Module 4: Negative & validation testing

Modules 1–3 tested the happy path — valid input, expected success. But the highest-value testing is often the opposite: send bad input and confirm the API rejects it correctly. A robust API answers bad input with the right error status and a helpful message — not a crash, not a silent success. In this module you’ll deliberately trigger 400, 401, 404, and 409, assert the API fails the right way, and — when it doesn’t — document the bug with xfail.

Keep TestMarket Lab running at http://localhost:3000 and your .venv active.

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Module 4: negative testing at a glance — slide guide Slides — opens in a new tab

Why negative testing matters

Happy-path tests prove the feature works when everything is right. Negative tests prove it fails safely when something is wrong — and that’s where real bugs hide. A user will eventually submit an empty form, a wrong password, or a duplicate email; your tests should pin down exactly what the API does when they do.

Two things make a good negative test:

The right status code — 400 for bad input, 401 for bad credentials, 404 for a missing resource, 409 for a conflict. The wrong code (or a 500 crash) is itself a bug.
A useful error body — a message that tells the caller what went wrong.

Remember: negative testing asserts the system fails correctly — the right 4xx status and a helpful message. Untested error paths are where bugs live.

400 Bad Request — missing or invalid input

400 means “your request was malformed” — a required field is missing or a value is invalid. TestMarket Lab validates several endpoints:

import requests

BASE_URL = "http://localhost:3000"


def test_login_missing_password():
    response = requests.post(f"{BASE_URL}/api/auth/login", json={"email": "[email protected]"})
    assert response.status_code == 400


def test_create_product_missing_price():
    response = requests.post(f"{BASE_URL}/api/products", json={"name": "No Price"})
    assert response.status_code == 400


def test_register_password_too_short():
    response = requests.post(
        f"{BASE_URL}/api/auth/register",
        json={"name": "Sam", "email": "[email protected]", "password": "x"},   # < 6 chars
    )
    assert response.status_code == 400

Each of these would-be writes is rejected, so nothing is added — the tests are safe to re-run without resetting.

Remember: 400 = malformed request (missing field, invalid value). It’s the API’s first line of defense; assert it wherever a field is required or constrained.

401 Unauthorized — bad credentials

401 means “I don’t know who you are” — authentication failed. A wrong password and an unknown email both return 401 (deliberately the same response, so an attacker can’t tell which emails exist):

import requests

BASE_URL = "http://localhost:3000"


def test_login_wrong_password():
    response = requests.post(
        f"{BASE_URL}/api/auth/login",
        json={"email": "[email protected]", "password": "not-my-password"},
    )
    assert response.status_code == 401


def test_login_unknown_email():
    response = requests.post(
        f"{BASE_URL}/api/auth/login",
        json={"email": "[email protected]", "password": "whatever"},
    )
    assert response.status_code == 401

Remember: 401 = authentication failed (bad or missing credentials). Note the security touch: wrong-password and unknown-email give the identical 401, so the API never leaks which emails are registered.

404 Not Found — missing resource

404 means the thing you asked for doesn’t exist. It applies to reads and writes against a missing id:

import requests

BASE_URL = "http://localhost:3000"

MISSING_ID = 99999


def test_get_missing_product():
    assert requests.get(f"{BASE_URL}/api/products/{MISSING_ID}").status_code == 404


def test_update_missing_product():
    response = requests.put(f"{BASE_URL}/api/products/{MISSING_ID}", json={"price": 9.99})
    assert response.status_code == 404


def test_delete_missing_product():
    assert requests.delete(f"{BASE_URL}/api/products/{MISSING_ID}").status_code == 404

Remember: 404 = the resource (by id or URL) doesn’t exist — for GET, PUT, and DELETE alike. Use an id you know is absent (a huge number like 99999).

409 Conflict — duplicate

409 means “this conflicts with what already exists.” Registering an email that’s already taken is the classic case:

import requests

BASE_URL = "http://localhost:3000"


def test_register_duplicate_email():
    # [email protected] is a seeded account, so registering it again conflicts
    response = requests.post(
        f"{BASE_URL}/api/auth/register",
        json={"name": "Impostor", "email": "[email protected]", "password": "secret123"},
    )
    assert response.status_code == 409

Remember: 409 = a conflict with existing state (a duplicate). It’s distinct from 400: the request is well-formed, it just clashes with a resource that already exists.

