from typing import Union

from fastapi import FastAPI, Header

app = FastAPI()


@app.get("/items/")
async def read_items(user_agent: Union[str, None] = Header(default=None)):

from typing import Union

from fastapi import FastAPI, Header

app = FastAPI()


@app.get("/items/")
async def read_items(
    strange_header: Union[str, None] = Header(default=None, convert_underscores=False),
):

        def header_example(
            data: Union[str, None] = Header(
                default=None,
                example="header1",
            ),
        ):
            return data

    @app.get("/header_examples/")
    def header_examples(
        data: Union[str, None] = Header(
            default=None,
            examples=[
                "header1",
                "header2",

                "value": "query2",
            },
        },
    ),
):
    return data


@app.get("/header_examples/")
def header_examples(
    data: Union[str, None] = Header(
        default=None,
        examples=[
            "json_schema_header1",
            "json_schema_header2",
        ],
        openapi_examples={
            "Header One": {

        ),
        deprecated(
            """
            The `extra` kwargs is deprecated. Use `json_schema_extra` instead.
            """
        ),
    ],
) -> Any:
    return params.Header(
        default=default,
        default_factory=default_factory,
        alias=alias,
        alias_priority=alias_priority,
        validation_alias=validation_alias,
        serialization_alias=serialization_alias,

@app.get("/items/{item_id}")
def main(
    item_id: int = Path(default=..., gt=0),
    query: str = Query(default=..., max_length=10),
    session: str = Cookie(default=..., min_length=3),
    x_trace: str = Header(default=..., title="Tracing header"),
):
    return {"message": "Hello World"}
```

...all these parameters are required because the default value is `...` (Ellipsis).