### Path operation functions

When you declare a *path operation function* with normal `def` instead of `async def`, it is run in an external threadpool that is then awaited, instead of being called directly (as it would block the server).

# OpenAPI Webhooks

There are cases where you want to tell your API **users** that your app could call *their* app (sending a request) with some data, normally to **notify** of some type of **event**.

This means that instead of the normal process of your users sending requests to your API, it's **your API** (or your app) that could **send requests to their system** (to their API, their app).

This is normally called a **webhook**.

## Webhooks steps

# Form Data

When you need to receive form fields instead of JSON, you can use `Form`.

!!! info
    To use forms, first install <a href="https://github.com/Kludex/python-multipart" class="external-link" target="_blank">`python-multipart`</a>.

    E.g. `pip install python-multipart`.

## Import `Form`

Import `Form` from `fastapi`:

=== "Python 3.9+"

    ```Python hl_lines="3"

```Python hl_lines="7  21  23"
{!../../../docs_src/custom_response/tutorial004.py!}
```

In this example, the function `generate_html_response()` already generates and returns a `Response` instead of returning the HTML in a `str`.

By returning the result of calling `generate_html_response()`, you are already returning a `Response` that will override the default **FastAPI** behavior.

In summary, to apply partial updates you would:

* (Optionally) use `PATCH` instead of `PUT`.
* Retrieve the stored data.
* Put that data in a Pydantic model.
* Generate a `dict` without default values from the input model (using `exclude_unset`).
    * This way you can update only the values actually set by the user, instead of overriding values already stored with default values in your model.

                automatically cancel the request and send the client an error.

                If `auto_error` is set to `False`, when the query parameter is not
                available, instead of erroring out, the dependency result will be
                `None`.

                This is useful when you want to have optional authentication.

For example, as in the `Image` model we have a `url` field, we can declare it to be an instance of Pydantic's `HttpUrl` instead of a `str`:

=== "Python 3.10+"

    ```Python hl_lines="2  8"
    {!> ../../../docs_src/body_nested_models/tutorial005_py310.py!}
    ```

=== "Python 3.9+"

    ```Python hl_lines="4  10"

```Python hl_lines="10-19"
{!../../../docs_src/sql_databases_peewee/sql_app/database.py!}
```

This class inherits from a special internal class used by Peewee.

It has all the logic to make Peewee use `contextvars` instead of `threading.local`.

`contextvars` works a bit differently than `threading.local`. But the rest of Peewee's internal code assumes that this class works with `threading.local`.

The way HTML forms (`<form></form>`) sends the data to the server normally uses a "special" encoding for that data, it's different from JSON.

**FastAPI** will make sure to read that data from the right place instead of JSON.

!!! note "Technical Details"
    Data from forms is normally encoded using the "media type" `application/x-www-form-urlencoded` when it doesn't include files.

#### `lru_cache` Technical Details

`@lru_cache` modifies the function it decorates to return the same value that was returned the first time, instead of computing it again, executing the code of the function every time.