# Without aliases


@app.get("/required-list-str")
async def read_required_list_str(p: Annotated[list[str], Header()]):
    return {"p": p}


class HeaderModelRequiredListStr(BaseModel):
    p: list[str]


@app.get("/model-required-list-str")
def read_model_required_list_str(p: Annotated[HeaderModelRequiredListStr, Header()]):
    return {"p": p.p}

from typing import Union

from fastapi import FastAPI
from pydantic import BaseModel, EmailStr

app = FastAPI()


class BaseUser(BaseModel):
    username: str
    email: EmailStr
    full_name: Union[str, None] = None


class UserIn(BaseUser):
    password: str


@app.post("/user/")
async def create_user(user: UserIn) -> BaseUser:

from fastapi import Cookie, FastAPI
from pydantic import BaseModel

app = FastAPI()


class Cookies(BaseModel):
    model_config = {"extra": "forbid"}

    session_id: str
    fatebook_tracker: str | None = None
    googall_tracker: str | None = None


@app.get("/items/")
async def read_items(cookies: Annotated[Cookies, Cookie()]):

from fastapi import Cookie, FastAPI
from pydantic import BaseModel

app = FastAPI()


class Cookies(BaseModel):
    model_config = {"extra": "forbid"}

    session_id: str
    fatebook_tracker: Union[str, None] = None
    googall_tracker: Union[str, None] = None


@app.get("/items/")
async def read_items(cookies: Annotated[Cookies, Cookie()]):

class ContributorsResults(BaseModel):
    contributors: Counter[str]
    translation_reviewers: Counter[str]
    translators: Counter[str]
    authors: dict[str, Author]


def get_contributors(pr_nodes: list[PullRequestNode]) -> ContributorsResults:
    contributors = Counter[str]()
    translation_reviewers = Counter[str]()
    translators = Counter[str]()
    authors: dict[str, Author] = {}

    for pr in pr_nodes:

```Python hl_lines="1  4"
{!../../docs_src/python_types/tutorial009_py39.py!}
```

Using `Optional[str]` instead of just `str` will let the editor help you detect errors where you could be assuming that a value is always a `str`, when it could actually be `None` too.

`Optional[Something]` is actually a shortcut for `Union[Something, None]`, they are equivalent.

This also means that in Python 3.10, you can use `Something | None`:

    "path",
    ["/optional-list-str", "/model-optional-list-str"],
)
def test_optional_list_str_missing_empty_dict(path: str):
    client = TestClient(app)
    response = client.post(path, json={})
    assert response.status_code == 200, response.text
    assert response.json() == {"p": None}


@pytest.mark.parametrize(
    "path",
    ["/optional-list-str", "/model-optional-list-str"],
)

from fastapi import FastAPI
from pydantic import BaseModel

app = FastAPI()


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


@app.post("/items/")
async def create_item(item: Item) -> Item:
    return item


@app.get("/items/")
async def read_items() -> list[Item]:
    return [

app = FastAPI()


@app.get("/required-str/{p}")
async def read_required_str(p: Annotated[str, Path()]):
    return {"p": p}


@app.get("/required-alias/{p_alias}")
async def read_required_alias(p: Annotated[str, Path(alias="p_alias")]):
    return {"p": p}


@app.get("/required-validation-alias/{p_val_alias}")
def read_required_validation_alias(
    p: Annotated[str, Path(validation_alias="p_val_alias")],
):

app = FastAPI()


class RecursiveItem(BaseModel):
    sub_items: list["RecursiveItem"] = []
    name: str


class RecursiveSubitemInSubmodel(BaseModel):
    sub_items2: list["RecursiveItemViaSubmodel"] = []
    name: str


class RecursiveItemViaSubmodel(BaseModel):
    sub_items1: list[RecursiveSubitemInSubmodel] = []
    name: str


RecursiveItem.model_rebuild()
RecursiveSubitemInSubmodel.model_rebuild()