```python
from typing import Union


def say_hi(name: Union[str, None]):
        print(f"Hi {name}!")
```

`typing` also has a shortcut to declare that something could be `None`, with `Optional`.

Here's a tip from my very **subjective** point of view:

* 🚨 Avoid using `Optional[SomeType]`
* Instead ✨ **use `Union[SomeType, None]`** ✨.

from fastapi import FastAPI
from pydantic import BaseModel


class Item(BaseModel):
    name: str
    description: str | None = None
    price: float
    tax: float | None = None


app = FastAPI()


@app.put("/items/{item_id}")
async def update_item(item_id: int, item: Item, q: str | None = None):
    result = {"item_id": item_id, **item.model_dump()}
    if q:
        result.update({"q": q})

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 <******@****.***> <******@****.***>

app = FastAPI()


class Image(BaseModel):
    url: HttpUrl
    name: str


class Item(BaseModel):
    name: str
    description: str | None = None
    price: float
    tax: float | None = None
    tags: set[str] = set()
    image: Image | None = None


@app.put("/items/{item_id}")
async def update_item(item_id: int, item: Item):
    results = {"item_id": item_id, "item": item}

from fastapi import Body, FastAPI
from pydantic import BaseModel

app = FastAPI()


class Item(BaseModel):
    name: str
    description: str | None = None
    price: float
    tax: float | None = None


class User(BaseModel):
    username: str
    full_name: str | None = None


@app.put("/items/{item_id}")
async def update_item(item_id: int, item: Item, user: User, importance: int = Body()):

        return str(obj)
    if isinstance(obj, (str, int, float, type(None))):
        return obj
    if isinstance(obj, PydanticUndefinedType):
        return None
    if isinstance(obj, dict):
        encoded_dict = {}
        allowed_keys = set(obj.keys())
        if include is not None:
            allowed_keys &= set(include)
        if exclude is not None:
            allowed_keys -= set(exclude)

この場合（`Annotated` を使わない場合）、関数内のデフォルト値 `None` を `Query()` に置き換える必要があるため、`Query(default=None)` のパラメータでデフォルト値を設定する必要があります。これは（少なくとも FastAPI にとっては）そのデフォルト値を定義するのと同じ目的を果たします。

なので:

```Python
q: str | None = Query(default=None)
```

...はデフォルト値 `None` を持つオプショナルなパラメータになり、以下と同じです:


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

ただし `Query` のバージョンでは、クエリパラメータであることを明示的に宣言しています。

def get_lang_paths() -> list[Path]:
    return sorted(docs_path.iterdir())


def lang_callback(lang: str | None) -> str | None:
    if lang is None:
        return None
    lang = lang.lower()
    return lang


def complete_existing_lang(incomplete: str):
    lang_path: Path
    for lang_path in get_lang_paths():

    return f"{client_id}_key"


def _get_client_tag(client_id: str | None = Query(None)) -> str | None:
    if client_id is None:
        return None
    return f"{client_id}_tag"


@app.get("/foo")
def foo_handler(
    client_key: str = Depends(_get_client_key),
    client_tag: str | None = Depends(_get_client_tag),
):
    return {"client_id": client_key, "client_tag": client_tag}

class CallableGenDependency:
    def __call__(self, value: str) -> Generator[str, None, None]:
        yield value


class AsyncCallableDependency:
    async def __call__(self, value: str) -> str:
        return value


class AsyncCallableGenDependency:
    async def __call__(self, value: str) -> AsyncGenerator[str, None]:
        yield value


class MethodsDependency: