How does a yield fixture handle teardown?

Code before yield sets up and the yielded value goes to the test; code after yield runs after the test finishes, even if it failed.

What's the risk of session-scoped fixtures?

They're shared across tests; if a test mutates them, later tests see the changes and become order-dependent.

How do you skip slow tests in a fast CI stage?

Mark them (@pytest.mark.slow), register the marker, and run pytest -m 'not slow'.

pytest Fixtures, Parametrisation and Markers — Testing in Practice

What you'll build

A test suite for a coupon service: a fixture builds a fresh service per test (with teardown), tmp_path provides a temporary file, parametrize checks a table of cases in one test function, and a custom marker tags slow tests so CI can choose what to run.

The idea

Fixtures provide test setup by name; yield fixtures run teardown after the test. Scopes (function, module, session) control reuse.
Built-in fixtures: tmp_path (a temporary directory), monkeypatch (environment and attributes), capsys (captured output).
@pytest.mark.parametrize runs the same test with many inputs; each case reports separately.
Markers (@pytest.mark.slow) let you select or skip groups: pytest -m "not slow". Register them in configuration.

The code

Fixtures, parametrize and markers

# file: coupons.py
import json

class CouponService:
    def __init__(self, path):
        self.codes = json.loads(path.read_text())
        self.closed = False

    def discount(self, code: str, total: float) -> float:
        rule = self.codes.get(code.upper())
        if rule is None or total < rule["min_total"]:
            return 0.0
        return round(total * rule["percent"] / 100, 2)

    def close(self):
        self.closed = True

# file: conftest.py
import json
import pytest
from coupons import CouponService

def pytest_configure(config):
    config.addinivalue_line("markers", "slow: tests that take a while")

@pytest.fixture
def service(tmp_path):
    path = tmp_path / "coupons.json"
    path.write_text(json.dumps({"WELCOME10": {"percent": 10, "min_total": 0}, "BIG20": {"percent": 20, "min_total": 100}}))
    svc = CouponService(path)
    yield svc
    svc.close()                          # teardown runs after each test

# file: test_coupons.py
import pytest

@pytest.mark.parametrize(("code", "total", "expected"), [
    ("WELCOME10", 50, 5.0),
    ("welcome10", 50, 5.0),             # case-insensitive
    ("BIG20", 99.99, 0.0),               # below the minimum
    ("BIG20", 100, 20.0),                # boundary
    ("UNKNOWN", 100, 0.0),
])
def test_discount(service, code, total, expected):
    assert service.discount(code, total) == expected

@pytest.mark.slow
def test_many_lookups(service):
    assert sum(service.discount("WELCOME10", 10) for _ in range(10_000)) == pytest.approx(10_000)

# file: main.py
import pytest
pytest.main(["-v", "-p", "no:cacheprovider", "-m", "not slow", "--no-header"])

# file: coupons.py
import json

class CouponService:
    def __init__(self, path):
        self.codes = json.loads(path.read_text())
        self.closed = False

def discount(self, code: str, total: float) -> float:
        rule = self.codes.get(code.upper())
        if rule is None or total < rule["min_total"]:
            return 0.0
        return round(total * rule["percent"] / 100, 2)

def close(self):
        self.closed = True

# file: conftest.py
import json
import pytest
from coupons import CouponService

def pytest_configure(config):
    config.addinivalue_line("markers", "slow: tests that take a while")

@pytest.fixture
def service(tmp_path):
    path = tmp_path / "coupons.json"
    path.write_text(json.dumps({"WELCOME10": {"percent": 10, "min_total": 0}, "BIG20": {"percent": 20, "min_total": 100}}))
    svc = CouponService(path)
    yield svc
    svc.close()                          # teardown runs after each test

# file: test_coupons.py
import pytest

@pytest.mark.parametrize(("code", "total", "expected"), [
    ("WELCOME10", 50, 5.0),
    ("welcome10", 50, 5.0),             # case-insensitive
    ("BIG20", 99.99, 0.0),               # below the minimum
    ("BIG20", 100, 20.0),                # boundary
    ("UNKNOWN", 100, 0.0),
])
def test_discount(service, code, total, expected):
    assert service.discount(code, total) == expected

@pytest.mark.slow
def test_many_lookups(service):
    assert sum(service.discount("WELCOME10", 10) for _ in range(10_000)) == pytest.approx(10_000)

# file: main.py
import pytest
pytest.main(["-v", "-p", "no:cacheprovider", "-m", "not slow", "--no-header"])

Output

============================= test session starts ==============================
collecting ... collected 6 items / 1 deselected / 5 selected

test_coupons.py::test_discount[WELCOME10-50-5.0] PASSED                  [ 20%]
test_coupons.py::test_discount[welcome10-50-5.0] PASSED                  [ 40%]
test_coupons.py::test_discount[BIG20-99.99-0.0] PASSED                   [ 60%]
test_coupons.py::test_discount[BIG20-100-20.0] PASSED                    [ 80%]
test_coupons.py::test_discount[UNKNOWN-100-0.0] PASSED                   [100%]

======================= 5 passed, 1 deselected in <time> ========================

Run this on your computer — see “Get Started” for setup.

The slow test was deselected by the marker expression, and each parametrised case shows up as its own test.

Common mistakes

Shared mutable fixtures with module or session scope that leak state between tests.
Copy-pasted test functions that differ only in input values; use parametrize.
Unregistered markers (typos silently select nothing); register them and use --strict-markers.

Key points

Fixtures build setup by name, with teardown after yield.
parametrize turns tables of cases into separate tests.
Markers organise suites; register them.

Further reading: pytest fixtures · Parametrizing tests · Markers

What you'll build

The idea

The code

Common mistakes

Key points

Practice questions