pytest-dev/pytest · error · ValueError

@pytest.fixture is being applied more than once to the same

Error message

@pytest.fixture is being applied more than once to the same function {function.__name__!r}

What it means

Raised by `FixtureFunctionMarker.__call__` when the function it receives is already a `FixtureFunctionDefinition`, i.e. `@pytest.fixture` has been stacked twice on the same function. Applying the decorator a second time wraps the already-produced definition object, which pytest rejects because the scope/params/name would be ambiguous.

Source

Thrown at src/_pytest/fixtures.py:1430

@dataclasses.dataclass(frozen=True)
class FixtureFunctionMarker:
    scope: ScopeName | Callable[[str, Config], ScopeName]
    params: tuple[object, ...] | None
    autouse: bool = False
    ids: tuple[object | None, ...] | Callable[[Any], object | None] | None = None
    name: str | None = None

    _ispytest: dataclasses.InitVar[bool] = False

    def __post_init__(self, _ispytest: bool) -> None:
        check_ispytest(_ispytest)

    def __call__(self, function: FixtureFunction) -> FixtureFunctionDefinition:
        if inspect.isclass(function):
            raise ValueError("class fixtures not supported (maybe in the future)")

        if isinstance(function, FixtureFunctionDefinition):
            raise ValueError(
                f"@pytest.fixture is being applied more than once to the same function {function.__name__!r}"
            )

        if hasattr(function, "pytestmark"):
            fail(
                "Marks cannot be applied to fixtures.\n"
                "See docs: https://docs.pytest.org/en/stable/deprecations.html#applying-a-mark-to-a-fixture-function"
            )

        fixture_definition = FixtureFunctionDefinition(
            function=function, fixture_function_marker=self, _ispytest=True
        )

        name = self.name or function.__name__
        if name == "request":
            location = getlocation(function)
            fail(
                f"'request' is a reserved word for fixtures, use another name:\n  {location}",

View on GitHub (pinned to 98b357f69e)

Solutions

Use exactly one `@pytest.fixture(...)` decorator per function with all options in a single call.
If a helper adds fixture behavior, have it return a plain function and decorate once at the definition site.
Remove the redundant decorator.

Example fix

// before
@pytest.fixture(scope="session")
@pytest.fixture
def x():
    yield 1
// after
@pytest.fixture(scope="session")
def x():
    yield 1

Defensive patterns

Strategy: validation

Validate before calling

def ensure_single_fixture_decorator(fn):
    if isinstance(fn, FixtureFunctionDefinition):
        raise ValueError(f"{fn.__name__!r} already decorated with @pytest.fixture")
    return fn

Type guard

def is_already_fixture(fn) -> bool:
    from _pytest.fixtures import FixtureFunctionDefinition
    return isinstance(fn, FixtureFunctionDefinition)

Prevention

Apply @pytest.fixture exactly once per function.
Put all fixture options in a single decorator call.
Avoid helper decorators that re-apply pytest.fixture.

When it happens

Trigger: Stacking decorators: `@pytest.fixture(scope="session")\n@pytest.fixture\ndef x(): ...`, or a helper that reapplies `@pytest.fixture` to an already-decorated fixture.

Common situations: Copy-paste of decorators. A custom decorator that internally calls `pytest.fixture` applied to a function already decorated. Migrating old code that used `@pytest.yield_fixture` plus `@pytest.fixture`.

Related errors

AI-assisted analysis of pytest-dev/pytest@98b357f69e (2026-08-04). Data as JSON: /data/errors/f7b5755152b533cb.json. Report an issue: GitHub.