**FastAPI** will take that model, generate its JSON Schema and include it in the correct place in OpenAPI.

For example, to declare another response with a status code `404` and a Pydantic model `Message`, you can write:

{* ../../docs_src/additional_responses/tutorial001_py310.py hl[18,22] *}

/// note

Keep in mind that you have to return the `JSONResponse` directly.

///

      Request
        .Builder()
        .url("https://valid-isrgrootx1.letsencrypt.org/robots.txt")
        .build()
    client.newCall(request).execute().use { response ->
      assertThat(response.code).isEqualTo(404)
      assertThat(response.protocol).isEqualTo(Protocol.HTTP_2)
    }
  }

   *
   * Note that transport-layer success (receiving a HTTP response code, headers and body) does not
   * necessarily indicate application-layer success: `response` may still indicate an unhappy HTTP
   * response code like 404 or 500.
   */
  @Throws(IOException::class)
  fun onResponse(
    call: Call,
    response: Response,
  )

@app.get("/items/{item_id}")
def get_item(item_id: str, username: Annotated[str, Depends(get_username)]):
    if item_id not in data:
        raise HTTPException(status_code=404, detail="Item not found")
    item = data[item_id]
    if item["owner"] != username:
        raise OwnerError(username)

* など

これらの場合、通常は **400**（400から499）の範囲内の **HTTPステータスコード** を返すことになります。

これは200のHTTPステータスコード（200から299）に似ています。これらの「200」ステータスコードは、何らかの形でリクエスト「成功」であったことを意味します。

400の範囲にあるステータスコードは、クライアントからのエラーがあったことを意味します。

**"404 Not Found"** のエラー（およびジョーク）を覚えていますか？

## `HTTPException`の使用 { #use-httpexception }

HTTPレスポンスをエラーでクライアントに返すには、`HTTPException`を使用します。

### `HTTPException`のインポート { #import-httpexception }

input size | lib | MB/s | Allocated
--- | --- | --- | ---
regular | standard | 97 | 3.6MB
regular | jstream | 175 | 2.1MB
large | standard | 92 | 305MB
large | jstream | 404 | 69MB

In a real world scenario, including initialization and reader overhead from varying blob sizes, performance can be expected as below:

    if x_token != fake_secret_token:
        raise HTTPException(status_code=400, detail="Invalid X-Token header")
    if item_id not in fake_db:
        raise HTTPException(status_code=404, detail="Item not found")
    return fake_db[item_id]


@app.post("/items/")
async def create_item(item: Item, x_token: str = Header()) -> Item:
    if x_token != fake_secret_token:

    Listener listener;
    synchronized (this) {
      listener = listeners.get(state);
    }

    if (code == null || listener == null) {
      return new MockResponse()
          .setResponseCode(404)
          .setBody("unexpected request");
    }

    try {
      OAuthSession session = slackApi.exchangeCode(code, redirectUrl());
      listener.sessionGranted(session);
    } catch (IOException e) {

    }

    @Test
    @DisplayName("Test status getter and setter")
    void testStatusGetterSetter() {
        assertEquals(0, response.getStatus());
        response.setStatus(404);
        assertEquals(404, response.getStatus());
    }

    @Test
    @DisplayName("Test numEntries getter and setter")
    void testNumEntriesGetterSetter() {
        assertEquals(0, response.getNumEntries());

    assert response.json() == {"detail": "Invalid X-Token header"}


def test_read_nonexistent_item():
    response = client.get("/items/baz", headers={"X-Token": "coneofsilence"})
    assert response.status_code == 404
    assert response.json() == {"detail": "Item not found"}


def test_create_item():
    response = client.post(
        "/items/",
        headers={"X-Token": "coneofsilence"},