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.

Without a data validation system, you would have to do all the checks by hand, in code.

    ```console
    $ which pip

    some/directory/fastapi/env/bin/pip
    ```

    </div>

=== "Windows PowerShell"

    <div class="termy">

    ```console
    $ Get-Command pip

    some/directory/fastapi/env/bin/pip
    ```

    </div>

If it shows the `pip` binary at `env/bin/pip` then it worked. 🎉

But most importantly:

* It will **limit and filter** the output data to what is defined in the return type.
    * This is particularly important for **security**, we'll see more of that below.

## `response_model` Parameter

There are some cases where you need or want to return some data that is not exactly what the type declares.

    </div>

✅ ⚫️ 👷, ⚙️:

=== "💾, 🇸🇻, 🚪 🎉"

    <div class="termy">

    ```console
    $ which pip

    some/directory/fastapi/env/bin/pip
    ```

    </div>

=== "🚪 📋"

    <div class="termy">

    ```console
    $ Get-Command pip

    some/directory/fastapi/env/bin/pip
    ```

    </div>

🚥 ⚫️ 🎦 `pip` 💱 `env/bin/pip` ⤴️ ⚫️ 👷. 👶

```JSON hl_lines="4-8"
{
    "openapi": "3.1.0",
    // More stuff here
    "servers": [
        {
            "url": "/api/v1"
        }
    ],
    "paths": {
            // More stuff here
    }
}
```

"Operation" here refers to one of the HTTP "methods".

One of:

* `POST`
* `GET`
* `PUT`
* `DELETE`

...and the more exotic ones:

* `OPTIONS`
* `HEAD`
* `PATCH`
* `TRACE`

In the HTTP protocol, you can communicate to each path using one (or more) of these "methods".

---

When building APIs, you normally use these specific HTTP methods to perform a specific action.

    response = client.post(
        "/items/", json={"name": "Foo", "price": "50.5", "description": "Some Foo"}
    )
    assert response.status_code == 200
    assert response.json() == {
        "name": "Foo",
        "price": 50.5,
        "description": "Some Foo",
        "tax": None,
    }


@needs_py310
def test_post_with_str_float_description_tax(client: TestClient):

    response = client.post(
        "/items/", json={"name": "Foo", "price": "50.5", "description": "Some Foo"}
    )
    assert response.status_code == 200
    assert response.json() == {
        "name": "Foo",
        "price": 50.5,
        "description": "Some Foo",
        "tax": None,
    }


def test_post_with_str_float_description_tax(client: TestClient):
    response = client.post(

## Client and SDK Generators - Sponsor

There are also some **company-backed** Client and SDK generators based on OpenAPI (FastAPI), in some cases they can offer you **additional features** on top of high-quality generated SDKs/clients.

    collected using form data (instead of JSON) and that it should have the specific
    fields `username` and `password`.

    All the initialization parameters are extracted from the request.

    Read more about it in the
    [FastAPI docs for Simple OAuth2 with Password and Bearer](https://fastapi.tiangolo.com/tutorial/security/simple-oauth2/).

    ## Example

    ```python
    from typing import Annotated