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Sarj Code Standards

prefer-pytest-dependency-fixtures

Reuse supplied pytest fixtures and pass test collaborators through fixture dependencies.

SARJ526 Rule source Tests Catalog JSON

Why

Constructing collaborators in a test bypasses pytest's dependency graph, caching and overrides even when only one test currently needs the setup.

Fix

Request function-scoped store, service and settings fixtures by parameter. Use a typed factory fixture for variants or multiple instances. Keep construction under test explicit and preserve setup ordering and the actual owner's cleanup; do not create global singletons or autouse fixtures.

Examples

Before — flagged A test constructs its service's store
tests/test_service.py
from typing import Protocol




class Port(Protocol):
    def read(self): ...




class Store:
    def read(self):
        return 1




class Service:
    def __init__(self, store: Port):
        self.store = store


    def read(self):
        return self.store.read()




def test_read():
    store = Store()
    service = Service(store)
    assert service.read() == 1
After — preferred The store arrives through pytest
tests/test_service.py
from typing import Protocol




class Port(Protocol):
    def read(self): ...




class Store:
    def read(self):
        return 1




class Service:
    def __init__(self, store: Port):
        self.store = store


    def read(self):
        return self.store.read()




def test_read(store):
    service = Service(store)
    assert service.read() == 1
Before — flagged A second store bypasses the supplied fixture
tests/test_store.py
from typing import Protocol




class Port(Protocol):
    def read(self): ...




class Store:
    def read(self):
        return 1




class Service:
    def __init__(self, store: Port):
        self.store = store


    def read(self):
        return self.store.read()




def test_read(store: Store):
    replacement = Store()
    assert replacement.read() == 1
After — preferred Use the supplied store fixture
tests/test_store.py
from typing import Protocol




class Port(Protocol):
    def read(self): ...




class Store:
    def read(self):
        return 1




class Service:
    def __init__(self, store: Port):
        self.store = store


    def read(self):
        return self.store.read()




def test_read(store: Store):
    assert store.read() == 1
Before — flagged A visible plugin fixture owns the collaborator
app/__init__.py
"""Example application package."""
app/stores.py
from abc import ABC, abstractmethod




class Queue(ABC):
    @abstractmethod
    def retry(self) -> bool: ...




class QueueStore(Queue):
    def __init__(self, resource: object) -> None:
        self.resource = resource


    def retry(self) -> bool:
        return True




class Owner:
    def __init__(self, resource: object) -> None:
        self.resource = resource


    def read(self) -> bool:
        return True
tests/__init__.py
"""Example test package."""
tests/conftest.py
pytest_plugins = ["tests.fixtures"]
tests/fixtures.py
import pytest
from app.stores import Owner, QueueStore




@pytest.fixture
def resource() -> object:
    return object()




@pytest.fixture
def owner(resource: object) -> Owner:
    return Owner(resource)




@pytest.fixture
def queue_store(resource: object) -> QueueStore:
    return QueueStore(resource)
tests/test_retry.py
from app.stores import Owner, QueueStore




def test_retry(owner: Owner):
    queue = QueueStore(owner.resource)
    queue.retry()
    assert owner.read()
After — preferred Request the collaborator through pytest
app/__init__.py
"""Example application package."""
app/stores.py
from abc import ABC, abstractmethod




class Queue(ABC):
    @abstractmethod
    def retry(self) -> bool: ...




class QueueStore(Queue):
    def __init__(self, resource: object) -> None:
        self.resource = resource


    def retry(self) -> bool:
        return True




class Owner:
    def __init__(self, resource: object) -> None:
        self.resource = resource


    def read(self) -> bool:
        return True
tests/__init__.py
"""Example test package."""
tests/conftest.py
pytest_plugins = ["tests.fixtures"]
tests/fixtures.py
import pytest
from app.stores import Owner, QueueStore




@pytest.fixture
def resource() -> object:
    return object()




@pytest.fixture
def owner(resource: object) -> Owner:
    return Owner(resource)




@pytest.fixture
def queue_store(resource: object) -> QueueStore:
    return QueueStore(resource)
tests/test_retry.py
def test_retry(queue_store):
    assert queue_store.retry()