</div>

This means that it will **always have a value**, it's just that sometimes the value could be `None` (or `null` in JSON).

That means that, clients using your API don't have to check if the value exists or not, they can **assume the field will always be there**, but just that in some cases it will have the default value of `None`.

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

    "path,expected_status,expected_response",
    [
        ("/items", 200, {"q": None, "skip": 0, "limit": 100}),
        ("/items?q=foo", 200, {"q": "foo", "skip": 0, "limit": 100}),
        ("/items?q=foo&skip=5", 200, {"q": "foo", "skip": 5, "limit": 100}),
        ("/items?q=foo&skip=5&limit=30", 200, {"q": "foo", "skip": 5, "limit": 30}),
        ("/users", 200, {"q": None, "skip": 0, "limit": 100}),
    ],
)

from typing import Union

from fastapi import APIRouter, FastAPI
from pydantic import BaseModel, HttpUrl

app = FastAPI()


class Invoice(BaseModel):
    id: str
    title: Union[str, None] = None
    customer: str
    total: float


class InvoiceEvent(BaseModel):
    description: str
    paid: bool


class InvoiceEventReceived(BaseModel):
    ok: bool


invoices_callback_router = APIRouter()

    assert response.json() == {
        "detail": [
            {
                "type": "missing",
                "loc": ["cookie", "p"],
                "msg": "Field required",
                "input": IsOneOf(None, {}),
            }
        ]
    }


@pytest.mark.parametrize(
    "path",
    ["/required-str", "/model-required-str"],
)
def test_required_str(path: str):
    client = TestClient(app)

def get_client():
    app = FastAPI()
    with pytest.warns(FastAPIDeprecationWarning):

        @app.get("/items/")
        async def read_items(
            q: Annotated[str | None, Query(regex="^fixedquery$")] = None,
        ):
            if q:
                return f"Hello {q}"
            else:
                return "Hello World"

    client = TestClient(app)
    return client


@needs_py310

class DummyClient:
    async def get_people(self) -> list:
        return ["John Doe", "Jane Doe"]

    async def close(self) -> None:
        pass


async def get_client() -> AsyncGenerator[DummyClient, None]:
    client = DummyClient()
    yield client
    await client.close()


Client = Annotated[DummyClient, Depends(get_client)]


@pytest.fixture(name="client")

class Session:
    def __init__(self) -> None:
        self.data = ["foo", "bar", "baz"]
        self.open = True

    def __iter__(self) -> Generator[str, None, None]:
        for item in self.data:
            if self.open:
                yield item
            else:
                raise ValueError("Session closed")


@contextmanager
def acquire_session() -> Generator[Session, None, None]:
    session = Session()
    try:

    assert response.json() == {
        "detail": [
            {
                "type": "missing",
                "loc": ["body", "p"],
                "msg": "Field required",
                "input": None,
            }
        ]
    }


@pytest.mark.parametrize(
    "path",
    [
        "/required-bytes",
        "/required-uploadfile",
    ],
)
def test_required(path: str):

    * Вы можете задать его URL с помощью параметра `docs_url`.
    * Вы можете отключить это с помощью настройки `docs_url=None`.
* **ReDoc**: отображаемый по адресу `/redoc`.
    * Вы можете задать его URL с помощью параметра `redoc_url`.
    * Вы можете отключить это с помощью настройки `redoc_url=None`.

К примеру, чтобы задать отображение Swagger UI по адресу `/documentation` и отключить ReDoc:

                "loc": ["body", "grant_type"],
                "msg": "Field required",
                "input": None,
            },
            {
                "type": "missing",
                "loc": ["body", "username"],
                "msg": "Field required",
                "input": None,
            },
            {
                "type": "missing",
                "loc": ["body", "password"],