"""
            ),
        ],
    ) -> None:
        """
        Move to a position in the file.

        Any next read or write will be done from that position.

        To be awaitable, compatible with async, this is run in threadpool.
        """
        return await super().seek(offset)

    async def close(self) -> None:
        """
        Close the file.

app = FastAPI()


@app.post("/files/")
async def create_file(file: bytes | None = File(default=None)):
    if not file:
        return {"message": "No file sent"}
    else:
        return {"file_size": len(file)}


@app.post("/uploadfile/")
async def create_upload_file(file: UploadFile | None = None):
    if not file:
        return {"message": "No upload file sent"}
    else:

from typing import Union

from fastapi import FastAPI
from pydantic import BaseModel


class Item(BaseModel):
    name: str
    description: Union[str, None] = None
    price: float
    tax: Union[float, None] = None


app = FastAPI()


@app.post("/items/")
async def create_item(item: Item):
    item_dict = item.dict()
    if item.tax:
        price_with_tax = item.price + item.tax

    summary: Optional[str] = None,
    description: Optional[str] = None,
    routes: Sequence[BaseRoute],
    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,

* `limit=10`：使用默认值

## 可选参数

同理，把默认值设为 `None` 即可声明**可选的**查询参数：

//// tab | Python 3.10+

```Python hl_lines="7"
{!> ../../docs_src/query_params/tutorial002_py310.py!}
```

////

//// tab | Python 3.8+

```Python hl_lines="9"
{!> ../../docs_src/query_params/tutorial002.py!}
```

////

本例中，查询参数 `q` 是可选的，默认值为 `None`。

/// check | "检查"

					/></label>
				<div class="col-lg-5 col-md-8 col-sm-7 col-xs-6">
					<input class="form-control" type="text" id="as_q" name="as.q" value="${f:h(fe:join(as.q))}">
				</div>
				<div class="col-lg-4 d-none d-lg-flex align-items-center">
				</div>
			</div>
			<div class="form-group row">
				<label for="as_epq" class="col-lg-3 col-md-4 col-sm-5 col-12 col-form-label"><la:message

        "name": "Baz",
        "description": None,
        "price": 50.2,
        "tax": 10.5,
        "tags": [],
    }


@needs_py39
def test_put(client: TestClient):
    response = client.put(
        "/items/bar", json={"name": "Barz", "price": 3, "description": None}
    )
    assert response.json() == {
        "name": "Barz",
        "description": None,
        "price": 3,
        "tax": 10.5,

from contextvars import ContextVar

import peewee

DATABASE_NAME = "test.db"
db_state_default = {"closed": None, "conn": None, "ctx": None, "transactions": None}
db_state = ContextVar("db_state", default=db_state_default.copy())


class PeeweeConnectionState(peewee._ConnectionState):
    def __init__(self, **kwargs):
        super().__setattr__("_state", db_state)
        super().__init__(**kwargs)

    def __setattr__(self, name, value):

app = FastAPI()


@app.post("/files/")
async def create_file(file: Annotated[Union[bytes, None], File()] = None):
    if not file:
        return {"message": "No file sent"}
    else:
        return {"file_size": len(file)}


@app.post("/uploadfile/")
async def create_upload_file(file: Union[UploadFile, None] = None):
    if not file:
        return {"message": "No upload file sent"}
    else:

app = FastAPI()


@app.post("/files/")
async def create_file(file: Annotated[Union[bytes, None], File()] = None):
    if not file:
        return {"message": "No file sent"}
    else:
        return {"file_size": len(file)}


@app.post("/uploadfile/")
async def create_upload_file(file: Union[UploadFile, None] = None):
    if not file:
        return {"message": "No upload file sent"}
    else: