pytest Fixtures Guide

Basic Fixtures & Scope

import pytest

# function scope (default) — new instance per test
@pytest.fixture
def db_connection():
    conn = create_connection("sqlite:///:memory:")
    yield conn
    conn.close()

# module scope — shared across all tests in the file
@pytest.fixture(scope="module")
def http_client():
    client = TestClient(app)
    return client

# session scope — shared for the entire test session
@pytest.fixture(scope="session")
def app_config():
    return load_config("test.yaml")

# class scope — shared within a test class
@pytest.fixture(scope="class")
def browser(request):
    driver = webdriver.Chrome()
    request.cls.driver = driver
    yield driver
    driver.quit()

# Scope priority (broadest to narrowest):
# session > package > module > class > function

conftest.py — Shared Fixtures

# conftest.py (at project root or test directory)
import pytest
from myapp import create_app, db as _db

@pytest.fixture(scope="session")
def app():
    """Create application with test configuration."""
    app = create_app({"TESTING": True, "DATABASE_URL": "sqlite:///:memory:"})
    return app

@pytest.fixture(scope="session")
def db(app):
    """Initialize database once for the session."""
    with app.app_context():
        _db.create_all()
        yield _db
        _db.drop_all()

@pytest.fixture
def client(app):
    """HTTP test client, fresh per test."""
    return app.test_client()

@pytest.fixture
def auth_headers():
    """JWT auth headers for API tests."""
    token = generate_test_token(user_id=1, role="admin")
    return {"Authorization": f"Bearer {token}"}

# conftest.py is automatically discovered by pytest
# No import needed in test files — just use fixture names as params

yield Fixtures — Setup & Teardown

import pytest from unittest.mock import patch @pytest.fixture def mock_external_api(): """Patch external API calls for the duration of the test.""" with patch("myapp.services.requests.get") as mock_get: mock_get.return_value.json.return_value = {"status": "ok"} mock_get.return_value.status_code = 200 yield mock_get # cleanup happens after yield automatically @pytest.fixture def temp_user(db): """Create a user, clean up after test.""" user = User(name="Test User", email="[email protected]") db.session.add(user) db.session.commit() yield user db.session.delete(user) db.session.commit() # Using multiple fixtures def test_user_profile(client, temp_user, auth_headers): response = client.get(f"/users/{temp_user.id}", headers=auth_headers) assert response.status_code == 200 assert response.json["name"] == "Test User"

Parametrize with Fixtures

import pytest

@pytest.fixture(params=["sqlite", "postgres", "mysql"])
def database(request):
    """Run tests against multiple DB backends."""
    db_type = request.param
    conn = connect_to_db(db_type)
    yield conn
    conn.close()

def test_insert_record(database):
    # This test runs 3 times, once per database type
    database.execute("INSERT INTO users (name) VALUES ('Alice')")
    result = database.execute("SELECT * FROM users").fetchall()
    assert len(result) == 1

# Named parameter sets with ids
@pytest.fixture(params=[
    pytest.param({"timeout": 5}, id="fast"),
    pytest.param({"timeout": 30}, id="slow"),
])
def config(request):
    return request.param

autouse Fixtures

import pytest

# Automatically used by all tests in scope
@pytest.fixture(autouse=True)
def reset_database(db):
    """Roll back all changes after each test."""
    yield
    db.session.rollback()

# autouse=True scoped to a class
class TestPayments:
    @pytest.fixture(autouse=True)
    def setup_payment_gateway(self):
        with patch("myapp.payments.stripe") as mock_stripe:
            mock_stripe.charge.return_value = {"id": "ch_test"}
            self.stripe = mock_stripe
            yield

    def test_charge_card(self):
        result = process_payment(amount=100)
        assert self.stripe.charge.called

tmpdir / tmp_path — Temporary Files

import pytest
from pathlib import Path

# tmp_path (modern, pathlib.Path, recommended)
def test_writes_output_file(tmp_path):
    output_file = tmp_path / "output.txt"
    write_report(output_file)
    assert output_file.exists()
    assert output_file.read_text() == "Report done\n"

# tmp_path_factory for session-scoped temp dirs
@pytest.fixture(scope="session")
def shared_data_dir(tmp_path_factory):
    base = tmp_path_factory.mktemp("data")
    (base / "sample.csv").write_text("id,name\n1,Alice\n")
    return base

# tmpdir (legacy, py.path.local)
def test_config_loading(tmpdir):
    cfg = tmpdir.join("config.yaml")
    cfg.write("debug: true\nport: 8080\n")
    loaded = load_config(str(cfg))
    assert loaded["port"] == 8080