Module 5: Test data setup & teardown

A test is only trustworthy if you control the data it runs against. Two things break that control: tests that depend on each other (one leaves data that changes another’s result), and setup done through the UI (slow and brittle). This module fixes both — reset to a known baseline, and arrange test data through the API with fixtures that build exactly what each test needs.

Keep TestMarket Lab running at http://localhost:3000 and your .venv active. This module builds directly on the fixtures from Module 2 and the POST calls from Module 3.

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Module 5: test data setup & teardown at a glance — slide guide Slides — opens in a new tab

The problem: tests that lean on ambient state

Suppose one test adds a product and a later test asserts “there are 15 products.” Run them in that order and the second fails — not because of a bug, but because the first test left data behind. Tests that pass or fail depending on what ran before them are the classic flaky suite.

The cure is isolation: every test starts from a known state and sets up its own data.

Remember: a test that depends on another test’s leftovers (or on the exact seed) is fragile. Each test should control its own starting state and its own data.

A known baseline: reset

You met reset_db in Module 2 — it POSTs to /api/reset, reseeding the database to its 15-product starting state. A test requests it to guarantee a clean slate:

import pytest
import requests


@pytest.fixture(scope="session")
def base_url():
    return "http://localhost:3000"


@pytest.fixture
def api(base_url):
    session = requests.Session()
    yield session
    session.close()


@pytest.fixture
def reset_db(base_url):
    requests.post(f"{base_url}/api/reset")
    yield

If you want every test isolated without remembering to ask, make it autouse=True — pytest then runs it before each test automatically:

@pytest.fixture(autouse=True)
def reset_db(base_url):
    requests.post(f"{base_url}/api/reset")
    yield

The trade-off: autouse reseeds even for read-only tests (a little slower), but guarantees no test can ever see another’s data. Opt-in is more surgical; autouse is safer by default — pick per suite.

Remember: reset gives a known baseline. Request reset_db where you need isolation, or make it autouse=True to reseed before every test — trading a little speed for guaranteed independence.

Arrange via the API, not the UI

To test “an order shows the right total,” you could click through the UI: log in, open a product, add to cart, check out. That’s slow and breaks whenever the UI changes. The professional pattern is to arrange through the API — build the state directly with POST, then assert:

import pytest


def test_order_total_via_api(reset_db, base_url, api):
    # arrange: place an order straight through the API — no clicking
    api.post(
        f"{base_url}/api/orders",
        json={"email": "customer@test.io", "items": [{"product_id": 1, "quantity": 2}]},
    )

    # assert: the order is there with the computed total (Wireless Mouse 29.99 x 2)
    orders = api.get(f"{base_url}/api/orders").json()
    assert any(o["total"] == pytest.approx(29.99 * 2) for o in orders)   # computed float -> approx

API setup is fast (milliseconds, not seconds) and precise — you build the exact state you want, nothing more. (Module 6 uses this to seed data for UI tests.)

Remember: arrange test data through the API, not the UI — it’s 10–50× faster and far less brittle. Set up state with POST, then assert. Save the UI for what you’re actually testing.

A data-building fixture

When several tests need “a product to work with,” move that setup into a fixture that makes one and hands back its data. Have it depend on reset_db so the fresh product’s name can’t clash with a previous run:

# tests/conftest.py (add to it)
@pytest.fixture
def product(reset_db, base_url, api):
    """Make a fresh product and give the test its data."""
    created = api.post(
        f"{base_url}/api/products",
        json={"name": "Fixture Product", "price": 19.99, "category": "electronics"},
    )
    return created.json()

def test_new_product_is_listed(base_url, api, product):
    listing = api.get(f"{base_url}/api/products").json()
    assert product["id"] in [p["id"] for p in listing]

The test never asks for reset_db directly — requesting product pulls it in, because product depends on it. That dependency also orders them correctly: reset_db runs first, then the product is made.

