Fixtures

Q: Where do fixtures live?

conftest.py in tests/ or subdirectories. Auto-discovered by pytest.

Q: What is yield vs return?

yield enables teardown code after the test. return has no teardown.

Q: What is autouse?

@pytest.fixture(autouse=True) runs for every test in scope without being requested.

Q: How do I override a fixture?

Redefine it in a subdirectory conftest.py or use @pytest.fixture(name="...").

Q: Can fixtures be async?

Yes with pytest-asyncio: @pytest.fixture + async def.

Q: What is params on fixtures?

@pytest.fixture(params=[1, 2, 3]) runs the test once per param value.

Q: How do I access a fixture without injecting?

request.getfixturevalue("name") inside a fixture or test.

Fixtures provide test dependencies - database connections, API clients, sample data - via dependency injection. Define once in conftest.py, inject by parameter name.

Recipe

# tests/conftest.py
import pytest
 
@pytest.fixture
def sample_user():
    return {"id": 1, "email": "ada@example.com"}
 
# tests/test_users.py
def test_user_email(sample_user):
    assert "@" in sample_user["email"]

When to reach for this:

Multiple tests share the same setup
Tests need teardown (close connections, delete temp files)
You want composable test dependencies

Working Example

# tests/conftest.py
import pytest
from sqlalchemy import create_engine
from sqlalchemy.orm import Session
 
@pytest.fixture(scope="session")
def engine():
    eng = create_engine("sqlite:///:memory:")
    yield eng
    eng.dispose()
 
@pytest.fixture
def db_session(engine):
    connection = engine.connect()
    transaction = connection.begin()
    session = Session(bind=connection)
    yield session
    session.close()
    transaction.rollback()
    connection.close()
 
@pytest.fixture
def user_factory(db_session):
    def _make(email: str = "test@example.com"):
        user = User(email=email)
        db_session.add(user)
        db_session.flush()
        return user
    return _make

def test_create_user(user_factory):
    user = user_factory(email="ada@example.com")
    assert user.id is not None

What this demonstrates:

scope="session" shares the engine across all tests
Per-test db_session rolls back for isolation
Factory fixture returns a callable for flexible data
yield fixtures run teardown after the test

Deep Dive

Fixture Scopes

Scope	Lifetime	Use for
function	One test	Default, most fixtures
class	One test class	Shared class setup
module	One file	Expensive per-file setup
session	Entire run	DB engine, app instance

Composing Fixtures

@pytest.fixture
def api_client(app):
    return TestClient(app)

api_client automatically receives the app fixture.

Gotchas

Scope mismatch - session fixture depending on function fixture. Fix: broader scope cannot depend on narrower scope.
Mutable shared state - session-scoped dict modified by tests. Fix: function scope or copy-on-write.
Missing teardown - connections leak. Fix: use yield fixtures for cleanup.
Fixture name typos - injected as a parameter that pytest does not recognize. Fix: parameter name must match fixture name exactly.
Over-fixturing - one-line setup in a fixture. Fix: inline simple arrange; fixture only when reused.

Alternatives

Alternative	Use When	Don't Use When
setUp/tearDown	unittest style	pytest project
Manual setup in each test	One-off cases	Shared setup
Factory pattern (no fixture)	Simple data	Need teardown

FAQs

Where do fixtures live?

conftest.py in tests/ or subdirectories. Auto-discovered by pytest.

What is yield vs return?

yield enables teardown code after the test. return has no teardown.

How do I use a fixture in another fixture?

Declare it as a parameter: def api(app): ....

What is autouse?

@pytest.fixture(autouse=True) runs for every test in scope without being requested.

How do I override a fixture?

Redefine it in a subdirectory conftest.py or use @pytest.fixture(name="...").

Can fixtures be async?

Yes with pytest-asyncio: @pytest.fixture + async def.

What is params on fixtures?

@pytest.fixture(params=[1, 2, 3]) runs the test once per param value.

How do I access a fixture without injecting?

request.getfixturevalue("name") inside a fixture or test.

Session scope and test isolation?

Session fixtures persist state. Ensure tests do not mutate shared objects.

Fixture vs factory?

