| name | pytest-patterns |
| description | Pytest best practices including fixtures, parametrize, markers, and assertion patterns for Python test suites.
|
| requires | {"bins":["python"]} |
Pytest patterns and best practices for Python test suites.
When to activate
Use this skill when the project has pyproject.toml, setup.cfg, pytest.ini,
conftest.py, or a tests/ directory containing *_test.py or test_*.py
files.
Test file structure
- Name test files
test_<module>.py to mirror the source module they cover.
- Group related tests inside a class (
class TestParseConfig:) when they
share setup or conceptually belong together.
- Name test functions
test_<behavior_under_test> -- the name should read
like a specification (e.g. test_parse_config_raises_on_missing_key).
Fixtures
- Place shared fixtures in
conftest.py so they are automatically
discovered by pytest.
- Scope controls lifetime:
function (default, one per test), class,
module, session.
- Use
yield fixtures for setup/teardown -- code after yield runs as
cleanup even if the test fails.
- Fixture factories (a fixture that returns a callable) let tests customize
object creation per-call.
- Parametrized fixtures (
@pytest.fixture(params=[...])) run every
dependent test once per parameter value.
Parametrize
@pytest.mark.parametrize("x,expected", [(1, 2), (3, 6)]) runs the
test once per tuple.- Multiple
@pytest.mark.parametrize decorators stack -- pytest generates
the cartesian product.
- Use
indirect=True to route parameter values through a fixture before
they reach the test.
Markers
@pytest.mark.slow -- tag long-running tests so they can be skipped with
pytest -m "not slow".@pytest.mark.skip(reason="...") -- unconditionally skip.@pytest.mark.xfail(reason="...") -- expect failure; test still runs but
does not fail the suite.- Register custom markers in
pyproject.toml under [tool.pytest.ini_options]
to avoid warnings.
- Filter with
pytest -m "marker_name".
Assertions
- Use plain
assert -- pytest rewrites it to show detailed diffs on failure.
with pytest.raises(ValueError, match=r"expected .* pattern"): for
exception type and message verification.pytest.approx(0.1 + 0.2, abs=1e-9) for floating-point comparisons.- Use
match= on pytest.raises to verify the error message, not just the
exception type.
Running
pytest -v -- verbose output with one line per test.pytest -x -- fail fast, stop on first failure.pytest -k "pattern" -- run only tests whose name matches the pattern.pytest --tb=short -- shorter tracebacks for quick scanning.
MUST
- Always use descriptive test names that explain the expected behavior.
- Test one behavior per function -- a test should have a single reason to
fail.
- Use fixtures instead of setup/teardown methods.
- Parametrize repetitive test cases rather than copy-pasting test functions.
MUST NOT
- Do not use
unittest.TestCase unless the project already uses it.
- Do not hardcode file paths -- use
tmp_path or tmp_path_factory.
- Do not use
sleep() in tests -- use mocking or polling helpers.
- Do not leave
print() statements in committed tests -- use logging or
capsys if output matters.
