Pytest Testing

Guidance for writing effective tests with pytest.

Core Concepts

Fixtures

Reusable test setup/teardown. Use yield for cleanup.

Scopes:

Scope	Runs	Use Case
`function`	Each test	Default, isolated
`class`	Once per class	Shared state in class
`module`	Once per file	Expensive setup
`session`	Once total	DB connections, servers

Key concept: conftest.py makes fixtures available to all tests in directory.

Parametrize

Test multiple inputs without repeating code.

Key concept: Generates separate test for each parameter set - failures are isolated.

Use pytest.param() for:

Custom test IDs
Expected failures (marks=pytest.mark.xfail)
Conditional skips

Markers

Tag tests for filtering and special behavior.

Built-in	Purpose
`@pytest.mark.skip`	Always skip
`@pytest.mark.skipif(condition)`	Conditional skip
`@pytest.mark.xfail`	Expected failure
`@pytest.mark.asyncio`	Async test (with pytest-asyncio)

Custom markers: Define in pyproject.toml, run with pytest -m "marker"

Mocking

Tool	Use Case
`Mock()`	Create fake object
`patch()`	Replace import
`MagicMock()`	Mock with magic methods
`AsyncMock()`	Mock async functions

Key concept: Patch where the thing is used, not where it's defined.

Test Organization

tests/
├── conftest.py          # Shared fixtures
├── unit/                # Fast, isolated
├── integration/         # Multiple components
└── e2e/                 # Full system

Coverage

Key thresholds:

80%+ is good coverage
100% is often overkill
Branch coverage catches conditionals

Config in pyproject.toml: Set source, branch=true, fail_under

Common Commands

Command	Purpose
`pytest -v`	Verbose output
`pytest -x`	Stop on first failure
`pytest -k "pattern"`	Filter by name
`pytest -m "marker"`	Filter by marker
`pytest -s`	Show print output
`pytest -n auto`	Parallel (pytest-xdist)
`pytest --cov=src`	With coverage

Best Practices

Practice	Why
One assertion per test	Clear failures
Descriptive test names	Self-documenting
Arrange-Act-Assert	Consistent structure
Test behavior, not implementation	Refactor-proof
Use factories for test data	Maintainable
Keep tests fast	Run often

Resources

Pytest docs: https://docs.pytest.org/
pytest-asyncio: https://pytest-asyncio.readthedocs.io/