```

=== "Python 3.10+ non-Annotated"

    !!! tip
        Prefer to use the `Annotated` version if possible.

    ```Python hl_lines="2"
    {!> ../../../docs_src/body_fields/tutorial001_py310.py!}
    ```

=== "Python 3.8+ non-Annotated"

    !!! tip
        Prefer to use the `Annotated` version if possible.

    ```Python hl_lines="4"
    {!> ../../../docs_src/body_fields/tutorial001.py!}
    ```

This also means that if something changed it will be **reflected** on the client code automatically. And if you **build** the client it will error out if you have any **mismatch** in the data used.

* It can do something to that **response** or run any needed code.
* Then it returns the **response**.

!!! note "Technical Details"
    If you have dependencies with `yield`, the exit code will run *after* the middleware.

    If there were any background tasks (documented later), they will run *after* all the middleware.

## Create a middleware

The first thing you should do is to "pin" the version of **FastAPI** you are using to the specific latest version that you know works correctly for your application.

For example, let's say you are using version `0.45.0` in your app.

If you use a `requirements.txt` file you could specify the version with:

```txt
fastapi==0.45.0
```

that would mean that you would use exactly the version `0.45.0`.

Or you could also pin it with:

```txt

    ```

=== "Python 3.10+ non-Annotated"

    !!! tip
        Prefer to use the `Annotated` version if possible.

    ```Python
    {!> ../../../docs_src/app_testing/app_b_py310/main.py!}
    ```

=== "Python 3.8+ non-Annotated"

    !!! tip
        Prefer to use the `Annotated` version if possible.

    ```Python
    {!> ../../../docs_src/app_testing/app_b/main.py!}
    ```

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

The value of the path parameter `item_id` will be passed to your function as the argument `item_id`.

So, if you run this example and go to <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 response of:

```JSON
{"item_id":"foo"}
```

!!! info
    Keep in mind that dataclasses can't do everything Pydantic models can do.

    So, you might still need to use Pydantic models.

    But if you have a bunch of dataclasses laying around, this is a nice trick to use them to power a web API using FastAPI. 🤓

## Dataclasses in `response_model`

You can also use `dataclasses` in the `response_model` parameter:

    ```

=== "Python 3.10+ non-Annotated"

    !!! tip
        Prefer to use the `Annotated` version if possible.

    ```Python hl_lines="9-12"
    {!> ../../../docs_src/body_fields/tutorial001_py310.py!}
    ```

=== "Python 3.8+ non-Annotated"

    !!! tip
        Prefer to use the `Annotated` version if possible.

    ```Python hl_lines="11-14"
    {!> ../../../docs_src/body_fields/tutorial001.py!}
    ```

# External Links and Articles

**FastAPI** has a great community constantly growing.

There are many posts, articles, tools, and projects, related to **FastAPI**.

Here's an incomplete list of some of them.

!!! tip

    <div class="termy">

    ```console
    $ pip install hypercorn

    ---> 100%
    ```

    </div>

    ...or any other ASGI server.

## Run the Server Program

If you installed an ASGI server manually, you would normally need to pass an import string in a special format for it to import your FastAPI application:

=== "Uvicorn"

    <div class="termy">

    ```console