pytest

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Expert guidance for Python testing with pytest framework. Use when (1) writing unit tests or integration tests, (2) setting up test fixtures for setup/teardown, (3) parametrizing tests with multiple inputs, (4) using markers (skip, xfail, custom), (5) mocking/patching with monkeypatch or pytest-mock, (6) configuring pytest (pytest.ini, pyproject.toml, conftest.py), (7) running tests with coverage, (8) debugging test failures, or (9) testing async code, FastAPI, Django, or Flask apps.

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[Testing and QA]

When & Why to Use This Skill

This Claude skill provides expert-level guidance for Python testing using the pytest framework. It streamlines the development of robust test suites by assisting with unit and integration testing, complex fixture management, and advanced mocking techniques. It is designed to help developers improve code quality, automate verification, and optimize test configurations for modern web frameworks like FastAPI, Django, and Flask.

Use Cases

  • Architecting scalable test suites with reusable fixtures for efficient setup and teardown of database connections or API states.
  • Implementing data-driven testing using parametrization to validate multiple input scenarios and edge cases with minimal code duplication.
  • Isolating code under test by mocking external dependencies, environment variables, and third-party services using monkeypatch or pytest-mock.
  • Configuring advanced testing environments via pytest.ini or pyproject.toml, including coverage reporting and parallel test execution for CI/CD pipelines.
  • Debugging complex test failures and testing asynchronous code in high-performance web applications.
namepytest
descriptionExpert guidance for Python testing with pytest framework. Use when (1) writing unit tests or integration tests, (2) setting up test fixtures for setup/teardown, (3) parametrizing tests with multiple inputs, (4) using markers (skip, xfail, custom), (5) mocking/patching with monkeypatch or pytest-mock, (6) configuring pytest (pytest.ini, pyproject.toml, conftest.py), (7) running tests with coverage, (8) debugging test failures, or (9) testing async code, FastAPI, Django, or Flask apps.

Pytest Testing Guide

Write Python tests efficiently with pytest.

Quick Start

pip install pytest

# test_example.py
def test_addition():
    assert 1 + 1 == 2

Run: pytest or pytest -v

Core Concepts

Concept Example
Test function def test_something():
Assertion assert result == expected
Fixture @pytest.fixture - dependency injection
Marker @pytest.mark.skip - test metadata
Parametrize @pytest.mark.parametrize - multiple inputs

Essential Patterns

Basic Test

def test_function():
    result = my_function(1, 2)
    assert result == 3

Exception Testing

import pytest

def test_raises():
    with pytest.raises(ValueError, match="invalid"):
        raise ValueError("invalid input")

Fixture (Setup/Teardown)

@pytest.fixture
def database():
    db = create_db()
    yield db          # provide to test
    db.cleanup()      # teardown

def test_query(database):
    assert database.query("SELECT 1")

Parametrize

@pytest.mark.parametrize("input,expected", [
    (1, 2),
    (2, 4),
    (3, 6),
])
def test_double(input, expected):
    assert input * 2 == expected

Mocking

def test_mock(monkeypatch):
    monkeypatch.setattr("module.func", lambda: "mocked")
    monkeypatch.setenv("API_KEY", "test-key")

Reference Files

Topic File When to Use
Basics basics.md Test discovery, assertions, running tests, config
Fixtures fixtures.md Setup/teardown, scopes, factories, conftest.py
Markers markers.md skip, xfail, parametrize, custom markers
Mocking mocking.md monkeypatch, pytest-mock, spies, async mocks
Plugins plugins.md Coverage, parallel, async, Django, Flask, FastAPI

Common Commands

pytest                    # Run all tests
pytest -v                 # Verbose
pytest -s                 # Show prints
pytest -x                 # Stop on first failure
pytest -k "pattern"       # Run matching tests
pytest -m slow            # Run by marker
pytest --cov=mypackage    # With coverage
pytest -n auto            # Parallel (xdist)
pytest --lf               # Run last failed
pytest --pdb              # Debug on failure

Project Structure

project/
├── src/
│   └── mypackage/
│       └── module.py
├── tests/
│   ├── conftest.py      # Shared fixtures
│   ├── test_unit.py
│   └── test_integration.py
├── pytest.ini           # Or pyproject.toml
└── requirements-test.txt

Configuration (pyproject.toml)

[tool.pytest.ini_options]
testpaths = ["tests"]
addopts = "-v --tb=short"
markers = [
    "slow: slow tests",
    "integration: integration tests",
]

Built-in Fixtures

Fixture Purpose
tmp_path Temp directory (pathlib.Path)
tmpdir Temp directory (legacy)
capsys Capture stdout/stderr
caplog Capture logging
monkeypatch Patch attributes/env vars
request Test/fixture metadata

Fixture Scopes

@pytest.fixture(scope="function")  # Default - per test
@pytest.fixture(scope="class")     # Per test class
@pytest.fixture(scope="module")    # Per file
@pytest.fixture(scope="session")   # Per test run

Best Practices

  • Name tests descriptively: test_user_login_with_invalid_password_fails
  • One assertion concept per test
  • Use fixtures for reusable setup
  • Use conftest.py for shared fixtures
  • Use markers to categorize tests (unit, integration, slow)
  • Run with coverage: pytest --cov --cov-report=html
  • Use parametrize to reduce duplicate tests