python-testing

## Quick Reference

| pytest Command | Purpose |
|----------------|---------|
| `pytest` | Run all tests |
| `pytest -v` | Verbose output |
| `pytest -k "name"` | Run tests matching pattern |
| `pytest -x` | Stop on first failure |
| `pytest --lf` | Run last failed |
| `pytest -n auto` | Parallel execution |

| Fixture Scope | Duration |
|---------------|----------|
| `function` | Per test (default) |
| `class` | Per test class |
| `module` | Per test file |
| `session` | Entire test run |

| Mock Pattern | Code |
|--------------|------|
| Patch function | `mocker.patch("module.func")` |
| Return value | `mock.return_value = {...}` |
| Side effect | `mock.side_effect = [a, b, exc]` |
| Assert called | `mock.assert_called_once()` |

| Marker | Use Case |
|--------|----------|
| `@pytest.mark.asyncio` | Async tests |
| `@pytest.mark.parametrize` | Multiple inputs |
| `@pytest.mark.skip` | Skip test |
| `@pytest.mark.xfail` | Expected failure |

## When to Use This Skill

Use for **testing Python code**:
- Writing pytest tests with fixtures
- Mocking external dependencies
- Testing async code
- Setting up code coverage
- Testing FastAPI applications

**Related skills:**
- For FastAPI: see `python-fastapi`
- For async patterns: see `python-asyncio`
- For CI/CD: see `python-github-actions`

---

# Python Testing Best Practices (2025)

## Overview

Modern Python testing centers around pytest as the de facto standard, with additional tools for coverage, mocking, and async testing.

## Pytest Fundamentals

### Installation

```bash
# With uv
uv add --dev pytest pytest-cov pytest-asyncio pytest-xdist

# With pip
pip install pytest pytest-cov pytest-asyncio pytest-xdist
```

### Basic Test Structure

```python
# tests/test_calculator.py
import pytest
from mypackage.calculator import add, divide

def test_add_positive_numbers():
    assert add(2, 3) == 5

def test_add_negative_numbers():
    assert add(-2, -3) == -5

def test_add_mixed_numbers():
    assert add(-2, 3) == 1

de