Remember: a data-building fixture makes the resource a test needs and returns its data. Depend on reset_db inside it so the reset always runs before the data is built (fixture order follows the dependency chain).

Factory fixtures: make as much as you need

A plain fixture makes one thing. When a test needs several — or wants to customize each — return a factory: a small function the test calls as many times as it likes. Give each product a unique name so they never collide:

# tests/conftest.py (add to it)
@pytest.fixture
def make_product(reset_db, base_url, api):
    """Return a factory that makes products with unique names."""
    count = 0

    def _make(**overrides):
        nonlocal count
        count += 1
        payload = {
            "name": f"Widget {count}",           # unique per call
            "price": 9.99,
            "category": "electronics",
        }
        payload.update(overrides)                # let callers override any field
        return api.post(f"{base_url}/api/products", json=payload).json()

    return _make

def test_factory_makes_distinct_products(make_product):
    first = make_product()
    second = make_product(price=49.99)

    assert first["id"] != second["id"]
    assert second["price"] == 49.99

make_product returns a function, so the test controls how many products exist and what they look like — the flexible cousin of a one-shot fixture.

Remember: a factory fixture returns a function, letting one test build many (customized) resources. Generate unique values (names/ids) inside it so repeated calls don’t collide.

A note on teardown

Because reset_db reseeds before each test, most suites don’t need explicit teardown — the next test wipes the slate itself. If a suite doesn’t reset, clean up what you made instead: DELETE the resource after a yield (Module 2’s yield-teardown, with Module 3’s DELETE), so the data doesn’t leak into later tests.

Remember: reseeding before each test usually makes teardown unnecessary. Without a reset, tear down your own data (DELETE after yield) so it can’t pollute other tests.

Putting it together

Arrange a custom product with the factory, place an order for it through the API, and assert on the result — all setup via API, no UI:

import pytest


def test_order_uses_the_made_product(base_url, api, make_product):
    widget = make_product(price=25.00)                      # arrange: a $25 product

    api.post(                                               # arrange: order 2 of them
        f"{base_url}/api/orders",
        json={"email": "customer@test.io",
              "items": [{"product_id": widget["id"], "quantity": 2}]},
    )

    orders = api.get(f"{base_url}/api/orders").json()        # assert: total is right
    assert any(o["total"] == pytest.approx(50.00) for o in orders)

One test, fully in control: it makes its product, places its order, and checks the outcome — starting from a clean database and touching only the API.

Remember: the pro pattern is arrange (API) → act → assert. Fixtures build the data, reset guarantees the baseline, and the test reads top-to-bottom as the scenario it verifies.

Exercises

Work in your python-sdet project with TestMarket Lab running. Run with pytest -v.

Exercise 1 — Prove isolation (10 min)

Write two tests: the first adds a product; the second asserts GET /api/products returns exactly 15. Make them pass in any order by giving the counting test reset_db (so it always starts from the 15-product seed) — or by making reset autouse. Confirm that without the reset the counting test is flaky (it sees 16 if the adding test ran first).

Exercise 2 — A `product` fixture (10 min)

Add a product fixture to conftest.py that makes a product (depending on reset_db) and returns its JSON. Use it in a test that asserts the product’s id appears in GET /api/products.

Exercise 3 — Arrange an order via the API (10 min)

Without touching the UI, POST an order for product_id 1, quantity 3, then assert an order with total == pytest.approx(29.99 * 3) appears in GET /api/orders.

Exercise 4 — A `make_product` factory (15 min)

Add a make_product factory fixture that makes products with unique names. Write a test that makes three products and asserts they have three distinct ids.

Bonus — Teardown without reset

Write a fixture that makes a product and, after yield, DELETEs it — then assert (in the teardown’s own follow-up, or a second test) that the product is gone. Compare it with the reset-based approach: which is simpler here, and why?

In Module 6 we finally drive the browser with Playwright for Python — locators, actions, waits, and assertions — using this module’s API-arrange trick to seed data fast before each UI test.