/// tip | 豆知識

Pydantic は **Rust** 側でシリアライズを行うため、戻り値の型を宣言しない場合に比べて大幅に高い**パフォーマンス**が得られます。

///

### 非 async の *path operation 関数* { #non-async-path-operation-functions }

`async` を使わない通常の `def` 関数でも同様に `yield` を使えます。

FastAPI が適切に実行されるように処理するため、イベントループをブロックしません。

この場合は関数が async ではないので、適切な戻り値の型は `Iterable[Item]` です:

    return {"name": fake_items_db[item_id]["name"], "item_id": item_id}


@router.put(
    "/{item_id}",
    tags=["custom"],
    responses={403: {"description": "Operation forbidden"}},
)
async def update_item(item_id: str):
    if item_id != "plumbus":
        raise HTTPException(
            status_code=403, detail="You can only update the item: plumbus"
        )

	// mainly set for certain WRITE operations.
	SkipDecommissioned bool
	// SkipRebalancing should be set to 'true' if the call should skip pools
	// participating in a rebalance operation. Typically set for 'write' operations.
	SkipRebalancing bool

	SrcPoolIdx int // set by PutObject/CompleteMultipart operations due to rebalance; used to prevent rebalance src, dst pools to be the same

| *Type*     | *String* |
| *Required* | *Yes*    |

### DurationSeconds

The duration, in seconds. The value can range from 900 seconds (15 minutes) up to 365 days. If value is higher than this setting, then operation fails. By default, the value is set to 3600 seconds. If no *DurationSeconds* is specified expiry seconds is obtained from *Token*.

| Params        | Value                                              |

    /**
     * Select the least busy channel based on pending operations
     */
    LEAST_LOADED,

    /**
     * Weighted random selection based on channel scores
     */
    WEIGHTED_RANDOM,

    /**
     * Affinity-based selection for related operations
     */
    AFFINITY_BASED,

    /**
     * Adaptive selection based on operation type and performance
     */
    ADAPTIVE

    },
}


class EventSourceResponse(StreamingResponse):
    """Streaming response with `text/event-stream` media type.

    Use as `response_class=EventSourceResponse` on a *path operation* that uses `yield`
    to enable Server Sent Events (SSE) responses.

    Works with **any HTTP method** (`GET`, `POST`, etc.), which makes it compatible
    with protocols like MCP that stream SSE over `POST`.

   * described in the class documentation.
   *
   * @throws NullPointerException if any of {@code elements} is null
   * @since 6.0
   */
  public static <E> ImmutableMultiset<E> copyOf(E[] elements) {
    return copyFromElements(elements);
  }

  /**
   * Returns an immutable multiset containing the given elements, in the "grouped iteration order"

    return this.<T>trustedPut(type.rejectTypeVariables(), value);
  }

  /**
   * Not supported. Use {@link #putInstance} instead.
   *
   * @deprecated unsupported operation
   * @throws UnsupportedOperationException always
   */
  @CanIgnoreReturnValue
  @Deprecated
  @Override
  @DoNotCall("Always throws UnsupportedOperationException")

* tested (adjective): протестированный
* middleware: middleware (don't translate, but add `промежуточный слой` if clarification is needed)
* path operation: операция пути (optionally clarify as `обработчик пути`)
* path operation function: функция-обработчик пути (or `функция обработки пути`)
* proprietary: проприетарный

# ボディ - フィールド { #body-fields }

`Query`や`Path`、`Body`を使って *path operation関数* のパラメータに追加のバリデーションやメタデータを宣言するのと同じように、Pydanticの`Field`を使ってPydanticモデルの内部でバリデーションやメタデータを宣言することができます。

## `Field`のインポート { #import-field }

まず、以下のようにインポートします:

{* ../../docs_src/body_fields/tutorial001_an_py310.py hl[4] *}


/// warning | 注意

`Field`は他の全てのもの（`Query`、`Path`、`Body`など）とは違い、`fastapi`からではなく、`pydantic`から直接インポートされていることに注意してください。

///