var_name,
                       query_item,
                       enabled_by_default,
                       question=None,
                       yes_reply=None,
                       no_reply=None,
                       bazel_config_name=None):
  """Set boolean action_env variable.

  Ask user if query_item will be enabled. Default is used if no input is given.

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()):

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():

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:

Şimdi bunu FastAPI ile kullanmanın zamanı. 🚀

Şu tip anotasyonuna sahiptik:

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

Şimdi bunu `Annotated` ile saracağız; şöyle olacak:

```Python
q: Annotated[str | None] = None
```

Bu iki sürüm de aynı anlama gelir: `q`, `str` veya `None` olabilen bir parametredir ve varsayılan olarak `None`’dır.

Şimdi işin eğlenceli kısmına geçelim. 🎉

class User(BaseModel):
    username: str


def get_current_user(oauth_header: str | None = Security(api_key)):
    if oauth_header is None:
        return None
    user = User(username=oauth_header)
    return user


@app.get("/users/me")
def read_current_user(current_user: User = Depends(get_current_user)):
    if current_user is None:
        return {"msg": "Create an account first"}
    else:
        return current_user

例如，你可以宣告某個值可以是 `str` 或 `None`：

```python
from typing import Union


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

在 `typing` 中也有用 `Optional` 宣告某個值可以是 `None` 的速記法。

以下是我個人（非常主觀）的建議：

* 🚨 避免使用 `Optional[SomeType]`
* 改為 ✨ 使用 `Union[SomeType, None]` ✨。

    username: str
    password: str
    email: EmailStr
    full_name: str | None = None


class UserOut(BaseModel):
    username: str
    email: EmailStr
    full_name: str | None = None


class UserInDB(BaseModel):
    username: str
    hashed_password: str
    email: EmailStr
    full_name: str | None = None


def fake_password_hasher(raw_password: str):
    return "supersecret" + raw_password