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model: haiku created: 2025-12-16 modified: 2025-12-16 reviewed: 2025-12-16 name: python-testing description: | Python testing with pytest, coverage, fixtures, parametrization, and mocking. Covers test organization, conftest.py, markers, async testing, and TDD workflows. Use when user mentions pytest, unit tests, test coverage, fixtures, mocking, or writing Python tests.

Python Testing

Quick reference for Python testing with pytest, coverage, fixtures, and best practices.

When This Skill Applies

• Writing unit tests and integration tests
• Test-driven development (TDD)
• Test fixtures and parametrization
• Coverage analysis
• Mocking and patching
• Async testing

Quick Reference

Running Tests

Copy# Basic test run
uv run pytest

# Verbose output
uv run pytest -v

# Show print statements
uv run pytest -s

# Stop at first failure
uv run pytest -x

# Run specific test
uv run pytest tests/test_module.py::test_function

# Run by keyword
uv run pytest -k "test_user"

Test Coverage

Copy# Run with coverage
uv run pytest --cov

# HTML report
uv run pytest --cov --cov-report=html

# Show missing lines
uv run pytest --cov --cov-report=term-missing

# Coverage for specific module
uv run pytest --cov=mymodule tests/

Fixtures

Copyimport pytest

@pytest.fixture
def sample_data():
    return {"key": "value"}

@pytest.fixture(scope="module")
def db_connection():
    conn = create_connection()
    yield conn
    conn.close()

def test_with_fixture(sample_data):
    assert sample_data["key"] == "value"

Parametrize Tests

Copyimport pytest

@pytest.mark.parametrize("input,expected", [
    ("hello", "HELLO"),
    ("world", "WORLD"),
    ("test", "TEST"),
])
def test_uppercase(input: str, expected: str):
    assert input.upper() == expected

@pytest.mark.parametrize("value,is_valid", [
    (1, True),
    (0, False),
    (-1, False),
])
def test_validation(value, is_valid):
    assert validate(value) == is_valid

Markers

Copyimport pytest

@pytest.mark.slow
def test_slow_operation():
    # Long-running test
    pass

@pytest.mark.skip(reason="Not implemented yet")
def test_future_feature():
    pass

@pytest.mark.skipif(sys.platform == "win32", reason="Unix only")
def test_unix_specific():
    pass

@pytest.mark.xfail
def test_known_issue():
    # Expected to fail
    pass

Copy# Run only marked tests
uv run pytest -m slow
uv run pytest -m "not slow"

Async Testing

Copyimport pytest

@pytest.mark.asyncio
async def test_async_function():
    result = await async_operation()
    assert result == expected_value

@pytest.fixture
async def async_client():
    client = AsyncClient()
    await client.connect()
    yield client
    await client.disconnect()

Mocking

Copyfrom unittest.mock import Mock, patch, MagicMock

def test_with_mock():
    mock_obj = Mock()
    mock_obj.method.return_value = "mocked"
    assert mock_obj.method() == "mocked"

@patch('module.external_api')
def test_with_patch(mock_api):
    mock_api.return_value = {"status": "success"}
    result = call_external_api()
    assert result["status"] == "success"

# pytest-mock (cleaner)
def test_with_mocker(mocker):
    mock = mocker.patch('module.function')
    mock.return_value = 42
    assert function() == 42

Test Organization

Copyproject/
├── src/
│   └── myproject/
│       ├── __init__.py
│       └── module.py
└── tests/
    ├── __init__.py
    ├── conftest.py          # Shared fixtures
    ├── test_module.py
    └── integration/
        └── test_api.py

conftest.py

Copy# tests/conftest.py
import pytest

@pytest.fixture(scope="session")
def app_config():
    return {"debug": True, "testing": True}

@pytest.fixture(autouse=True)
def reset_db():
    setup_database()
    yield
    teardown_database()

pyproject.toml Configuration

Copy[tool.pytest.ini_options]
testpaths = ["tests"]
python_files = ["test_*.py", "*_test.py"]
python_classes = ["Test*"]
python_functions = ["test_*"]
addopts = [
    "-v",
    "--strict-markers",
    "--cov=src",
    "--cov-report=term-missing",
]
markers = [
    "slow: marks tests as slow",
    "integration: marks tests as integration tests",
]

[tool.coverage.run]
source = ["src"]
omit = ["*/tests/*", "*/test_*.py"]

[tool.coverage.report]
exclude_lines = [
    "pragma: no cover",
    "def __repr__",
    "raise AssertionError",
    "raise NotImplementedError",
    "if __name__ == .__main__.:",
]

Common Testing Patterns

Test Exceptions

Copyimport pytest

def test_raises_exception():
    with pytest.raises(ValueError):
        function_that_raises()

def test_raises_with_message():
    with pytest.raises(ValueError, match="Invalid input"):
        function_that_raises()

Test Warnings

Copyimport pytest

def test_deprecation_warning():
    with pytest.warns(DeprecationWarning):
        deprecated_function()

Temporary Files

Copydef test_with_tmp_path(tmp_path):
    file_path = tmp_path / "test.txt"
    file_path.write_text("content")
    assert file_path.read_text() == "content"

TDD Workflow

Copy# 1. RED: Write failing test
uv run pytest tests/test_new_feature.py
# FAILED

# 2. GREEN: Implement minimal code
uv run pytest tests/test_new_feature.py
# PASSED

# 3. REFACTOR: Improve code
uv run pytest  # All tests pass

See Also

• uv-project-management - Adding pytest to projects
• python-code-quality - Combining tests with linting
• python-development - Core Python development patterns

References

• Official docs: https://docs.pytest.org/
• Detailed guide: See REFERENCE.md in this skill directory