class CallableDependency:
    def __call__(self, value: str) -> str:
        return value


class CallableGenDependency:
    def __call__(self, value: str) -> Generator[str, None, None]:
        yield value


class AsyncCallableDependency:
    async def __call__(self, value: str) -> str:
        return value


class AsyncCallableGenDependency:
    async def __call__(self, value: str) -> AsyncGenerator[str, None]:

import time

from fastapi import FastAPI, Request

app = FastAPI()


@app.middleware("http")
async def add_process_time_header(request: Request, call_next):
    start_time = time.time()
    response = await call_next(request)
    process_time = time.time() - start_time
    response.headers["X-Process-Time"] = str(process_time)

@app.post("/")
async def post(
    foo: Union[Foo, None] = None,
):
    return foo


client = TestClient(app)


def test_call_invalid():
    response = client.post("/", json={"foo": {"bar": "baz"}})
    assert response.status_code == 422


def test_call_valid():
    response = client.post("/", json={})
    assert response.status_code == 200
    assert response.json() == {}

假设要创建校验查询参数 `q` 是否包含固定内容的依赖项。

但此处要把待检验的固定内容定义为参数。

## **可调用**实例

Python 可以把类实例变为**可调用项**。

这里说的不是类本身（类本就是可调用项），而是类实例。

为此，需要声明 `__call__` 方法：

```Python hl_lines="10"
{!../../../docs_src/dependencies/tutorial011.py!}
```

本例中，**FastAPI**  使用 `__call__` 检查附加参数及子依赖项，稍后，还要调用它向*路径操作函数*传递值。

## 参数化实例

接下来，使用 `__init__` 声明用于**参数化**依赖项的实例参数：

```Python hl_lines="7"

    ):
        self.model = HTTPBaseModel(scheme=scheme, description=description)
        self.scheme_name = scheme_name or self.__class__.__name__
        self.auto_error = auto_error

    async def __call__(
        self, request: Request
    ) -> Optional[HTTPAuthorizationCredentials]:
        authorization = request.headers.get("Authorization")
        scheme, credentials = get_authorization_scheme_param(authorization)

✋️ 👥 💚 💪 🔗 👈 🔧 🎚.

##  "🇧🇲" 👐

🐍 📤 🌌 ⚒ 👐 🎓 "🇧🇲".

🚫 🎓 ⚫️ (❔ ⏪ 🇧🇲), ✋️ 👐 👈 🎓.

👈, 👥 📣 👩‍🔬 `__call__`:

```Python hl_lines="10"
{!../../../docs_src/dependencies/tutorial011.py!}
```

👉 💼, 👉 `__call__` ⚫️❔ **FastAPI** 🔜 ⚙️ ✅ 🌖 🔢 & 🎧-🔗, & 👉 ⚫️❔ 🔜 🤙 🚶‍♀️ 💲 🔢 👆 *➡ 🛠️ 🔢* ⏪.

## 🔗 👐

& 🔜, 👥 💪 ⚙️ `__init__` 📣 🔢 👐 👈 👥 💪 ⚙️ "🔗" 🔗:

from fastapi import Depends, FastAPI

app = FastAPI()


class FixedContentQueryChecker:
    def __init__(self, fixed_content: str):
        self.fixed_content = fixed_content

    def __call__(self, q: str = ""):
        if q:
            return self.fixed_content in q
        return False


checker = FixedContentQueryChecker("bar")


@app.get("/query-checker/")

    dependency: Annotated[
        Optional[Callable[..., Any]],
        Doc(
            """
            A "dependable" callable (like a function).

            Don't call it directly, FastAPI will call it for you, just pass the object
            directly.
            """
        ),
    ] = None,
    *,
    use_cache: Annotated[
        bool,
        Doc(
            """

    This is a special class that you can define in a parameter in a dependency to
    obtain the OAuth2 scopes required by all the dependencies in the same chain.

    This way, multiple dependencies can have different scopes, even when used in the
    same *path operation*. And with this, you can access all the scopes required in
    all those dependencies in a single place.

    Read more about it in the

    If there were any background tasks (documented later), they will run *after* all the middleware.

## Create a middleware

To create a middleware you use the decorator `@app.middleware("http")` on top of a function.

The middleware function receives:

* The `request`.
* A function `call_next` that will receive the `request` as a parameter.