* `main.py` adında bir dosya oluşturup içine şu kodu yapıştıralım:

```Python
from typing import Union

from fastapi import FastAPI

app = FastAPI()


@app.get("/")
def read_root():
    return {"Hello": "World"}


@app.get("/items/{item_id}")
def read_item(item_id: int, q: Union[str, None] = None):
    return {"item_id": item_id, "q": q}
```

<details markdown="1">

*.gen.go linguist-generated=true
*.gen.yaml linguist-generated=true
*.gen.json linguist-generated=true
*_pb2.py linguist-generated=true
manifests/charts/**/profile*.yaml linguist-generated=true
go.sum merge=union
vendor/**  linguist-vendored
common/**  linguist-vendored
archive/**  linquist-vendored

    webhooks: Optional[Sequence[BaseRoute]] = None,
    tags: Optional[List[Dict[str, Any]]] = None,
    servers: Optional[List[Dict[str, Union[str, Any]]]] = None,
    terms_of_service: Optional[str] = None,
    contact: Optional[Dict[str, Union[str, Any]]] = None,
    license_info: Optional[Dict[str, Union[str, Any]]] = None,
    separate_input_output_schemas: bool = True,
) -> Dict[str, Any]:

## Exemple

### Créez

* Créez un fichier `main.py` avec :

```Python
from typing import Union

from fastapi import FastAPI

app = FastAPI()


@app.get("/")
def read_root():
    return {"Hello": "World"}


@app.get("/items/{item_id}")
def read_item(item_id: int, q: Union[str, None] = None):
    return {"item_id": item_id, "q": q}
```

<details markdown="1">

## 範例

### 建立

- 建立一個 python 檔案 `main.py`，並寫入以下程式碼：

```Python
from typing import Union

from fastapi import FastAPI

app = FastAPI()


@app.get("/")
def read_root():
    return {"Hello": "World"}


@app.get("/items/{item_id}")
def read_item(item_id: int, q: Union[str, None] = None):
    return {"item_id": item_id, "q": q}
```

<details markdown="1">

    this.selfLoopCount = checkNonNegative(selfLoopCount);
    checkState(selfLoopCount <= inEdgeMap.size() && selfLoopCount <= outEdgeMap.size());
  }

  @Override
  public Set<N> adjacentNodes() {
    return Sets.union(predecessors(), successors());
  }

  @Override
  public Set<E> incidentEdges() {
    return new AbstractSet<E>() {
      @Override
      public UnmodifiableIterator<E> iterator() {

            _dst.enc_ndr_long(resume_handle);
        }
        public void decode_out(NdrBuffer _src) throws NdrException {
            level = (int)_src.dec_ndr_long();
            _src.dec_ndr_long(); /* union discriminant */
            int _infop = _src.dec_ndr_long();
            if (_infop != 0) {
                if (info == null) { /* YOYOYO */
                    info = new ShareInfoCtr0();
                }

```Python hl_lines="9  15 16  19 20  23 24"
{!../../docs_src/extra_models/tutorial002.py!}
```

## `Union`または`anyOf`

レスポンスを２つの型の`Union`として宣言することができます。

OpenAPIでは`anyOf`で定義されます。

そのためには、標準的なPythonの型ヒント<a href="https://docs.python.org/3/library/typing.html#typing.Union" class="external-link" target="_blank">`typing.Union`</a>を使用します:

```Python hl_lines="1 14 15 18 19 20 33"
{!../../docs_src/extra_models/tutorial003.py!}

        status_code: Optional[int] = None,
        tags: Optional[List[Union[str, Enum]]] = None,
        dependencies: Optional[Sequence[Depends]] = None,
        summary: Optional[str] = None,
        description: Optional[str] = None,
        response_description: str = "Successful Response",
        responses: Optional[Dict[Union[int, str], Dict[str, Any]]] = None,
        deprecated: Optional[bool] = None,

假设要触发的自定义异常叫作 `UnicornException`。

且需要 FastAPI 实现全局处理该异常。

此时，可以用 `@app.exception_handler()` 添加自定义异常控制器：

```Python hl_lines="5-7  13-18  24"
{!../../docs_src/handling_errors/tutorial003.py!}

```

请求 `/unicorns/yolo` 时，路径操作会触发 `UnicornException`。

但该异常将会被 `unicorn_exception_handler` 处理。

接收到的错误信息清晰明了，HTTP 状态码为 `418`，JSON 内容如下：

```JSON
{"message": "Oops! yolo did something. There goes a rainbow..."}

```