When a request contains invalid data, **FastAPI** internally raises a `RequestValidationError`.

And it also includes a default exception handler for it.

To override it, import the `RequestValidationError` and use it with `@app.exception_handler(RequestValidationError)` to decorate the exception handler.

The exception handler will receive a `Request` and the exception.

```Python hl_lines="2  14-16"

Sie können diese Logik beim *Hochfahren* und *Herunterfahren* mithilfe des `lifespan`-Parameters der `FastAPI`-App und eines „Kontextmanagers“ definieren (ich zeige Ihnen gleich, was das ist).

Beginnen wir mit einem Beispiel und sehen es uns dann im Detail an.

Wir erstellen eine asynchrone Funktion `lifespan()` mit `yield` wie folgt:

```Python hl_lines="16  19"
{!../../../docs_src/events/tutorial003.py!}
```

from fastapi.testclient import TestClient


# TODO: remove when deprecating Pydantic v1
def test_upload_file_invalid():
    with pytest.raises(ValueError):
        UploadFile.validate("not a Starlette UploadFile")


def test_upload_file_invalid_pydantic_v2():
    with pytest.raises(ValueError):
        UploadFile._validate("not a Starlette UploadFile", {})


def test_default_placeholder_equals():

    response = client.get("/items/plumbus")
    assert response.status_code == 200, response.text
    assert response.json() == "plumbus"


def test_fastapi_error(client: TestClient):
    with pytest.raises(FastAPIError) as exc_info:
        client.get("/items/portal-gun")
    assert "No response object was returned" in exc_info.value.args[0]


def test_internal_server_error():

    response = client.get("/items/plumbus")
    assert response.status_code == 200, response.text
    assert response.json() == "plumbus"


def test_fastapi_error(client: TestClient):
    with pytest.raises(FastAPIError) as exc_info:
        client.get("/items/portal-gun")
    assert "No response object was returned" in exc_info.value.args[0]


def test_internal_server_error():

    response = client.get("/items/plumbus")
    assert response.status_code == 200, response.text
    assert response.json() == "plumbus"


@needs_py39
def test_fastapi_error(client: TestClient):
    with pytest.raises(FastAPIError) as exc_info:
        client.get("/items/portal-gun")
    assert "No response object was returned" in exc_info.value.args[0]


@needs_py39
def test_internal_server_error():

    assert response.json() == "plumbus"


@needs_py39
def test_internal_error(client: TestClient):
    from docs_src.dependencies.tutorial008d_an_py39 import InternalError

    with pytest.raises(InternalError) as exc_info:
        client.get("/items/portal-gun")
    assert (
        exc_info.value.args[0] == "The portal gun is too dangerous to be owned by Rick"
    )


@needs_py39

=== "Python 3.10+"

    ```Python hl_lines="16  21"
    {!> ../../../docs_src/response_model/tutorial001_01_py310.py!}
    ```

=== "Python 3.9+"

    ```Python hl_lines="18  23"
    {!> ../../../docs_src/response_model/tutorial001_01_py39.py!}
    ```

=== "Python 3.8+"

    ```Python hl_lines="18  23"
    {!> ../../../docs_src/response_model/tutorial001_01.py!}
    ```

    def return_fast_uuid():
        asyncpg_uuid = MyUuid("a10ff360-3b1e-4984-a26f-d3ab460bdb51")
        assert isinstance(asyncpg_uuid, uuid.UUID)
        assert type(asyncpg_uuid) != uuid.UUID
        with pytest.raises(TypeError):
            vars(asyncpg_uuid)
        return {"fast_uuid": asyncpg_uuid}

    class SomeCustomClass(BaseModel):
        model_config = {"arbitrary_types_allowed": True}

        a_uuid: MyUuid

### Normales **FastAPI**

Schreiben Sie zunächst wie gewohnt Ihre ganze **FastAPI**-Anwendung:

```Python hl_lines="1  4  7-9"
{!../../../docs_src/extending_openapi/tutorial001.py!}
```

### Das OpenAPI-Schema generieren