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,

from fastapi import FastAPI, Query

app = FastAPI()


@app.get("/items/")
async def read_items(q: str | None = Query(default=None, max_length=50)):
    results = {"items": [{"item_id": "Foo"}, {"item_id": "Bar"}]}
    if q:
        results.update({"q": q})

from fastapi import FastAPI, Header

app = FastAPI()


@app.get("/items/")
async def read_items(user_agent: str | None = Header(default=None)):

The way to describe this in OpenAPI, is to mark that field as **required**, because it will always be there.

Because of that, the JSON Schema for a model can be different depending on if it's used for **input or output**:

* for **input** the `description` will **not be required**
* for **output** it will be **required** (and possibly `None`, or in JSON terms, `null`)

/// note

FastAPI will know that the value of `q` is not required because of the default value `= None`.

The `str | None` (Python 3.10+) or `Union` in `Union[str, None]` (Python 3.9+) is not used by FastAPI to determine that the value is not required, it will know it's not required because it has a default value of `= None`.

But adding the type annotations will allow your editor to give you better support and detect errors.

///

from typing import Annotated, Union

from fastapi import FastAPI, Query

app = FastAPI()


@app.get("/items/")
async def read_items(
    hidden_query: Annotated[Union[str, None], Query(include_in_schema=False)] = None,
):
    if hidden_query:
        return {"hidden_query": hidden_query}
    else:

from fastapi import FastAPI, Header

app = FastAPI()


@app.get("/items/")
async def read_items(x_token: list[str] | None = Header(default=None)):

from fastapi import FastAPI, Query

app = FastAPI()


@app.get("/items/")
async def read_items(q: list[str] | None = Query(default=None)):
    query_items = {"q": q}

    Args:
      app (ASGI application): ASGI application
      max_content_size (optional): the maximum content size allowed in bytes, None for no limit
    """

    def __init__(self, app: APIRouter, max_content_size: Optional[int] = None):
        self.app = app
        self.max_content_size = max_content_size

    def receive_wrapper(self, receive):
        received = 0

Paul Merlin <******@****.***> <******@****.***>
Peter Niederwieser <******@****.***> <pniederw@.(none)>
Peter Niederwieser <******@****.***> <pniederw@pniederw-PC.(none)>
Piotr Jagielski <******@****.***> <Piotr Jagielski>
Rene Groeschke <******@****.***> <Rene@DevWin7.(none)>
Rene Groeschke <******@****.***> <******@****.***>
Rene Groeschke <******@****.***> <******@****.***>