Move fixture cache from FixtureDef to SetupState by seifertm · Pull Request #14770 · pytest-dev/pytest

seifertm · 2026-07-23T10:16:40Z

This PR removes FixtureDef.cached_result in favor of a cache that is attached to SetupState. FixtureRequest received methods to manipulate the fixture cache.

The PR is easiest reviewed commit by commit.

Open issues

Previously, fixture values were cached as part of FixtureDef objects and the cache entries contained a cache_key. The cache key is either None for non-parametrized fixtures or SubRequest.param. This allows invalidating cache entries when fixture parameters change.

Hoisting up the fixture cache to SetupState technically makes FixtureDef part of the cache key. This means we should use a two-dimensional index (e.g. tuple[FixtureDef, param]) to store entries in the fixture cache. Unfortunately, this is tricky, because TopRequest doesn't know about the current fixture param and therefore doesn't know the cache key.

This work has been done as part of the pytest sprint 2026, sponsored by Omicron.

RonnyPfannschmidt · 2026-07-23T11:16:41Z

Good find, thanks @RonnyPfannschmidt !

I pushed 5 additional commits. The tracking of the currently active request parameter for --setup-show is no longer attached to FixtureDef. Instead, --setup-show uses the existing SetupState.fixture_cache, which already contains the active fixture param as a cache key. The variable naming was changed from "cache_key" to "param" or "active_param".

In order to distinguish between "no param" and "param=None", I introduced a sentinel value. This prevents --setup-show from printing my_fixture[None] for unparametrized fixtures.

bluetech · 2026-07-23T11:32:13Z

Previous stalled PR you might want to use as a reference/comparison: #14104

seifertm · 2026-07-23T16:00:52Z

Thanks @bluetech! This topic was raised during the pytest sprint and I wasn't aware that there have been previous attempts at this. I'll check out the PR.

…of FixtureDef.

Use _invalidate_fixture_cache instead.

… to FixtureRequest.

…Def to SubRequest

…the result is successful or an exception.

…he responsibility of FixtureRequest.

… traceback

…he_value and _cache_exception, respectively

…of SetupState, instead of FixtureDef.

…it about the contents of the cache key.

…hing fixture results to distinguish between *no param* and *None*.

…arams for --setup-show.

seifertm · 2026-07-24T09:51:06Z

Previous stalled PR you might want to use as a reference/comparison: #14104

The linked PR appears to rework fixture evaluation logic and move that to runner.py. This PR reduces the mutability of FixtureDef by moving the fixture cache from FixtureDef to SetupState. On first sight, both PRs seem to be complementary rather than competing with each other.

seifertm · 2026-07-24T10:03:25Z

+if TYPE_CHECKING and sys.version_info >= (3, 11):
+
+    class _FixtureResult(NamedTuple, Generic[FixtureValue]):
+        value: FixtureValue
+        param: object
+        exception_and_traceback: None


This code is marked as uncovered or partially covered, even though it's only used for type checking.

Do you have any suggestions how I can deal with that?

Lets add a # pragma: no cover on the if line.

Or better yet, update the exceptions in

pytest/pyproject.toml

Lines 476 to 487 in ef052d0

exclude_lines = [

'\#\s*pragma: no cover',

'^\s*raise NotImplementedError\b',

'^\s*return NotImplemented\b',

'^\s*assert False(,|$)',

'^\s*case unreachable:',

'^\s*assert_never\(',

'^\s*if TYPE_CHECKING:',

'^\s*(el)?if TYPE_CHECKING:',

'^\s*@overload( |$)',

'^\s*def .+: \.\.\.$',

'^\s*@pytest\.mark\.xfail',

to if TYPE_CHECKING\s.*

…ty in pytest_fixture_post_finalizer.

…ather than fetching it from the cache.

for more information, see https://pre-commit.ci

nicoddemus · 2026-07-24T14:07:22Z

+if TYPE_CHECKING and sys.version_info >= (3, 11):
+
+    class _FixtureResult(NamedTuple, Generic[FixtureValue]):
+        value: FixtureValue
+        param: object
+        exception_and_traceback: None


Lets add a # pragma: no cover on the if line.

Or better yet, update the exceptions in

pytest/pyproject.toml

Lines 476 to 487 in ef052d0

exclude_lines = [

'\#\s*pragma: no cover',

'^\s*raise NotImplementedError\b',

'^\s*return NotImplemented\b',

'^\s*assert False(,|$)',

'^\s*case unreachable:',

'^\s*assert_never\(',

'^\s*if TYPE_CHECKING:',

'^\s*(el)?if TYPE_CHECKING:',

'^\s*@overload( |$)',

'^\s*def .+: \.\.\.$',

'^\s*@pytest\.mark\.xfail',

to if TYPE_CHECKING\s.*

nicoddemus · 2026-07-24T14:09:47Z

-        # If the fixture was executed, the current value of the fixture.
-        # Can change if the fixture is executed with different parameters.
-        self.cached_result: _FixtureCachedResult[FixtureValue] | None = None
        self._finalizers: Final[list[Callable[[], object]]] = []


Unrelated to the PR, more of a question (@RonnyPfannschmidt): what about the finalizers? We likely would also want finalizers to be hosted per-thread?

nicoddemus · 2026-07-24T14:11:39Z

+        # Importing the appropriate types from the fixtures module lead to circular
+        # imports, so we leave the cache untyped for now
+        self.fixture_cache = {}  # type: ignore[var-annotated]


I think having the type checking here is important, let's move the type definitions we need to a new fixtures_types module, containing only the type definitions, which allows us to import them here without cycles.

nicoddemus · 2026-07-24T14:12:34Z

    )


+def test_suppress_capturing(pytester: Pytester) -> None:


Why is this test defined here?

