pytest

## When to use this skill

Use this skill whenever the user wants to:
- Write and run Python unit tests with pytest
- Use fixtures for test setup, teardown, and dependency injection
- Parametrize tests to run with multiple input sets
- Mock dependencies with `pytest-mock` or `unittest.mock`
- Configure pytest via `pytest.ini`, `pyproject.toml`, or `conftest.py`
- Generate coverage reports with `pytest-cov`
- Run tests in parallel with `pytest-xdist`

## How to use this skill

### Workflow

1. **Write test functions** with `test_` prefix or `Test` class
2. **Use fixtures** in `conftest.py` for shared setup and dependency injection
3. **Run tests** with `pytest` CLI, applying markers and filters as needed
4. **Analyze results** with coverage and reporting plugins

### 1. Basic Test

```python
def test_addition():
    assert 1 + 1 == 2

def test_string_upper():
    assert "hello".upper() == "HELLO"
```

### 2. Fixtures

```python
import pytest

@pytest.fixture
def sample_user():
    return {"name": "Alice", "email": "alice@example.com"}

def test_user_name(sample_user):
    assert sample_user["name"] == "Alice"

@pytest.fixture(autouse=True)
def reset_database(db):
    db.clear()
    yield
    db.clear()
```

### 3. Parametrize

```python
import pytest

@pytest.mark.parametrize("input,expected", [
    ("hello", 5),
    ("", 0),
    ("pytest", 6),
])
def test_string_length(input, expected):
    assert len(input) == expected
```

### 4. Mocking

```python
def test_api_call(mocker):
    mock_get = mocker.patch("requests.get")
    mock_get.return_value.json.return_value = {"status": "ok"}

    result = fetch_status()
    assert result == "ok"
    mock_get.assert_called_once()
```

### 5. Running Tests

```bash
# Run all tests
pytest

# Verbose with pattern filter
pytest -v -k "test_user"

# With coverage
pytest --cov=mypackage --cov-report=html

# Parallel execution
pytest -n auto
```

## Best Practices

- Keep tests independent with no shared mutable state; use fixtures for setup and teardown
- Use clear, descriptive test names that explain the expected behavior
- Avoid depending on test execution order; use `conftest.py` for shared fixtures
- Run the full test suite in CI; use `--cov` for coverage and `--maxfail=1` to fail fast
- Use markers (`@pytest.mark.slow`) to categorize tests and run subsets

## Resources

- Official documentation: https://docs.pytest.org/
- pytest-cov: https://pytest-cov.readthedocs.io/
- pytest-xdist: https://github.com/pytest-dev/pytest-xdist

## Keywords

pytest, Python testing, fixtures, parametrize, mocking, conftest, coverage, pytest-cov, pytest-xdist, unit testing, markers