🐛 fix: pytest 9 compatibility for the class-scoped d fixture

[nstarman]

Problem

The test suite fails under pytest 9.x (12 ERRORs at fixture setup) while passing under 8.x. Since pyproject.toml declares pytest>=8.4.2 with no upper bound, CI breaks the moment uv.lock picks up pytest 9.

Root cause

The visible AssertionError at _pytest/fixtures.py:1221 (assert not self._finalizers) is a cascade, not the cause. The real error is on the first test in the class:

PytestRemovedIn10Warning('Class-scoped fixture defined as instance method is deprecated.
Instance attributes set in this fixture will NOT be visible to test methods,
as each test gets a new instance while the fixture runs only once per class.
Use @classmethod decorator and set attributes on cls instead.')

pytest 9 added this deprecation (_pytest/deprecated.py, CLASS_FIXTURE_INSTANCE_METHOD). Because we set filterwarnings = ["error"], it is raised during FixtureDef.execute, which leaves the FixtureDef half-initialised — every subsequent test in the class then trips the internal assertion. Hence 1 real error plus 11 cascading ones.

So this is a deprecated pattern on our side, not a pytest regression.

Fix

Drop scope="class" from the d fixture. It just builds ImmutableMap(a=1, b=2), so class scope bought nothing.

Preventing a repeat

CI runs uv run --frozen, so a breaking release is invisible until the lock is refreshed. Added a tests-latest job that re-resolves to the newest versions (uv run --upgrade pytest) on every run. It is deliberately not in the status gate, so a bad upstream release can't block merges — it's an early-warning signal, not a required check.

No upper bound was added to pytest: the cap would have hidden a genuine deprecation rather than surfaced it, and uv.lock already gives CI reproducibility.

Verification

Full suite (README + src/ + tests/):

pytest	result
8.4.2	76 passed
9.1.1	76 passed

The canary command was also run locally: resolves pytest 9.1.1, 76 passed. pre-commit passes on both changed files.

Note for #66 / #67

Based on main rather than on the #66→#67 stack. The d fixture is byte-identical in main, #66 and #67, so basing this on the stack would block a CI fix behind two feature PRs. Merging here first leaves both PRs a one-line rebase.

Possible upstream report

The cascading internal assertion is arguably a pytest bug worth filing separately — pytest's own deprecation warning corrupts fixture state when raised as an error, and the resulting AssertionError completely hides the actionable message. Minimal repro under pytest 9.1.1 with -W error:

import pytest

class TestX:
    @pytest.fixture(scope="class")
    def d(self):
        return 1

    def test_a(self, d): assert d == 1
    def test_b(self, d): assert d == 1

test_a shows the deprecation; test_b shows only AssertionError.

🤖 Generated with Claude Code

[github-code-quality]

Code Coverage Overview

Languages: Python

Python / code-coverage-agent

The overall line coverage in commit 5b6d8c3 in the fix/pytest9-class-fi... branch remains at 100%, unchanged from commit 7fc1341 in the main branch.