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