Python Library Testing

Quick Start

pytest # Run tests pytest --cov=my_library # With coverage pytest -x # Stop on first failure pytest -k "test_encode" # Run matching tests

Pytest Configuration

pyproject.toml

[tool.pytest.ini_options] testpaths = ["tests"] addopts = "-ra -q --cov=my_library --cov-fail-under=85"

[tool.coverage.run] branch = true source = ["src/my_library"]

Test Structure

tests/ ├── conftest.py # Shared fixtures ├── test_encoding.py └── test_decoding.py

Essential Patterns

Basic test:

def test_encode_valid_input(): result = encode(37.7749, -122.4194) assert isinstance(result, str) assert len(result) == 12

Parametrization:

@pytest.mark.parametrize("lat,lon,expected", [ (37.7749, -122.4194, "9q8yy"), (40.7128, -74.0060, "dr5ru"), ]) def test_known_values(lat, lon, expected): assert encode(lat, lon, precision=5) == expected

Fixtures:

@pytest.fixture def sample_data(): return [(37.7749, -122.4194), (40.7128, -74.0060)]

def test_batch(sample_data): results = batch_encode(sample_data) assert len(results) == 2

Mocking:

def test_api_call(mocker): mocker.patch("my_lib.client.fetch", return_value={"data": []}) result = my_lib.get_data() assert result == []

Exception testing:

def test_invalid_raises(): with pytest.raises(ValueError, match="latitude"): encode(91.0, 0.0)

For detailed patterns, see:

• FIXTURES.md - Advanced fixture patterns

• HYPOTHESIS.md - Property-based testing

• CI.md - CI/CD test configuration

Test Principles

Principle Meaning

Independent No shared state between tests

Deterministic Same result every run

Fast Unit tests < 100ms each

Focused Test behavior, not implementation

Checklist

Testing:

• Tests exist for public API
• Edge cases covered (empty, boundary, error)
• No external service dependencies (mock them)
• Coverage > 85%
• Tests run in CI

Learn More

This skill is based on the Code Quality section of the Guide to Developing High-Quality Python Libraries by Will McGinnis.