testing-python-libraries

# Python Library Testing

## Quick Start

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

## Pytest Configuration

```toml
# 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:**
```python
def test_encode_valid_input():
    result = encode(37.7749, -122.4194)
    assert isinstance(result, str)
    assert len(result) == 12
```

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

**Exception testing:**
```python
def test_invalid_raises():
    with pytest.raises(ValueError, match="latitude"):
        encode(91.0, 0.0)
```

For detailed patterns, see:
- **[FIXTURES.md](FIXTURES.md)** - Advanced fixture patterns
- **[HYPOTHESIS.md](HYPOTHESIS.md)** - Property-based testing
- **[CI.md](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 |