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pytest testing framework conventions and practices. Invoke whenever task involves any interaction with pytest — writing tests, configuring pytest, fixtures, parametrize, mocking, debugging test failures, or coverage.
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| name | pytest |
| description | pytest testing framework conventions and practices. Invoke whenever task involves any interaction with pytest — writing tests, configuring pytest, fixtures, parametrize, mocking, debugging test failures, or coverage. |
Test behavior, not implementation. Tests are executable documentation — if the test name doesn't explain what the code does, rewrite it.
pytest is Python's standard testing framework. It uses plain assert statements, fixtures for setup/teardown, and a
rich plugin ecosystem. All patterns target Python 3.14+.
${CLAUDE_SKILL_DIR}/references/fixtures.md]: Fixture lifecycle,
yield fixtures, factory pattern, request object, parametrized fixtures${CLAUDE_SKILL_DIR}/references/parametrize.md]: Multi-parameter examples,
indirect fixtures, custom IDs, stacking decorators${CLAUDE_SKILL_DIR}/references/monkeypatch.md]: API overview,
attribute/env/dict patching, scoped monkeypatch, common recipes${CLAUDE_SKILL_DIR}/references/plugins.md]: pytest-asyncio, pytest-mock,
pytest-xdist, pytest-cov configuration patternstest_*.py or *_test.py. Prefer test_<module>.py matching source module.test_<behavior> — describe the behavior, not the method: test_returns_empty_list_when_no_matches
not test_search.TestClassName__init__Structure every test in three phases:
def test_user_creation_sets_defaults():
# Arrange
data = {"name": "Alice", "email": "alice@example.com"}
# Act
user = User.from_dict(data)
# Assert
assert user.name == "Alice"
assert user.is_active is True
assert user.roles == []
@pytest.mark.slow.setUp/tearDown from
unittest.function scope (fresh per test). Use broader scopes (class, module,
session) only for expensive resources.autouse=True sparingly. Only for setup that genuinely applies to every test in scope (e.g., database transaction
rollback, temp directory cleanup).@pytest.fixture
def db_connection():
conn = create_connection()
yield conn
conn.close()
@pytest.fixture
def temp_config(tmp_path: Path):
config_file = tmp_path / "config.toml"
config_file.write_text('[app]\ndebug = true\n')
yield config_file
# cleanup automatic — tmp_path handles it
yield separates setup from teardown. Code after yield runs even if the test fails.yield over addfinalizer — clearer control flow.try/except and log.When tests need multiple instances with varying configuration:
@pytest.fixture
def make_user():
def _make_user(name: str = "Alice", *, active: bool = True) -> User:
return User(name=name, is_active=active)
return _make_user
def test_inactive_users_excluded(make_user):
active = make_user("Alice", active=True)
inactive = make_user("Bob", active=False)
assert filter_active([active, inactive]) == [active]
function — Each test (default). Most fixtures — cheap setup, isolation.
class — All tests in a class. Shared expensive setup within a test class.
module — All tests in a file. Database connection per test file.
session — Entire test run. Server startup, heavy resource initialization.
Session-scoped fixtures must be in conftest.py at the root test directory.
Don't mix scopes carelessly. A function-scoped fixture cannot depend on a function-scoped fixture that modifies state from a broader scope.
tmp_path — Path to a temporary directory unique to the test (function scope)tmp_path_factory — Factory for creating temp directories (session scope)capsys — Capture sys.stdout/sys.stderr writescapfd — Capture file descriptor 1/2 output (catches C-level writes)caplog — Capture logging output with access to recordsmonkeypatch — Dynamic attribute/env/dict patching with automatic restorerequest — Fixture metadata: .param, .node, .config, .fspathpytestconfig — Access to the pytest config objectSee ${CLAUDE_SKILL_DIR}/references/fixtures.md for fixture lifecycle details, parametrized fixtures, and advanced
patterns.
