**FastAPI** はこれを実現するために内部的に使用しています。

///

## `yield`と`HTTPException`を持つ依存関係

`yield`と例外をキャッチする`try`ブロックを持つことができる依存関係を使用することができることがわかりました。

`yield`の後の終了コードで`HTTPException`などを発生させたくなるかもしれません。しかし**それはうまくいきません**

`yield`を持つ依存関係の終了コードは[例外ハンドラ](../handling-errors.md#_4){.internal-link target=_blank}の*後に*実行されます。依存関係によって投げられた例外を終了コード（`yield`の後）でキャッチするものはなにもありません。

 *       byte sequences under the covers.
 *   <li><b>hash code:</b> each hash function always yields hash codes of the same fixed bit length
 *       (given by {@link #bits}). For example, {@link Hashing#sha1} produces a 160-bit number,
 *       while {@link Hashing#murmur3_32()} yields only 32 bits. Because a {@code long} value is
 *       clearly insufficient to hold all hash code values, this API represents a hash code as an

 *       byte sequences under the covers.
 *   <li><b>hash code:</b> each hash function always yields hash codes of the same fixed bit length
 *       (given by {@link #bits}). For example, {@link Hashing#sha1} produces a 160-bit number,
 *       while {@link Hashing#murmur3_32()} yields only 32 bits. Because a {@code long} value is
 *       clearly insufficient to hold all hash code values, this API represents a hash code as an

	// at this moment, and if not, yield. This is to catch other CNIs which might have mutated the file between
	// the (theoretical) window after we initially install/write, but before we actually start the filewatch.
	if err := checkValidCNIConfig(in.cfg, in.cniConfigFilepath); err != nil {
		return nil
	}

	// If a file we are watching has a change event, yield and let caller check validity
	select {

 * Runs a [TaskRunner] in a controlled environment so that everything is sequential and
 * deterministic.
 *
 * This class ensures that at most one thread is running at a time. This is initially the JUnit test
 * thread, which yields its execution privilege while calling [runTasks], [runNextTask], or
 * [advanceUntil]. These functions don't return until the task threads are all idle.
 *
 * Task threads release their execution privilege in these ways:
 *

   * all subscribers in the order they are posted.
   *
   * <p>When all subscribers are dispatched to using a <i>direct</i> executor (which dispatches on
   * the same thread that posts the event), this yields a breadth-first dispatch order on each
   * thread. That is, all subscribers to a single event A will be called before any subscribers to
   * any events B and C that are posted to the event bus by the subscribers to A.
   */

Using resources from dependencies with `yield` in background tasks is no longer supported.

This change is what supports the new features, read below. 🤓

### Dependencies with `yield`, `HTTPException` and Background Tasks

Dependencies with `yield` now can raise `HTTPException` and other exceptions after `yield`. 🎉

`shutdown` 🔜 🔨 🕐❔ 👆 **⛔️** 🈸.

🎲 👆 💪 ▶️ 🆕 ⏬, ⚖️ 👆 🤚 🎡 🏃 ⚫️. 🤷

///

### 🔆 🔢

🥇 👜 👀, 👈 👥 ⚖ 🔁 🔢 ⏮️ `yield`. 👉 📶 🎏 🔗 ⏮️ `yield`.

```Python hl_lines="14-19"
{!../../docs_src/events/tutorial003.py!}
```

🥇 🍕 🔢, ⏭ `yield`, 🔜 🛠️ **⏭** 🈸 ▶️.

& 🍕 ⏮️ `yield` 🔜 🛠️ **⏮️** 🈸 ✔️ 🏁.

### 🔁 🔑 👨‍💼

🚥 👆 ✅, 🔢 🎀 ⏮️ `@asynccontextmanager`.

👈 🗜 🔢 🔘 🕳 🤙 "**🔁 🔑 👨‍💼**".

from fastapi import Depends


async def dependency_a():
    dep_a = generate_dep_a()
    try:
        yield dep_a
    finally:
        dep_a.close()


async def dependency_b(dep_a: Annotated[DepA, Depends(dependency_a)]):
    dep_b = generate_dep_b()
    try:
        yield dep_b
    finally:
        dep_b.close(dep_a)


async def dependency_c(dep_b: Annotated[DepB, Depends(dependency_b)]):

client = TestClient(app)


def test_dependency_contextvars():
    """
    Check that custom middlewares don't affect the contextvar context for dependencies.

    The code before yield and the code after yield should be run in the same contextvar
    context, so that request_state_context_var.reset(contextvar_token).

    If they are run in a different context, that raises an error.
    """