Pytest Testing Skill
Core Patterns
Basic Test
import pytest
def test_addition():
assert 2 + 3 == 5
def test_exception():
with pytest.raises(ValueError, match="invalid"):
int("not_a_number")
class TestCalculator:
def test_add(self):
calc = Calculator()
assert calc.add(2, 3) == 5
def test_divide_by_zero(self):
with pytest.raises(ZeroDivisionError):
Calculator().divide(10, 0)
Fixtures
@pytest.fixture
def calculator():
return Calculator()
@pytest.fixture
def db_connection():
conn = Database.connect("test_db")
yield conn # teardown after yield
conn.rollback()
conn.close()
@pytest.fixture(scope="module")
def api_client():
client = APIClient(base_url="http://localhost:8000")
yield client
client.logout()
# conftest.py - shared fixtures
@pytest.fixture(autouse=True)
def reset_state():
State.reset()
yield
State.cleanup()
# Usage
def test_add(calculator):
assert calculator.add(2, 3) == 5
Parametrize
@pytest.mark.parametrize("input,expected", [
("hello", 5), ("", 0), ("pytest", 6),
])
def test_string_length(input, expected):
assert len(input) == expected
@pytest.mark.parametrize("a,b,expected", [
(2, 3, 5), (-1, 1, 0), (0, 0, 0),
])
def test_add(calculator, a, b, expected):
assert calculator.add(a, b) == expected
Markers
@pytest.mark.slow
def test_large_dataset(): ...
@pytest.mark.skip(reason="Not implemented")
def test_future_feature(): ...
@pytest.mark.skipif(sys.platform == "win32", reason="Unix only")
def test_unix_permissions(): ...
@pytest.mark.xfail(reason="Known bug #123")
def test_known_bug(): ...
Mocking
from unittest.mock import patch, MagicMock
def test_send_email(mocker):
mock_smtp = mocker.patch("myapp.email.smtplib.SMTP")
send_welcome_email("user@test.com")
mock_smtp.return_value.sendmail.assert_called_once()
def test_api_call(mocker):
mock_response = mocker.Mock()
mock_response.status_code = 200
mock_response.json.return_value = {"users": [{"name": "Alice"}]}
mocker.patch("myapp.service.requests.get", return_value=mock_response)
users = get_users()
assert len(users) == 1
@patch("myapp.service.database")
def test_save_user(mock_db):
mock_db.save.return_value = True
assert save_user({"name": "Alice"}) is True
mock_db.save.assert_called_once()
Assertions
assert x == y
assert x != y
assert x in collection
assert isinstance(obj, MyClass)
assert 0.1 + 0.2 == pytest.approx(0.3)
with pytest.raises(ValueError) as exc_info:
raise ValueError("bad")
assert "bad" in str(exc_info.value)
Anti-Patterns
| Bad | Good | Why |
|---|
self.assertEqual() | assert x == y | pytest rewrites give better output |
Setup in __init__ | @pytest.fixture | Lifecycle management |
| Global state | Fixture with yield | Proper cleanup |
| Huge test functions | Small focused tests | Easier debugging |
Quick Reference
| Task | Command |
|---|
| Run all | pytest |
| Run file | pytest tests/test_login.py |
| Run specific | pytest tests/test_login.py::test_login_success |
| By marker | pytest -m slow |
| By keyword | pytest -k "login and not invalid" |
| Verbose | pytest -v |
| Stop first fail | pytest -x |
| Last failed | pytest --lf |
| Coverage | pytest --cov=myapp --cov-report=html |
| Parallel | pytest -n auto (pytest-xdist) |
pyproject.toml
[tool.pytest.ini_options]
testpaths = ["tests"]
markers = ["slow: slow tests", "integration: integration tests"]
addopts = "-v --tb=short"
Deep Patterns
For production-grade patterns, see reference/playbook.md:
| Section | What's Inside |
|---|
| §1 Config | pytest.ini + pyproject.toml with markers, coverage |
| §2 Fixtures | Scoping, factories, teardown, autouse, tmp_path |
| §3 Parametrize | Basic, with IDs, cartesian, indirect |
| §4 Mocking | pytest-mock, monkeypatch, spies, env vars |
| §5 Async | pytest-asyncio, async fixtures, async client |
| §6 Exceptions | pytest.raises(match=), warnings |
| §7 Markers & Plugins | Custom markers, collection hooks |
| §8 Class-Based | Nested classes, autouse setup |
| §9 CI/CD | GitHub Actions matrix, coverage gates |
| §10 Debugging Table | 10 common problems with fixes |
| §11 Best Practices | 15-item production checklist |
