The benefit of raising an exception over `return`ing a value will be more evident in the section about Dependencies and Security.

In this example, when the client requests an item by an ID that doesn't exist, raise an exception with a status code of `404`:

### Always `raise` in Dependencies with `yield` and `except`

If you catch an exception in a dependency with `yield`, unless you are raising another `HTTPException` or similar, you should re-raise the original exception.

    response = client.get("/query/int?query=42.5")
    assert response.status_code == 422
    assert response.json() == IsDict(
        {
            "detail": [
                {
                    "type": "int_parsing",
                    "loc": ["query", "query"],
                    "msg": "Input should be a valid integer, unable to parse string as an integer",
                    "input": "42.5",
                }
            ]

## Data validation

But if you go to the browser at <a href="http://127.0.0.1:8000/items/foo" class="external-link" target="_blank">http://127.0.0.1:8000/items/foo</a>, you will see a nice HTTP error of:

```JSON
{
  "detail": [
    {
      "type": "int_parsing",
      "loc": [
        "path",
        "item_id"
      ],

                },
                {
                    "type": "int_parsing",
                    "loc": ["query", "skip"],
                    "msg": "Input should be a valid integer, unable to parse string as an integer",
                    "input": "a",
                },
                {
                    "type": "int_parsing",
                    "loc": ["query", "limit"],

    Ceci vous permettra d'obtenir des fonctionnalités de l'éditeur dans votre fonction, telles
    que des vérifications d'erreur, de l'auto-complétion, etc.

## <abbr title="aussi appelé sérialisation, ou parfois parsing ou marshalling en anglais">Conversion</abbr> de données

```Python hl_lines="3"
{!../../../docs_src/behind_a_proxy/tutorial002.py!}
```

Passing the `root_path` to `FastAPI` would be the equivalent of passing the `--root-path` command line option to Uvicorn or Hypercorn.

### About `root_path`

Keep in mind that the server (Uvicorn) won't use that `root_path` for anything else than passing it to the app.

    assert response.status_code == 422, response.text
    assert response.json() == IsDict(
        {
            "detail": [
                {
                    "type": "int_parsing",
                    "loc": ["body", "size"],
                    "msg": "Input should be a valid integer, unable to parse string as an integer",
                    "input": "XL",
                }
            ],

* <abbr title="anche noto come: serializzazione, parsing, marshalling">Conversione</abbr> dei dati di input: da risorse esterne a dati e tipi di Python. È possibile leggere da:
    * JSON.
    * Path parameters.
    * Query parameters.
    * Cookies.
    * Headers.
    * Form.
    * File.

## 0.110.0

### Breaking Changes

* 🐛 Fix unhandled growing memory for internal server errors, refactor dependencies with `yield` and `except` to require raising again as in regular Python. PR [#11191](https://github.com/tiangolo/fastapi/pull/11191) by [@tiangolo](https://github.com/tiangolo).