Fixture provides an instance. Factory fixture returns a function that creates instances.

pytest Setup - conftest discovery
Parametrization - multiple test inputs
Mocking & Patching - mock fixtures
Testing Web APIs - TestClient fixtures

Stack versions: This page was written for Python 3.14.0, FastAPI 0.115+, Django 5.2, Flask 3.1, Pydantic 2, PyTorch 2.6+, pandas 2.2+, Polars 1.x, ruff 0.9+, and uv 0.6+.

Recipe

# tests/conftest.py
import pytest
 
@pytest.fixture
def sample_user():
    return {"id": 1, "email": "ada@example.com"}
 
# tests/test_users.py
def test_user_email(sample_user):
    assert "@" in sample_user["email"]

When to reach for this:

Multiple tests share the same setup
Tests need teardown (close connections, delete temp files)
You want composable test dependencies

Working Example

# tests/conftest.py
import pytest
from sqlalchemy import create_engine
from sqlalchemy.orm import Session
 
@pytest.fixture(scope="session")
def engine():
    eng = create_engine("sqlite:///:memory:")
    yield eng
    eng.dispose()
 
@pytest.fixture
def db_session(engine):
    connection = engine.connect()
    transaction = connection.begin()
    session = Session(bind=connection)
    yield session
    session.close()
    transaction.rollback()
    connection.close()
 
@pytest.fixture
def user_factory(db_session):
    def _make(email: str = "test@example.com"):
        user = User(email=email)
        db_session.add(user)
        db_session.flush()
        return user
    return _make

def test_create_user(user_factory):
    user = user_factory(email="ada@example.com")
    assert user.id is not None

What this demonstrates:

scope="session" shares the engine across all tests
Per-test db_session rolls back for isolation
Factory fixture returns a callable for flexible data
yield fixtures run teardown after the test

Deep Dive

Fixture Scopes

Scope	Lifetime	Use for
function	One test	Default, most fixtures
class	One test class	Shared class setup
module	One file	Expensive per-file setup
session	Entire run	DB engine, app instance

Composing Fixtures

@pytest.fixture
def api_client(app):
    return TestClient(app)

api_client automatically receives the app fixture.

Gotchas

Scope mismatch - session fixture depending on function fixture. Fix: broader scope cannot depend on narrower scope.
Mutable shared state - session-scoped dict modified by tests. Fix: function scope or copy-on-write.
Missing teardown - connections leak. Fix: use yield fixtures for cleanup.
Fixture name typos - injected as a parameter that pytest does not recognize. Fix: parameter name must match fixture name exactly.
Over-fixturing - one-line setup in a fixture. Fix: inline simple arrange; fixture only when reused.

Alternatives

Alternative	Use When	Don't Use When
setUp/tearDown	unittest style	pytest project
Manual setup in each test	One-off cases	Shared setup
Factory pattern (no fixture)	Simple data	Need teardown

FAQs

Where do fixtures live?

conftest.py in tests/ or subdirectories. Auto-discovered by pytest.

What is yield vs return?

yield enables teardown code after the test. return has no teardown.

How do I use a fixture in another fixture?

Declare it as a parameter: def api(app): ....

What is autouse?

@pytest.fixture(autouse=True) runs for every test in scope without being requested.

How do I override a fixture?

Redefine it in a subdirectory conftest.py or use @pytest.fixture(name="...").

Can fixtures be async?

Yes with pytest-asyncio: @pytest.fixture + async def.

What is params on fixtures?

@pytest.fixture(params=[1, 2, 3]) runs the test once per param value.

How do I access a fixture without injecting?

request.getfixturevalue("name") inside a fixture or test.

Session scope and test isolation?

Session fixtures persist state. Ensure tests do not mutate shared objects.

Fixture vs factory?

Fixture provides an instance. Factory fixture returns a function that creates instances.

pytest Setup - conftest discovery
Parametrization - multiple test inputs
Mocking & Patching - mock fixtures
Testing Web APIs - TestClient fixtures

Stack versions: This page was written for Python 3.14.0, FastAPI 0.115+, Django 5.2, Flask 3.1, Pydantic 2, PyTorch 2.6+, pandas 2.2+, Polars 1.x, ruff 0.9+, and uv 0.6+.

Fixtures

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Fixtures