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

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

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

import 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

import 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

import 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

Run only marked tests

uv run pytest -m slow uv run pytest -m "not slow"

Async Testing

import 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

from 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

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

conftest.py

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

[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

import 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

import pytest

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

Temporary Files

def 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

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