Another big feature needed by APIs is data validation, making sure that the data is valid, given certain parameters. For example, that some field is an `int`, and not some random string. This is especially useful for incoming data.

* <a href="https://github.com/Kludex/python-multipart" target="_blank"><code>python-multipart</code></a> - Richiesto se vuoi supportare il <abbr title="convertire la stringa che proviene da una richiesta HTTP in dati Python">"parsing"</abbr> con `request.form()`.
* <a href="https://pythonhosted.org/itsdangerous/" target="_blank"><code>itsdangerous</code></a> - Richiesto per usare `SessionMiddleware`.

    [
        ("/items", None, 200, {"strange_header": None}),
        ("/items", {"X-Header": "notvalid"}, 200, {"strange_header": None}),
        (
            "/items",
            {"strange_header": "FastAPI test"},
            200,
            {"strange_header": "FastAPI test"},
        ),
        (
            "/items",
            {"strange-header": "Not really underscore"},
            200,

from typing import Annotated

from fastapi import FastAPI, Header

app = FastAPI()


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

    - **name**: each item must have a name
    - **description**: a long description
    - **price**: required
    - **tax**: if the item doesn't have tax, you can omit this
    - **tags**: a set of unique tag strings for this item
    """

    - **name**: each item must have a name
    - **description**: a long description
    - **price**: required
    - **tax**: if the item doesn't have tax, you can omit this
    - **tags**: a set of unique tag strings for this item
    \f
    :param item: User input.
    """

{!../../../docs_src/nosql_databases/tutorial001.py!}
```

### f-strings

If you are not familiar with the `f"userprofile::{username}"`, it is a Python "<a href="https://docs.python.org/3/glossary.html#term-f-string" class="external-link" target="_blank">f-string</a>".

Any variable that is put inside of `{}` in an f-string will be expanded / injected in the string.

### `dict` unpacking

# Pfad-Parameter

Sie können Pfad-„Parameter“ oder -„Variablen“ mit der gleichen Syntax deklarieren, welche in Python-<abbr title="Format-String – Formatierter String: Der String enthält Variablen, die mit geschweiften Klammern umschlossen sind. Solche Stellen werden durch den Wert der Variable ersetzt">Format-Strings</abbr> verwendet wird:

```Python hl_lines="6-7"
{!../../../docs_src/path_params/tutorial001.py!}
```

Allgemeine Validierungen und Metadaten:

* `alias`
* `title`
* `description`
* `deprecated`

Validierungen spezifisch für Strings:

* `min_length`
* `max_length`
* `pattern`

In diesen Beispielen haben Sie gesehen, wie Sie Validierungen für Strings hinzufügen.

In diesem Fall macht es Sinn, die Tags in einem `Enum` zu speichern.

**FastAPI** unterstützt diese genauso wie einfache Strings:

```Python hl_lines="1  8-10  13  18"
{!../../../docs_src/path_operation_configuration/tutorial002b.py!}
```

## Zusammenfassung und Beschreibung