Unit Testing ConfigurableResource Classes

[rover-wagnar]

Hi,

I'm looking for help with a better workaround or solution for handling mocking with classes that inherit from ConfigurableResource. Here's the issue I'm running into with an example.

Say I have this class, a very simple API client class that's a ConfigurableResource and handles setting a bearer token value for paginated API calls:

class RestApiClient(ConfigurableResource):
    _api_token: Optional[ApiToken] = None

    @property
    def api_token(self) -> ApiToken:
        if not self._api_token or self._api_token.has_expired():
            self._api_token = self._refresh_access_token()
        return self._api_token

     def refresh_access_token(self) -> ApiToken:
        # Code for handling a POST call for an API token value

    def get_paginated(self, api_path: str) -> List[Dict]:
        headers = {}
        headers["Authorization"] = "Bearer {}".format(self.api_token.value)
        
        response = requests.Session().get(request_url, headers=headers)
        response.raise_for_status()
        # code for handling response data and paginated calls, etc.

If I want to test that calling get_paginated means refresh_access_token is called, I would write a unit test like this, where I patch the object to mock a method so I can make assertions on call behavior:

@pytest.fixture
def rest_api_client():
    return RestApiClient(...)

def test_calls_refresh_access_token_if_no_access_token(rest_api_client, mock_response):
    # After initial construction, no api_token should be present so we expect a call to refresh_access_token
    with (
        patch('code_location.resources.rest_api.requests.Session.get') as mock_get,
        patch.object(rest_api_client, 'refresh_access_token') as mock_refresh_access_token
    ):
        mock_get.return_value = mock_response
        
        rest_api_client.get_paginated(FAKE_API_ENDPOINT)
        
    mock_refresh_access_token.assert_called_once()

The problem is, this test will fail. It fails because the way unittest.mock.patch apparently handles mocking is by using assignment to make a method a Mock object (which records interactions so we can call methods like .assert_called_once()). But ConfigurableResource objects are Pydantic BaseModels with Frozen=True which don't allow re-assignment like this, so the mock fails and the test fails with a Pydantic ValidationError.

One workaround for this is to autospec a MagicMock object with the ApiClient class like MagicMock(spec=RestApiClient). But this object does not have the same method definitions so the mocked version of get_paginated does not call refresh_access_token, and so I can't assert the method was called (unless I redefine mock methods for the MagicMock which defeats the purpose of the test).

Currently, I just don't write those kinds of unit tests for ConfigurableResource classes, but I feel like this limitation around mocking for ConfigurableResources limits test coverage. Am I missing a better way to unit test ConfigurableResources or a better workaround?