value=99):
        with patch.object(exa, '_subtract', return_value=66):
            return exa

class TestPatchingWorks:

    def test_unpatched_works(self):
        exa = ExampleClass()
        exa.add(5)
        exa.subtract(2)
        assert exa.what_is_my_value() == 3

    def test_patching_works_with_patch_class(self):
        exa = ExampleClass()
        with patch.object(ExampleClass, '_add', return_value=30):
            with patch.object(ExampleClass, '_subtract', return_value=10):
                assert exa.what_is_my_value() == 0
                exa.add(5)
                assert exa.what_is_my_value() == 30
                exa.subtract(2)
                assert exa.what_is_my_value() == 20

    def test_patching_works_with_patch_instance(self):
        exa = ExampleClass()
        with patch.object(exa, '_add', return_value=40):
            with patch.object(exa, '_subtract', return_value=30):
                assert exa.what_is_my_value() == 0
                exa.add(5)
                assert exa.what_is_my_value() == 40
                exa.subtract(2)
                assert exa.what_is_my_value() == 10

    def test_patching_works_with_function(self):
        exa = ExampleClass()
        exa = patch_me(exa)
        assert exa.what_is_my_value() == 0
        exa.add(5)
        assert exa.what_is_my_value() == 99
        exa.subtract(2)
        assert exa.what_is_my_value() == 33

Answer 1

Well, the patches are unwound as soon as you exit from the with blocks, which happens when you return.

To not do that, you can make your patcher function a context manager, most easily with the contextlib.contextmanager decorator.

@contextlib.contextmanager
def _patched_service(self):
  service = Service()
  with patch.object(service, "_send_to_third_party") as patch_send:
    with patch.object(service, "_notify_listeners") as patch_notify:
      yield service, patch_send, patch_notify

...

def test_patched(self):
  with self._patched_service() as (service, patch_send, patch_notify):
    service.do_something()
    patch_send.assert_called_once()
    patch_notify.assert_called_once()

should do the trick.

The same thing in Pytest fixtures would be

@pytest.fixture
def patched_service():
  service = Service()
  with patch.object(service, "_send_to_third_party") as patch_send:
    with patch.object(service, "_notify_listeners") as patch_notify:
      yield service, patch_send, patch_notify

def test_patched(patched_service):
  (service, patch_send, patch_notify) = patched_service
  service.do_something()
  patch_send.assert_called_once()
  patch_notify.assert_called_once()