from contextlib import asynccontextmanager as asynccontextmanager
from typing import AsyncGenerator, ContextManager, TypeVar

import anyio
from anyio import CapacityLimiter
from starlette.concurrency import iterate_in_threadpool as iterate_in_threadpool  # noqa
from starlette.concurrency import run_in_threadpool as run_in_threadpool  # noqa
from starlette.concurrency import (  # noqa
    run_until_first_complete as run_until_first_complete,
)

from contextlib import asynccontextmanager

from fastapi import FastAPI


def fake_answer_to_everything_ml_model(x: float):
    return x * 42


ml_models = {}


@asynccontextmanager
async def lifespan(app: FastAPI):
    # Load the ML model
    ml_models["answer_to_everything"] = fake_answer_to_everything_ml_model
    yield
    # Clean up the ML models and release the resources
    ml_models.clear()

from contextlib import asynccontextmanager
from typing import AsyncGenerator, Dict

import pytest
from fastapi import APIRouter, FastAPI
from fastapi.testclient import TestClient
from pydantic import BaseModel


class State(BaseModel):
    app_startup: bool = False
    app_shutdown: bool = False
    router_startup: bool = False
    router_shutdown: bool = False
    sub_router_startup: bool = False
    sub_router_shutdown: bool = False

    * <a href="https://docs.python.org/3/library/contextlib.html#contextlib.asynccontextmanager" class="external-link" target="_blank">`@contextlib.asynccontextmanager`</a>

    будет корректно использоваться в качестве **FastAPI**-зависимости.

    На самом деле, FastAPI использует эту пару декораторов "под капотом".

    * <a href="https://docs.python.org/3/library/contextlib.html#contextlib.asynccontextmanager" class="external-link" target="_blank">`@contextlib.asynccontextmanager`</a>

    都可以作为 **FastAPI** 的依赖项。

    实际上，FastAPI内部就使用了这两个装饰器。


## 使用 `yield` 的数据库依赖项

例如，您可以使用这种方式创建一个数据库会话，并在完成后关闭它。

在发送响应之前，只会执行 `yield` 语句及之前的代码：

    * <a href="https://docs.python.org/3/library/contextlib.html#contextlib.asynccontextmanager" class="external-link" target="_blank">`@contextlib.asynccontextmanager`</a>

    kann auch als gültige **FastAPI**-Abhängigkeit verwendet werden.

    Tatsächlich verwendet FastAPI diese beiden Dekoratoren intern.

    * <a href="https://docs.python.org/3/library/contextlib.html#contextlib.asynccontextmanager" class="external-link" target="_blank">`@contextlib.asynccontextmanager`</a>

    これらは **FastAPI** の依存関係として使用するのに有効です。

    実際、FastAPIは内部的にこれら２つのデコレータを使用しています。

## `yield`を持つデータベースの依存関係

例えば、これを使ってデータベースセッションを作成し、終了後にそれを閉じることができます。

    * <a href="https://docs.python.org/3/library/contextlib.html#contextlib.asynccontextmanager" class="external-link" target="_blank">`@contextlib.asynccontextmanager`</a>

    would be valid to use as a **FastAPI** dependency.

    In fact, FastAPI uses those two decorators internally.

## A database dependency with `yield`

    * <a href="https://docs.python.org/3/library/contextlib.html#contextlib.asynccontextmanager" class="external-link" target="_blank">`@contextlib.asynccontextmanager`</a>

    🔜 ☑ ⚙️ **FastAPI** 🔗.

    👐, FastAPI ⚙️ 📚 2️⃣ 👨‍🎨 🔘.

## 💽 🔗 ⏮️ `yield`

🖼, 👆 💪 ⚙️ 👉 ✍ 💽 🎉 &amp; 🔐 ⚫️ ⏮️ 🏁.

And the part after the `yield` will be executed **after** the application has finished.

### Async Context Manager

If you check, the function is decorated with an `@asynccontextmanager`.

That converts the function into something called an "**async context manager**".

```Python hl_lines="1  13"
{!../../../docs_src/events/tutorial003.py!}
```