Testing Python Libraries

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

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. See these posts for deeper coverage:

• Testing with Pytest
• Testing Coverage
• Testing with Tox
• Testing with Mocking

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