Deprecate `self` in class-scoped fixtures on base classes (#10819) (#14011) by yastcher · Pull Request #14071 · pytest-dev/pytest

yastcher · 2025-12-28T15:27:52Z

closes Inherit fixture from baseclass with class scope fails #14011
closes Fixture with scope=class and autouse=True is not executed from mother's test class #10819

RonnyPfannschmidt · 2026-01-12T08:37:18Z

testing/test_inherit_class_fixture.py

+        pass
+
+    @pytest.fixture(scope="class")
+    def fix(self) -> typing.Generator[None]:


reminds me that we need to ensure that class scope fixtures do not actually ever get a instance - class scope is above instances and should get a class

Addressed: added _ClassScopeInstance proxy that redirects attribute writes to the class, so class-scoped fixtures never work with actual test instances

Implemented _ClassScopeInstance proxy that redirects attribute writes to the class, so class-scoped fixtures work correctly without actual test instances.

However, I noticed your comment in #10819 about deprecating non-classmethod class-scoped fixtures. If that's the preferred direction, this fix might be unnecessary — we could add a deprecation warning instead and guide users toward @ classmethod fixtures.

Should I proceed with the proxy approach, or switch to deprecation warning?

technically speaking its a bug to even try to pass a instance to a class scope fixture as no instance is available in that context

that a instance is actually avaliable is a bug an a artifact of how setupstate and scope initially developed

before there where fixtures we had the pytest_funcarg__ARGNAME hooks and no scopes at all

Got it. So the fix is correct — class-scoped fixtures should never receive an actual instance. The _ClassScopeInstance proxy ensures they work with the class directly, which is the intended behavior

We should error in future and warn right now

Instance methods are always wrong

Class scope requires classmethods

Understood. Will remove the proxy and add deprecation warning instead.

Proposed implementation:

In src/_pytest/deprecated.py:

CLASS_FIXTURE_INSTANCE_METHOD = PytestRemovedIn10Warning( "Class-scoped fixture defined as instance method is deprecated.\n" "Use @classmethod decorator instead.\n" "See https://docs.pytest.org/en/stable/deprecations.html#class-scoped-fixture-as-instance-method" )

Then warn in resolve_fixture_function when class-scoped fixture is an instance method.

Does this look right?

the deprecation message should be very aggressive about the fact that any changes to the instance would in fact not transfer to subsequent instances

CLASS_FIXTURE_INSTANCE_METHOD = PytestRemovedIn10Warning( "Class-scoped fixture defined as instance method is deprecated.\n" "Instance attributes set in this fixture will NOT be visible to test methods,\n" "as each test gets a new instance while the fixture runs only once per class.\n" "Use @classmethod decorator and set attributes on cls instead.\n" "See https://docs.pytest.org/en/stable/deprecations.html#class-scoped-fixture-as-instance-method" )

?

finished and squashed

nicoddemus · 2026-01-30T16:39:44Z

src/_pytest/deprecated.py

+    "Instance attributes set in this fixture will NOT be visible to test methods,\n"
+    "as each test gets a new instance while the fixture runs only once per class.\n"
+    "Use @classmethod decorator and set attributes on cls instead.\n"
+    "See https://docs.pytest.org/en/stable/deprecations.html#class-scoped-fixture-as-instance-method"


We need to introduce a new section in deprecations.rst explaning this deprecation, including examples of how to update the code.

.. _class-scoped-fixture-as-instance-method: Class-scoped fixture as instance method ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ .. deprecated:: 9.1 Defining a class-scoped fixture as an instance method (without ``@classmethod``) is deprecated and will be removed in pytest 10.0. When a class-scoped fixture is defined as an instance method, any attributes set on ``self`` will not be visible to test methods. This happens because pytest creates a new instance of the test class for each test method, while the fixture runs only once per class on a different instance. **Before** (deprecated): .. code-block:: python class TestExample: @pytest.fixture(scope="class") def setup_data(self): self.data = [1, 2, 3] # This won't be visible to tests! def test_something(self, setup_data): assert self.data == [1, 2, 3] # AttributeError: 'TestExample' object has no attribute 'data' **After** (recommended): .. code-block:: python class TestExample: @pytest.fixture(scope="class") @classmethod def setup_data(cls): cls.data = [1, 2, 3] def test_something(self, setup_data): assert self.data == [1, 2, 3] # Works correctly Using ``@classmethod`` ensures attributes are set on the class itself, making them accessible to all test methods.

nicoddemus · 2026-01-30T16:40:39Z

testing/test_inherit_class_fixture.py

@@ -0,0 +1,92 @@
+from __future__ import annotations


Not sure why we need this test module? I feel the deprecation and its test is enough for this change.

Removed. This test file was created for the initial approach (fixing the behavior), but became redundant once we decided to deprecate instead. The deprecation warning is already covered by the test in deprecated_test.py

webknjaz · 2026-02-03T09:10:43Z

Could you rephrase the PR title? It's rather vague.

…v#10819) (pytest-dev#14011)

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yastcher force-pushed the fix/14011-10819-correct-class-fixtures branch 3 times, most recently from 6c5967f to 9772d0c Compare December 28, 2025 17:21

psf-chronographer bot added the bot:chronographer:provided (automation) changelog entry is part of PR label Dec 28, 2025

yastcher force-pushed the fix/14011-10819-correct-class-fixtures branch from 9772d0c to acd357c Compare December 29, 2025 18:41

yastcher force-pushed the fix/14011-10819-correct-class-fixtures branch from acd357c to ff1759c Compare January 12, 2026 02:25

RonnyPfannschmidt reviewed Jan 12, 2026

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yastcher force-pushed the fix/14011-10819-correct-class-fixtures branch 9 times, most recently from 98b0e10 to 395b55c Compare January 20, 2026 01:17

yastcher changed the title ~~correct-class-fixtures (#10819) (#14011)~~ Resolves by deprecating the incorrect usage pattern and documenting the correct approach (#10819) (#14011) Jan 22, 2026

yastcher force-pushed the fix/14011-10819-correct-class-fixtures branch 3 times, most recently from d8935c9 to 959c305 Compare January 29, 2026 20:45

nicoddemus requested changes Jan 30, 2026

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yastcher force-pushed the fix/14011-10819-correct-class-fixtures branch from 959c305 to 1100ad2 Compare January 30, 2026 18:46

yastcher requested a review from nicoddemus January 30, 2026 19:05

yastcher force-pushed the fix/14011-10819-correct-class-fixtures branch 2 times, most recently from 5cafff1 to 0f3894c Compare February 3, 2026 01:23

yastcher force-pushed the fix/14011-10819-correct-class-fixtures branch from 0f3894c to df2e44d Compare February 3, 2026 15:35

yastcher changed the title ~~Resolves by deprecating the incorrect usage pattern and documenting the correct approach (#10819) (#14011)~~ Deprecate self in class-scoped fixtures on base classes (#10819) (#14011) Feb 3, 2026

yastcher and others added 2 commits February 3, 2026 23:07

deprecate: warn on class-scoped fixture as instance method (pytest-de…

c914da9

…v#10819) (pytest-dev#14011)

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yastcher force-pushed the fix/14011-10819-correct-class-fixtures branch from df2e44d to cdc35a9 Compare February 3, 2026 20:07

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webknjaz commented Feb 3, 2026

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yastcher Jan 17, 2026 •

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