@pytest.mark.parametrize("input_val, expected", [
("hello", 5),
("", 0),
(" spaces ", 10),
])
def test_string_length(input_val: str, expected: int):
assert len(input_val) == expected
pytest.param("", 0, id="empty-string") for readable output.@pytest.mark.parametrize("x", [1, 2])
@pytest.mark.parametrize("y", [10, 20])
def test_combinations(x: int, y: int):
assert x + y > 0
# Generates: (1,10), (1,20), (2,10), (2,20)
Pass parameter values to fixtures instead of directly to the test:
@pytest.fixture
def user(request) -> User:
return User(name=request.param)
@pytest.mark.parametrize("user", ["Alice", "Bob"], indirect=True)
def test_user_greeting(user: User):
assert user.name in user.greet()
See ${CLAUDE_SKILL_DIR}/references/parametrize.md for multi-parameter patterns, conditional skipping within
parametrize, and dynamic parametrize generation.
@pytest.mark.skip(reason="...") — unconditionally skip.@pytest.mark.skipif(condition, reason="...") — skip when condition is true:
@pytest.mark.skipif(sys.platform == "win32", reason="Unix only").@pytest.mark.xfail(reason="...") — expected failure. Passes if the test fails, reports unexpected pass if it
succeeds. Use strict=True to fail on unexpected pass.@pytest.mark.usefixtures("fixture_name") — inject fixture without using its value.@pytest.mark.filterwarnings("ignore::DeprecationWarning") — per-test warning filter.Register in pyproject.toml to avoid warnings:
[tool.pytest.ini_options]
markers = [
"slow: marks tests as slow (deselect with '-m \"not slow\"')",
"integration: marks integration tests",
]
@pytest.mark.slow
def test_full_pipeline():
...
Run subsets: pytest -m "not slow", pytest -m "integration and not slow".
def test_reads_env_variable(monkeypatch):
monkeypatch.setenv("API_KEY", "test-key")
assert get_api_key() == "test-key"
def test_overrides_attribute(monkeypatch):
monkeypatch.setattr("myapp.config.DEBUG", True)
assert is_debug_mode() is True
monkeypatch auto-restores on test exit. No manual cleanup.sys.path.from unittest.mock import MagicMock, patch, AsyncMock
def test_service_calls_repository():
repo = MagicMock(spec=UserRepository)
repo.get.return_value = User(name="Alice")
service = UserService(repo=repo)
result = service.find_user("alice")
repo.get.assert_called_once_with("alice")
assert result.name == "Alice"
@patch("myapp.services.httpx.get")
def test_fetches_external_data(mock_get):
mock_get.return_value = MagicMock(json=lambda: {"status": "ok"})
assert fetch_status() == "ok"
spec= on MagicMock — catches attribute typos at test time.AsyncMock for async functions. Auto-detected when patching async targets.patch target is where the name is looked up, not where it's defined: @patch("myapp.services.httpx.get") not
@patch("httpx.get").def test_with_mocker(mocker):
mock_fetch = mocker.patch("myapp.services.fetch_data")
mock_fetch.return_value = {"key": "value"}
result = process_data()
mock_fetch.assert_called_once()
mocker auto-restores after each test. Prefer over manual patch context managers.mocker.patch("module.Class", autospec=True) — recursively specs all attributes and method signatures from the
real object. Catches signature mismatches at test time.mocker.spy(obj, "method") wraps the real method — tracks calls while preserving behavior.pytest rewrites assert statements to show detailed failure messages:
assert result == expected # shows both values on failure
assert "error" in message # shows the full string
assert len(items) == 3 # shows actual length
assert all(x > 0 for x in values) # shows the values
assert with pytest's rewrite engine gives clear failure messages.def test_raises_on_invalid_input():
with pytest.raises(ValueError, match=r"must be positive"):
calculate(-1)
def test_exception_attributes():
with pytest.raises(ValidationError) as exc_info:
validate(bad_data)
assert exc_info.value.field == "email"
assert "invalid format" in str(exc_info.value)
