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Use this skill when
Setting up pytest in a new project (pyproject.toml [tool.pytest.ini_options], testpaths, addopts, conftest.py)
Writing tests with assert, pytest.raises, pytest.warns, pytest.approx, pytest.mark.*
Task is JS/TS unit testing (Vitest/Jest semantics, not pytest) — use vitest
Task is browser E2E automation — use playwright
Task is Django built-in TestCase/Client without pytest involved — use django directly
Task is unittest module (stdlib) idiom without pytest — different runner, different conventions
Task is load testing, BDD (pytest-bdd only marginally — confirm scope), or contract testing
Purpose
pytest is the de-facto Python test framework in 2026. It replaces unittest boilerplate with plain assert, rich introspection on failure, and a fixture system that composes deterministic test state without inheritance hierarchies. Around pytest sits the most mature plugin ecosystem in Python testing: pytest-cov for coverage, pytest-asyncio for async def tests, pytest-xdist for parallel runs, pytest-mock for the mocker fixture, Hypothesis for property-based generation.
pytest 9 dropped Python 3.9, promoted PytestRemovedIn9Warning to errors, added native TOML support (pytest.toml / [tool.pytest] in pyproject.toml with real TOML types — not the legacy INI-compat mode), shipped first-party subtests, and tightened duplicate-path collection. This skill covers v9 specifics, the full fixture + parametrize + mark surface, mocking (with the common unittest.mock vs pytest-mock mocker confusion resolved), async testing, coverage, parallelism, and the most common failure modes encountered when scaling a test suite.
Capabilities
Test discovery & invocation
pytest discovers files named test_*.py or *_test.py, classes named Test* (no __init__), and functions/methods named test_*. Layout: src/<pkg>/ for code, tests/ for tests at repo root; tests/conftest.py shares fixtures across the tree. Run with pytest, narrow with pytest tests/test_foo.py::TestClass::test_method, filter by keyword -k 'login and not legacy', by marker -m 'slow', fail fast -x, show locals on failure -l, control traceback style --tb=short|long|line|native|no.
@pytest.fixture produces test inputs and cleanup. Scopes: function (default), class, module, package, session. yield-style fixtures run teardown after the yield. autouse=True applies a fixture to every test in its scope — use sparingly. Fixtures resolve by name in the test signature; conftest.py files share fixtures hierarchically (closest conftest.py wins). Factory pattern: a fixture returns a callable that builds objects on demand. Parametrized fixtures (params=[...]) multiply downstream tests; request.param reads the current value. indirect=True on @pytest.mark.parametrize routes params through the fixture.
@pytest.mark.parametrize("a,b,expected", [(1, 2, 3), ...]) produces one test per row. Stack multiple @parametrize decorators for cartesian product. pytest.param(value, marks=pytest.mark.xfail, id="case-name") attaches per-row marks and stable IDs. ids= accepts a list or callable. indirect=("fixture_name",) makes a param flow through the fixture instead of being injected directly.
Built-ins: skip, skipif(condition, reason=...), xfail (run but expect failure; strict=True fails on unexpected pass), parametrize, usefixtures, filterwarnings. Custom marks register in pyproject.toml under [tool.pytest.ini_options].markers, run via pytest -m slow. Always set --strict-markers so typos in mark names raise instead of silently passing.
unittest.mock.patch (stdlib): use as decorator @patch("pkg.mod.func"), context manager with patch(...), or patch.object(cls, "method"). Mocks are torn down automatically. autospec=True enforces the target's signature.
pytest-mockmocker fixture: mocker.patch("pkg.mod.func", return_value=...). No decorator stacking, no nested with, automatic teardown via fixture scope. Preferred in pytest-native code.
Both wrap the same MagicMock. side_effect=callable | exception | iterable, return_value=..., mock.assert_called_with(...), mock.assert_called_once_with(...), ANY for "don't care" args. Always patch where the name is looked up, not where it's defined.
Install pytest-asyncio. Default mode is strict — mark each async test with @pytest.mark.asyncio and async fixtures with @pytest_asyncio.fixture. Set asyncio_mode = "auto" in config to auto-mark every async def test_*. Control event-loop sharing with loop_scope="session" etc. For libraries using anyio, use the anyio_backend fixture instead.
Hypothesis generates inputs from @given(st.integers(), st.text()) and shrinks failing cases to minimal counter-examples. settings(max_examples=200, deadline=500) tunes per-test budget. Example database persists past failures for regression. Works with pytest fixtures via @given(...) applied after@pytest.fixture-using parameters in the signature.
syrupy provides a snapshot fixture: assert result == snapshot. First run records, subsequent runs compare. Update with --snapshot-update. Use snapshots for stable serialized output (HTML, JSON dumps, rendered text); avoid for values where any change is meaningful — use explicit assertions instead.
Common failures: collection errors from conftest.py import-time exceptions, fixture 'X' not found (typo or wrong conftest layer), async test hangs (missing pytest-asyncio install or wrong mode), parametrize ID encoding for non-ASCII values, flaky tests from shared module state or time-dependent assertions, slow suite detection with --durations=10.
Use pytest-randomly always (order independence is a feature, not a chore). Use pyproject.toml for config. Keep --strict-markers and xfail_strict = true in addopts. Put conftest.py at the lowest common ancestor of tests that share fixtures — not at the repo root by default. Treat autouse as a code smell unless the fixture is truly always-on (e.g., DB transaction rollback).
