correct-class-fixtures (#10819) (#14011)

[yastcher]

• 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]

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

[yastcher]

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

[yastcher]

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?

[RonnyPfannschmidt]

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

[yastcher]

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

[RonnyPfannschmidt]

We should error in future and warn right now

Instance methods are always wrong

Class scope requires classmethods

[yastcher]

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?

[RonnyPfannschmidt]

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

[yastcher]

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"
)

?