Pytest

pytest

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+.

References

• Fixture patterns, scope, factories, teardown — [${CLAUDE_SKILL_DIR}/references/fixtures.md]: Fixture lifecycle, yield fixtures, factory pattern, request object, parametrized fixtures
• Parametrize patterns, indirect, IDs — [${CLAUDE_SKILL_DIR}/references/parametrize.md]: Multi-parameter examples, indirect fixtures, custom IDs, stacking decorators
• Monkeypatch patterns, scoped patches — [${CLAUDE_SKILL_DIR}/references/monkeypatch.md]: API overview, attribute/env/dict patching, scoped monkeypatch, common recipes
• Plugin ecosystem and configuration — [${CLAUDE_SKILL_DIR}/references/plugins.md]: pytest-asyncio, pytest-mock, pytest-xdist, pytest-cov configuration patterns

Test Structure

Discovery and Naming

• Files: test_*.py or *_test.py. Prefer test_<module>.py matching source module.
• Functions: test_<behavior> — describe the behavior, not the method: test_returns_empty_list_when_no_matches not test_search.
• Classes: TestClassName groups related tests. No __init__ method. Use classes when tests share setup; use bare functions for independent tests.
• conftest.py is auto-discovered — no import needed. Place shared fixtures at the appropriate directory level.

Arrange-Act-Assert

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 == []

• One act per test. If you need multiple acts, write multiple tests.
• Comments optional when phases are obvious. Add when the test is long enough that phases aren't immediately clear.

Test Granularity

• One concept per test. Multiple assertions are fine when they verify the same behavior. Separate tests when behaviors are independent.
• Fast by default. Unit tests should run in milliseconds. Gate slow tests (network, DB) behind markers: @pytest.mark.slow.
• Isolation is mandatory. Tests must not depend on execution order or shared mutable state. Each test sets up its own world.

Fixtures

Core Rules

• Fixtures over setup methods. Fixtures are composable, scoped, and explicit. Never use setUp/tearDown from unittest.
• Explicit injection. Request fixtures by name in test parameters. Every dependency is visible in the test signature.
• Smallest viable scope. Default is 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).

Yield Fixtures (Setup + Teardown)

@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.
• Prefer yield over addfinalizer — clearer control flow.
• Teardown must not raise. If cleanup can fail, wrap in try/except and log.

Fixture Factories

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]

Fixture Scope

• 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.

Built-in Fixtures

• 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 writes
• capfd — Capture file descriptor 1/2 output (catches C-level writes)
• caplog — Capture logging output with access to records
• monkeypatch — Dynamic attribute/env/dict patching with automatic restore
• request — Fixture metadata: .param, .node, .config, .fspath
• pytestconfig — Access to the pytest config object

See ${CLAUDE_SKILL_DIR}/references/fixtures.md for fixture lifecycle details, parametrized fixtures, and advanced patterns.

Parametrize

Basic Usage

@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

• Use descriptive IDs: pytest.param("", 0, id="empty-string") for readable output.
• Each row is a distinct test. Failures report which parameter combination failed.

Stacking Decorators

@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)

Indirect Parametrize

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.

Markers

Built-in Markers

• @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.

Custom Markers

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".

Mocking

monkeypatch (Preferred for Simple Cases)

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.
• Use for: environment variables, module attributes, dictionary entries, sys.path.

unittest.mock (For Complex Mocking)

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"

• Always use 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").

pytest-mock (mocker Fixture)

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.

Mocking Rules

• Mock at boundaries. Mock external services, databases, filesystems, clocks — not internal functions.
• Don't mock what you own when a fake or in-memory implementation is available.
• Prefer dependency injection over patching. Pass collaborators as parameters, mock in tests.
• Never mock the thing you're testing. If you need to mock part of the SUT, the SUT has too many responsibilities — split it.

Assertions

Plain Assert

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

• No assertion library needed. Plain assert with pytest's rewrite engine gives clear failure messages.
• Multiple assertions per test are fine when they verify the same behavior.

Exception Testing

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)

• Always use match= when the exception type is broad — validates the message.
• Access .value for exception attributes via exc_info.
• pytest.raises is a context manager. The code that raises must be inside the with.

Approximate Comparisons

assert result == pytest.approx(3.14, abs=0.01)
assert results == pytest.approx([1.0, 2.0, 3.0], rel=1e-3)

Warning Testing

def test_deprecation_warning():
    with pytest.warns(DeprecationWarning, match="use new_func"):
        old_func()

Async Testing

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).
• Async fixtures work with yield for teardown — same pattern as sync fixtures.

conftest.py Patterns

Hierarchy

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

• Fixtures cascade downward. A fixture in tests/conftest.py is available to all tests. A fixture in tests/unit/conftest.py is available only to unit tests.
• Don't import from conftest. pytest discovers and injects conftest fixtures automatically.
• Split by concern. Root conftest for shared utilities (factories, settings). Subdirectory conftest for environment-specific setup (database, external services).

Output Capture

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"

Configuration

[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.

Plugin Ecosystem

• pytest-asyncio — Async test support with @pytest.mark.asyncio
• pytest-mock — mocker fixture wrapping unittest.mock
• pytest-cov — Coverage reporting (--cov=src)
• pytest-xdist — Parallel test execution (-n auto)
• pytest-httpx — Mock httpx requests in tests
• pytest-randomly — Randomize test order to catch hidden dependencies

See ${CLAUDE_SKILL_DIR}/references/plugins.md for configuration patterns and usage details.

Application

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"

Integration

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.