match= when the exception type is broad — validates the message..value for exception attributes via exc_info.pytest.raises is a context manager. The code that raises must be inside the with.assert result == pytest.approx(3.14, abs=0.01)
assert results == pytest.approx([1.0, 2.0, 3.0], rel=1e-3)
def test_deprecation_warning():
with pytest.warns(DeprecationWarning, match="use new_func"):
old_func()
With pytest-asyncio:
import pytest
@pytest.mark.asyncio
async def test_async_fetch():
result = await fetch_data("https://api.example.com")
assert result.status == 200
@pytest.fixture
async def async_client():
async with AsyncClient() as client:
yield client
@pytest.mark.asyncio
async def test_with_async_client(async_client):
response = await async_client.get("/health")
assert response.status_code == 200
@pytest.mark.asyncio on every async test (or configure asyncio_mode = "auto" in pyproject.toml).yield for teardown — same pattern as sync fixtures.tests/
├── conftest.py # session/root fixtures
├── unit/
│ ├── conftest.py # unit test fixtures
│ └── test_models.py
└── integration/
├── conftest.py # integration fixtures (DB, services)
└── test_api.py
tests/conftest.py is available to all tests. A fixture in
tests/unit/conftest.py is available only to unit tests.def test_prints_greeting(capsys):
greet("Alice")
captured = capsys.readouterr()
assert "Hello, Alice" in captured.out
def test_logs_warning(caplog):
with caplog.at_level(logging.WARNING):
process_legacy_data()
assert "deprecated" in caplog.text
assert caplog.records[0].levelname == "WARNING"
[tool.pytest.ini_options]
testpaths = ["tests"]
addopts = "-ra -q --strict-markers"
markers = [
"slow: slow tests",
"integration: integration tests",
]
filterwarnings = [
"error", # treat all warnings as errors
"ignore::DeprecationWarning", # except deprecations from deps
]
asyncio_mode = "auto" # pytest-asyncio: auto-detect async tests
--strict-markers — fail on unregistered markers. Catches typos.-ra — show summary of all non-passing tests at the end.filterwarnings = ["error"] — catch hidden warnings early.pytest-asyncio — Async test support with @pytest.mark.asynciopytest-mock — mocker fixture wrapping unittest.mockpytest-cov — Coverage reporting (--cov=src)pytest-xdist — Parallel test execution (-n auto)pytest-httpx — Mock httpx requests in testspytest-randomly — Randomize test order to catch hidden dependenciesSee ${CLAUDE_SKILL_DIR}/references/plugins.md for configuration patterns and usage details.
When writing tests: apply all conventions silently — don't narrate each rule being followed. Match the project's existing test style. If an existing codebase contradicts a convention, follow the codebase and flag the divergence once.
When reviewing tests: cite the specific issue and show the fix inline. Don't lecture — state what's wrong and how to fix it.
Bad: "According to pytest best practices, you should use fixtures
instead of setUp methods..."
Good: "setUp/tearDown -> @pytest.fixture with yield"
The python skill governs language choices; this skill governs pytest testing decisions. The coding skill governs workflow (discovery, planning, verification).
Test behavior, not implementation. When in doubt, mock less.
Design Claude Code output styles: role, voice, and behavioral rules injected into the system prompt. Invoke whenever task involves any interaction with output styles — creating, editing, evaluating, or changing how Claude communicates.
CLAUDE.md instruction quality: writing effective project instructions, diagnosing why Claude ignores rules, routing content to the right layer, progressive disclosure, and systematic improvement. Invoke whenever task involves any interaction with CLAUDE.md files — creating, writing, reviewing, auditing, improving, optimizing, or debugging instruction compliance.
Prompt design techniques for LLMs: structure, examples, reasoning patterns, and optimization. Invoke whenever task involves any interaction with AI instructions — crafting, debugging, improving, or evaluating prompts for skills, agents, output styles, or system configurations.