```Python hl_lines="26-27  30"
{!> ../../docs_src/dependency_testing/tutorial001_an_py310.py!}
```

////

//// tab | Python 3.9+

```Python hl_lines="28-29  32"
{!> ../../docs_src/dependency_testing/tutorial001_an_py39.py!}
```

////

//// tab | Python 3.8+

```Python hl_lines="29-30  33"
{!> ../../docs_src/dependency_testing/tutorial001_an.py!}
```

////

//// tab | Python 3.10+ non-Annotated

import pytest
from dirty_equals import IsDict
from fastapi.testclient import TestClient

from ...utils import needs_py39


@pytest.fixture(name="client")
def get_client():
    from docs_src.request_files.tutorial001_an_py39 import app

    client = TestClient(app)
    return client


@needs_py39
def test_post_form_no_body(client: TestClient):
    response = client.post("/files/")

import pytest
from dirty_equals import IsDict
from fastapi.testclient import TestClient

from tests.utils import needs_py39


@pytest.fixture(name="client")
def get_client():
    from docs_src.request_form_models.tutorial001_an_py39 import app

    client = TestClient(app)
    return client


@needs_py39
def test_post_body_form(client: TestClient):
    response = client.post("/login/", data={"username": "Foo", "password": "secret"})

**FastAPI** 会确保每个带有 `yield` 的依赖中的"退出代码"按正确顺序运行。

例如，`dependency_c` 可以依赖于 `dependency_b`，而 `dependency_b` 则依赖于 `dependency_a`。

//// tab | Python 3.9+

```Python hl_lines="6  14  22"
{!> ../../docs_src/dependencies/tutorial008_an_py39.py!}
```

////

//// tab | Python 3.8+

```Python hl_lines="5  13  21"
{!> ../../docs_src/dependencies/tutorial008_an.py!}
```

////

//// tab | Python 3.8+ non-Annotated

```Python hl_lines="22-27"
{!> ../../docs_src/schema_extra_example/tutorial003_an_py310.py!}
```

////

//// tab | Python 3.9+

```Python hl_lines="22-27"
{!> ../../docs_src/schema_extra_example/tutorial003_an_py39.py!}
```

////

//// tab | Python 3.8+

```Python hl_lines="23-28"
{!> ../../docs_src/schema_extra_example/tutorial003_an.py!}
```

////

//// tab | Python 3.10+ non-Annotated

    params=[
        "tutorial001",
        pytest.param("tutorial001_py39", marks=needs_py39),
        pytest.param("tutorial001_py310", marks=needs_py310),
        "tutorial001_an",
        pytest.param("tutorial001_an_py39", marks=needs_py39),
        pytest.param("tutorial001_an_py310", marks=needs_py310),
    ],
)
def get_client(request: pytest.FixtureRequest):
    clear_sqlmodel()

Например, `dependency_c` может иметь зависимость от `dependency_b`, а `dependency_b` от `dependency_a`:

//// tab | Python 3.9+

```Python hl_lines="6  14  22"
{!> ../../docs_src/dependencies/tutorial008_an_py39.py!}
```

////

//// tab | Python 3.6+

```Python hl_lines="5  13  21"
{!> ../../docs_src/dependencies/tutorial008_an.py!}
```

////

//// tab | Python 3.6+ без Annotated

**FastAPI** knows what to do in each case and how to reuse the same object, so that all the background tasks are merged together and are run in the background afterwards:


{* ../../docs_src/background_tasks/tutorial002_an_py310.py hl[13,15,22,25] *}


In this example, the messages will be written to the `log.txt` file *after* the response is sent.

If there was a query in the request, it will be written to the log in a background task.

我们建议使用 **FastAPI** 的安全工具。

## 概览

首先，看看下面的代码是怎么运行的，然后再回过头来了解其背后的原理。

## 创建 `main.py`

把下面的示例代码复制到 `main.py`：

//// tab | Python 3.9+

```Python
{!> ../../docs_src/security/tutorial001_an_py39.py!}
```

////

//// tab | Python 3.8+

```Python
{!> ../../docs_src/security/tutorial001_an.py!}
```

////

//// tab | Python 3.8+ non-Annotated

/// tip