Injecting Pytest fixtures without cluttering test signatures

Sometimes, when writing tests in Pytest, I find myself using fixtures that the test function/method doesn’t directly reference. Instead, Pytest runs the fixture, and the test function implicitly leverages its side effects. For example:

import os
from collections.abc import Iterator
from unittest.mock import Mock, patch
import pytest


# Define an implicit environment mock fixture that patches os.environ
@pytest.fixture
def mock_env() -> Iterator[None]:
    with patch.dict("os.environ", {"IMPLICIT_KEY": "IMPLICIT_VALUE"}):
        yield


# Define an explicit service mock fixture
@pytest.fixture
def mock_svc() -> Mock:
    service = Mock()
    service.process.return_value = "Explicit Mocked Response"
    return service


# IDEs tend to dim out unused parameters like mock_env
def test_stuff(mock_svc: Mock, mock_env: Mock) -> None:
    # Use the explicit mock
    response = mock_svc.process()
    assert response == "Explicit Mocked Response"
    mock_svc.process.assert_called_once()

    # Assert the environment variable patched by mock_env
    assert os.environ["IMPLICIT_KEY"] == "IMPLICIT_VALUE"

In the test_stuff function above, we directly use the mock_svc fixture but not mock_env. Instead, we expect Pytest to run mock_env, which modifies the environment variables. This works, but IDEs often mark mock_env as an unused parameter and dims it out.

One way to avoid this is by marking the mock_env fixture with @pytest.fixture(autouse=True) and omitting it from the test function’s parameters. However, I prefer not to use autouse=True because it can make reasoning about tests harder.

TIL that you can use @pytest.mark.usefixtures¹ to inject these implicit fixtures without cluttering the test function signature or using autouse. Here’s the same test marked with usefixtures:

# ... same as above


@pytest.mark.usefixtures("mock_env")
def test_stuff(mock_svc: Mock) -> None:
    # Use the explicit mock
    response = mock_svc.process()
    assert response == "Explicit Mocked Response"
    mock_svc.process.assert_called_once()

    # Assert the environment variable patched by mock_env
    assert os.environ["IMPLICIT_KEY"] == "IMPLICIT_VALUE"

Now, the mock_env fixture is applied without cluttering the test function’s signature, and no more greyed-out unused parameter warnings! The usefixtures marker also accepts multiple fixtures as variadic arguments: @pytest.mark.usefixtures("fixture_a", "fixture_b").

One thing to keep in mind is that it won’t work if you try to mark another fixture with the usefixtures decorator. The pytest documentation includes a warning² about this.

Fin!

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