async def get_db():
    db = DBSession()
    try:
        yield db
    finally:

    def __enter__(self):
        return self.db

    def __exit__(self, exc_type, exc_value, traceback):
        self.db.close()


async def get_db():
    with MySuperContextManager() as db:

👥 🚮 🏗 `SessionLocal()` & 🚚 📨 `try` 🍫.

 & ⤴️ 👥 🔐 ⚫️ `finally` 🍫.

👉 🌌 👥 ⚒ 💭 💽 🎉 🕧 📪 ⏮️ 📨. 🚥 📤 ⚠ ⏪ 🏭 📨.

✋️ 👆 💪 🚫 🤚 ➕1️⃣ ⚠ ⚪️➡️ 🚪 📟 (⏮️ `yield`). 👀 🌖 [🔗 ⏮️ `yield` & `HTTPException`](dependencies/dependencies-with-yield.md#yield-httpexception){.internal-link target=_blank}

///

& ⤴️, 🕐❔ ⚙️ 🔗 *➡ 🛠️ 🔢*, 👥 📣 ⚫️ ⏮️ 🆎 `Session` 👥 🗄 🔗 ⚪️➡️ 🇸🇲.

    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()


app = FastAPI(lifespan=lifespan)


@app.get("/predict")
async def predict(x: float):

from typing import Annotated

from fastapi import Depends, FastAPI, HTTPException

app = FastAPI()


class InternalError(Exception):
    pass


def get_username():
    try:
        yield "Rick"
    except InternalError:
        print("We don't swallow the internal error here, we raise again 😎")
        raise


@app.get("/items/{item_id}")
def get_item(item_id: str, username: Annotated[str, Depends(get_username)]):

/// info

我们将`SessionLocal()`请求的创建和处理放在一个`try`块中。

然后我们在finally块中关闭它。

通过这种方式，我们确保数据库会话在请求后始终关闭。即使在处理请求时出现异常。

但是您不能从退出代码中引发另一个异常（在yield之后）。可以查阅 [Dependencies with yield and HTTPException](https://fastapi.tiangolo.com/zh/tutorial/dependencies/dependencies-with-yield/#dependencies-with-yield-and-httpexception)

///

*然后，当在路径操作函数*中使用依赖项时，我们使用`Session`，直接从 SQLAlchemy 导入的类型声明它。

				}

				// if the default concurrency yields zero results, throw an error.
				if throughputHighestResults[i].Downloads == 0 && opts.concurrencyStart == concurrency {
					errStr = fmt.Sprintf("no results for downloads upon first attempt, concurrency %d and duration %s",
						opts.concurrencyStart, opts.duration)
				}

				// if the default concurrency yields zero results, throw an error.

    }
    val result = plans[nextPlanIndex++]
    events += "take plan ${result.id}"

    if (result.yieldBeforePlanReturns) {
      taskFaker.yield()
    }

    val planningThrowable = result.planningThrowable
    if (planningThrowable != null) throw planningThrowable

    return result
  }

 * that order.
 *
 * <p>By default, {@code Splitter}'s behavior is simplistic and unassuming. The following
 * expression:
 *
 * <pre>{@code
 * Splitter.on(',').split(" foo,,,  bar ,")
 * }</pre>
 *
 * ... yields the substrings {@code [" foo", "", "", " bar ", ""]}. If this is not the desired
 * behavior, use configuration methods to obtain a <i>new</i> splitter instance with modified
 * behavior:
 *
 * <pre>{@code

	left, right *Tree[K, V]
	key         K
	value       V
}

func (t *Tree[K, V]) walk(yield func(key K, val V) bool) bool {
	return t == nil || t.left.walk(yield) && yield(t.key, t.value) && t.right.walk(yield)
}

func (t *Tree[K, V]) Walk(yield func(key K, val V) bool) {
	t.walk(yield)
}

// walk tree t in-order
var t Tree[string, int]
for k, v := range t.Walk {
	// process k, v