It was added to make the CI pass. The codecov job seems to fail, if the coverage of the PR is less than 100%. Apparently this also applies to files that are touched by the PR, but didn't have full coverage. This test specifically covers the negative branch of this if statement.

Co-authored-by: Bruno Oliveira <bruno@soliv.dev>

psf-chronographer Bot added the bot:chronographer:provided (automation) changelog entry is part of PR label Jul 23, 2026

seifertm force-pushed the move_fixture_cache_from_fixturedef_to_setupstate branch from c54967f to 6678e5d Compare July 23, 2026 10:19

seifertm marked this pull request as ready for review July 23, 2026 10:19

RonnyPfannschmidt requested changes Jul 23, 2026

View reviewed changes

The-Compiler force-pushed the main branch from cb2ee5b to ced9022 Compare July 24, 2026 09:22

seifertm added 18 commits July 24, 2026 11:34

fixtures, runner: Store cached fixture values in SetupState, instead …

87453b1

…of FixtureDef.

fixtures: Convert fixture results to NamedTuples

9204a14

fixtures: Extracted method to invalidate the cached result of a fixture.

21565b5

fixtures: Don't allow setting the cached fixture value to None.

8306a1f

Use _invalidate_fixture_cache instead.

fixtures: RequestFixtureDef.cache_key returns the correct cache key.

e09fda8

fixtures: Move methods for manipulating fixture cache from FixtureDef…

ee27c17

… to FixtureRequest.

fixtures: Move determination of fixture value cache keys from Fixture…

7ebde2f

…Def to SubRequest

fixtures: Split up methods for cache population depending on whether …

18813a7

…the result is successful or an exception.

fixtures: Selecting the correct cache key for fixture values is now t…

5497a67

…he responsibility of FixtureRequest.

fixtures: FixtureRequest._set_cached_exception no longer requires the…

c29c6e9

… traceback

fixtures: Rename _set_cached_result and _set_cached_exception to _cac…

c5b7e16

…he_value and _cache_exception, respectively

fixtures: Convert FixtureRequest._cache_key into a property

c5d2887

docs: Add news fragment

2a28f9e

setuponly, runner: Move track currently active fixture param as part …

f04e086

…of SetupState, instead of FixtureDef.

fixtures: Rename cache_key to active_param or param to be more explic…

c9555d4

…it about the contents of the cache key.

fixtures: Use sentinel value for non-existant fixture params when cac…

0269198

…hing fixture results to distinguish between *no param* and *None*.

fixtures, setuponly: Reuse SetupState.fixture_cache to track active p…

18f5852

…arams for --setup-show.

fixtures: Address typing error for generic namedtuples in Python 3.10

61286bd

seifertm force-pushed the move_fixture_cache_from_fixturedef_to_setupstate branch from cf3854d to 61286bd Compare July 24, 2026 09:34

seifertm commented Jul 24, 2026

View reviewed changes

seifertm added 2 commits July 24, 2026 15:33

fixtures: Replace mentions of FixtureDef.cached_result in the docs

dd68168

setuponly: Add test for suppressing capture.

b49c266

seifertm and others added 3 commits July 24, 2026 15:41

setuponly: Adds assertion that the cached fixture value cannot be emp…

8a17fc1

…ty in pytest_fixture_post_finalizer.

setuponly: _show_fixture_action directly receives the fixture param r…

052ad16

…ather than fetching it from the cache.

[pre-commit.ci] auto fixes from pre-commit.com hooks

8d136d4

for more information, see https://pre-commit.ci

nicoddemus requested changes Jul 24, 2026

View reviewed changes

seifertm and others added 2 commits July 24, 2026 16:14

changelog: Clarify wording in news fragment

0e24562

Co-authored-by: Bruno Oliveira <bruno@soliv.dev>

pyproject: Ignore additional TYPE_CHECKING expressions from coverage.

9720a75

seifertm force-pushed the move_fixture_cache_from_fixturedef_to_setupstate branch from 1c6a7d2 to 9720a75 Compare July 24, 2026 14:39

	exclude_lines = [
	'\#\s*pragma: no cover',
	'^\s*raise NotImplementedError\b',
	'^\s*return NotImplemented\b',
	'^\s*assert False(,\|$)',
	'^\s*case unreachable:',
	'^\s*assert_never\(',
	'^\s*if TYPE_CHECKING:',
	'^\s*(el)?if TYPE_CHECKING:',
	'^\s*@overload( \|$)',
	'^\s*def .+: \.\.\.$',
	'^\s*@pytest\.mark\.xfail',

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seifertm commented Jul 23, 2026 •

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