Assert the error, not just the status

The status code is the headline; the body is the detail. Assert both — but assert the message loosely (a key substring), so a harmless wording tweak doesn’t break your test:

import requests

BASE_URL = "http://localhost:3000"


def test_error_body_is_helpful():
    response = requests.post(f"{BASE_URL}/api/auth/login", json={})
    assert response.status_code == 400

    body = response.json()
    assert "error" in body                       # the API returns an error field
    assert "required" in body["error"].lower()   # …and it says what's wrong

Remember: assert the status precisely and the message loosely — check for a key word ("required", "password") rather than the exact string, so wording changes don’t cause false failures.

Putting it together: a validation matrix

Negative cases are a natural fit for parametrize (Module 2) — one test, a table of bad-input rows, each with its expected status. This “validation matrix” is the shape of most real negative-test suites:

import pytest
import requests

BASE_URL = "http://localhost:3000"


@pytest.mark.parametrize(
    "payload, expected_status",
    [
        ({"email": "[email protected]", "password": "customer123"}, 200),  # valid (control)
        ({"email": "[email protected]", "password": "wrong"}, 401),        # bad password
        ({"email": "[email protected]", "password": "whatever"}, 401),        # unknown email
        ({"email": "[email protected]"}, 400),                             # missing password
        ({}, 400),                                                         # missing everything
    ],
    ids=["valid", "bad-password", "unknown-email", "missing-password", "empty"],
)
def test_login_validation(payload, expected_status):
    response = requests.post(f"{BASE_URL}/api/auth/login", json=payload)
    assert response.status_code == expected_status

Keep one valid row as a control — it proves the endpoint works, so a failing negative row is really about the input, not a broken endpoint.

Remember: a validation matrix (parametrize of (bad_input, expected_status)) covers many failure modes in one readable test. Always include a valid control row.

When the API fails wrong: document a bug with `xfail`

Sometimes negative testing finds a real defect — the API doesn’t fail cleanly. TestMarket Lab has one: posting a product whose name duplicates an existing one returns 500 (a server crash) instead of a proper 409/400. That’s exactly the kind of bug negative testing exists to surface.

Don’t delete the finding — encode it with xfail (Module 2), asserting the behavior you want, and mark it strict=True. It stays green as a known issue today; the day the bug is fixed the test XPASSes and fails the run — forcing you to delete the now-stale marker (a plain, non-strict xfail would let the fix slip by as a silent summary line):

import pytest


@pytest.mark.xfail(reason="BUG: duplicate product name returns 500 instead of 409", strict=True)
def test_duplicate_product_name_should_conflict(reset_db, base_url, api):
    payload = {"name": "Repeat Item", "price": 9.99, "category": "electronics"}
    api.post(f"{base_url}/api/products", json=payload)   # arrange: this name now exists

    duplicate = api.post(f"{base_url}/api/products", json=payload)
    assert duplicate.status_code == 409                  # what SHOULD happen (today it's 500)

This test writes (it adds a product), so it takes reset_db from your Module 2 conftest.py to stay re-runnable. One caveat: xfail marks the whole test, so a failure anywhere in it counts as the expected failure — keep bug tests small and focused on the one defect, or a big one could quietly absorb an unrelated regression.

Remember: capture a known bug as a strict=True xfail test that asserts the correct behavior with a reason. It documents the defect, keeps the suite green today, and fails loudly the moment the fix lands — far better than a comment or a deleted test.

Exercises

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

Exercise 1 — Missing-field 400s (10 min)

Write tests that assert 400 for: POST /api/auth/login with no body {}, POST /api/products with only a name, and POST /api/auth/register with a missing name. Confirm each returns 400.

Exercise 2 — The 401 pair (5 min)

Assert that a wrong password and an unknown email both return 401 from POST /api/auth/login — and note they return the same status on purpose.

Exercise 3 — 404 across verbs (10 min)

For a product id you know doesn’t exist, assert 404 from GET, PUT, and DELETE on /api/products/<id>.

Exercise 4 — Assert the message (5 min)

POST /api/auth/register with a 3-character password. Assert 400, and that the error body’s message mentions "password" (case-insensitive, substring — not the exact string).

Exercise 5 — A register validation matrix (15 min)

Parametrize POST /api/auth/register over rows of (payload, expected_status): a valid new signup (201), a duplicate email (409), a short password (400), and a missing field (400). Give each row a readable id. (Tip: a valid signup writes — take reset_db so re-runs start clean.)

Bonus — Find and xfail a bug

Reproduce the duplicate-product-name 500 yourself, then write the xfail test that asserts the correct 409. Run pytest -rx to see the “expected failure” reason in the summary.

In Module 5 we go deep on test data setup & teardown — using POST /api/reset and arrange-via-API (POST /api/products / POST /api/orders) so every test controls its own starting state instead of depending on the seed.