```

由于我们必须用 `Query(default=None)` 替换默认值 `None`，`Query` 的第一个参数同样也是用于定义默认值。

所以：

```Python
q: Union[str, None] = Query(default=None)
```

...使得参数可选，等同于：

```Python
q: str = None
```

但是 `Query` 显式地将其声明为查询参数。

然后，我们可以将更多的参数传递给 `Query`。在本例中，适用于字符串的 `max_length` 参数：

```Python
q: Union[str, None] = Query(default=None, max_length=50)
```

将会校验数据，在数据无效时展示清晰的错误信息，并在 OpenAPI 模式的*路径操作*中记录该参​​数。

from ._compat import PYDANTIC_V2, UndefinedType, Url, _model_dump


# Taken from Pydantic v1 as is
def isoformat(o: Union[datetime.date, datetime.time]) -> str:
    return o.isoformat()


# Taken from Pydantic v1 as is
# TODO: pv2 should this return strings instead?
def decimal_encoder(dec_value: Decimal) -> Union[int, float]:
    """
    Encodes a Decimal as int of there's no exponent, otherwise float

pkg go/types, method (*Tuple) Variables() iter.Seq[*Var] #66626
pkg go/types, method (*TypeList) Types() iter.Seq[Type] #66626
pkg go/types, method (*TypeParamList) TypeParams() iter.Seq[*TypeParam] #66626

</div>

## 예제

### 만들기

* `main.py` 파일을 만드십시오:

```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">

    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]:

* `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

## 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">

            _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();
                }