Pytest: test only one instance of parametrized fixture

Question

Setting

Suppose I have conftest.py where I defined resources for tests

class ClassifyRequest:
    pass

class OCRRequest:
    pass

@pytest.fixture(params=[ClassifyRequest, OCRRequest])
def api_request(request):
    return request.param()  # return one of the request instances

Then, in test_api.py I use the parametrized fixture to test a service:

def test_service(api_request):
    response = send_request(api_request)
    assert response.ok()

All good, but then I want to test specialized fixture api_request[ClassifyRequest]:

@pytest.mark.usefixture("api_request[ClassifyRequest]")
def test_classification():
    # do something with the api_request fixture

Question

What is the way to specialize a parametrized fixture for a test function? I have two ideas:

1. Just use a non-parametrized fixture. This inevitably leads to boilerplate code.
2. Remove explicit parametrization from the fixture decorator and use indirect parametrization like

   @pytest.mark.parametrize("response_class", ["classify", "ocr"])
   def test_api(api_request):
       # do smth
   

   Replacing class parameters with strings equals creating two sources of configuration. What if I want to add another parameter? Do I have to design a new string for it, as well as instantiate an object inside api_request fixture indirectly?
3. The same, except keep class parametrization and move classes outside conftest.py as pytest can't import names from that module with import statements.

Additional info

pytest==6.0.1

Answer 1

If all your fixture does is instantiation, I'd opt for parametrizing the test, not the fixture. But, well, if you have to and specialize it manually anyway, you could simply add a wrapper to make the fixture callable:

import pytest
import types

class ClassifyRequest:
    def __init__(self):
        print("ClassifyRequest ctor")

class OCRRequest:
    def __init__(self):
        print("OCRRequest ctor")

def api_request_wrapper(request):
    return request.param()  # return one of the request instances

@pytest.fixture(params=[ClassifyRequest, OCRRequest])
def api_request(request):
    return api_request_wrapper(request)

def test_classification():
    request = types.SimpleNamespace()
    request.param = ClassifyRequest
    instance = api_request_wrapper(request)

Seems a bit hacky, though.