Aynı hata <a href="http://127.0.0.1:8000/items/4.2" class="external-link" target="_blank">http://127.0.0.1:8000/items/4.2</a> sayfasında olduğu gibi `int` yerine `float` bir değer verseydik de ortaya çıkardı.

!!! check "Ek bilgi"
    Böylece, aynı Python tip tanımlaması ile birlikte, **FastAPI** veri doğrulama özelliği sağlar.

* **Kodlaması Hızlı**: Geliştirme hızını yaklaşık %200 ile %300 aralığında arttırır. *
* **Daha az hata**: İnsan (geliştirici) kaynaklı hataları yaklaşık %40 azaltır. *
* **Sezgisel**: Muhteşem bir editör desteği. Her yerde <abbr title="Otomatik Tamamlama: auto-complete, autocompletion, IntelliSense">otomatik tamamlama</abbr>. Hata ayıklama ile daha az zaman harcayacaksınız.

### Eşzamanlılık paralellikten daha mı iyi?

Hayır!  Hikayenin ahlakı bu değil.

    @app.post("/login")
    def login(form_data: Annotated[OAuth2PasswordRequestForm, Depends()]):
        data = {}
        data["scopes"] = []
        for scope in form_data.scopes:
            data["scopes"].append(scope)
        if form_data.client_id:
            data["client_id"] = form_data.client_id
        if form_data.client_secret:
            data["client_secret"] = form_data.client_secret
        return data
    ```

    ```

FastAPI will use this return type to:

* **Validate** the returned data.
    * If the data is invalid (e.g. you are missing a field), it means that *your* app code is broken, not returning what it should, and it will return a server error instead of returning incorrect data. This way you and your clients can be certain that they will receive the data and the data shape expected.
* Add a **JSON Schema** for the response, in the OpenAPI *path operation*.

One of the main features needed by API systems is data "<abbr title="also called marshalling, conversion">serialization</abbr>" which is taking data from the code (Python) and converting it into something that can be sent through the network. For example, converting an object containing data from a database into a JSON object. Converting `datetime` objects into strings, etc.

    data = {"username": username, "password": password}
    if scope:
        data["scope"] = scope
    response = client.post("/token", data=data)
    content = response.json()
    access_token = content.get("access_token")
    return access_token


def test_login():
    response = client.post("/token", data={"username": "johndoe", "password": "secret"})

#### Data "schema"

The term "schema" might also refer to the shape of some data, like a JSON content.

In that case, it would mean the JSON attributes, and data types they have, etc.

#### OpenAPI and JSON Schema

        "description": None,
        "price": 50.5,
        "tax": None,
    }


@needs_py310
def test_wrong_headers(client: TestClient):
    data = '{"name": "Foo", "price": 50.5}'
    response = client.post(
        "/items/", content=data, headers={"Content-Type": "text/plain"}
    )
    assert response.status_code == 422, response.text
    assert response.json() == IsDict(
        {

        "name": "Foo",
        "description": None,
        "price": 50.5,
        "tax": None,
    }


def test_wrong_headers(client: TestClient):
    data = '{"name": "Foo", "price": 50.5}'
    response = client.post(
        "/items/", content=data, headers={"Content-Type": "text/plain"}
    )
    assert response.status_code == 422, response.text
    assert response.json() == IsDict(
        {