# pytest — The Python Testing Framework That Scales

> pytest makes it easy to write small tests, yet scales to support complex functional testing. Fixtures, parameterization, plugins, markers, and a rich assertion introspection system. The de facto testing standard for the Python ecosystem.

## Install

Save in your project root:

## Quick Use

```bash
pip install pytest
```

```python
# test_calc.py
def add(a, b):
    return a + b

def test_add():
    assert add(1, 2) == 3

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

def test_add_strings():
    assert add("hello ", "world") == "hello world"
```

```bash
pytest                       # Auto-discover and run tests
pytest -v                    # Verbose
pytest -x                    # Stop on first failure
pytest -k "negative"         # Filter by name pattern
pytest --cov=mypackage       # Coverage (needs pytest-cov)
pytest -n auto               # Parallel (needs pytest-xdist)
```

Fixtures:
```python
import pytest

@pytest.fixture
def db_session():
    session = create_session()
    yield session
    session.rollback()
    session.close()

def test_create_user(db_session):
    user = User(name="William")
    db_session.add(user)
    db_session.flush()
    assert user.id is not None

@pytest.mark.parametrize("a,b,expected", [(1,2,3), (0,0,0), (-1,1,0)])
def test_add_parametrized(a, b, expected):
    assert add(a, b) == expected
```

## Intro

pytest is the de facto testing framework for Python. Simple enough for small tests, powerful enough for complex functional testing. Features include fixtures (dependency injection), parametrization, markers, plugins, and an incredibly helpful assertion introspection system that shows you exactly what failed and why.

- **Repo**: https://github.com/pytest-dev/pytest
- **Stars**: 13K+
- **Language**: Python
- **License**: MIT

## What pytest Does

- **Auto-discovery** — finds `test_*.py` files and `test_*` functions
- **Assertion introspection** — plain `assert` with detailed failure diffs
- **Fixtures** — DI for test setup/teardown (function, class, module, session scope)
- **Parametrize** — run same test with multiple inputs
- **Markers** — `@pytest.mark.slow`, `@pytest.mark.skip`, custom
- **Plugins** — 1200+ on PyPI (pytest-cov, pytest-xdist, pytest-mock, pytest-asyncio)
- **Conftest** — shared fixtures across test directories
- **Temporary directories** — `tmp_path` fixture
- **Capture** — stdout/stderr capture per test
- **Parallel** — via pytest-xdist

## Comparison

| Framework | Style | Fixtures | Plugins |
|---|---|---|---|
| pytest | Function-based | DI (fixtures) | 1200+ |
| unittest | Class-based | setUp/tearDown | Limited |
| nose2 | Mixed | Limited | Some |
| doctest | Inline | None | None |

## 常见问题 FAQ

**Q: pytest vs unittest？**
A: pytest 更简洁（无需 class）、assertion 更好（不用 assertEqual）、fixture DI 更强。Python 社区绝大多数新项目用 pytest。

**Q: 怎么测异步代码？**
A: `pip install pytest-asyncio`，然后 `@pytest.mark.asyncio async def test_xxx()` 即可 await 异步函数。

**Q: fixture 的 scope？**
A: function（每个测试一次）、class、module、session（整个测试跑一次）。scope 越大，setup 开销越小但隔离越弱。

## 来源与致谢 Sources

- Docs: https://docs.pytest.org
- GitHub: https://github.com/pytest-dev/pytest
- License: MIT

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