# Handling Errors

There are many situations in which you need to notify an error to a client that is using your API.

This client could be a browser with a frontend, a code from someone else, an IoT device, etc.

You could need to tell the client that:

* The client doesn't have enough privileges for that operation.
* The client doesn't have access to that resource.
* The item the client was trying to access doesn't exist.
* etc.

For that, you can use the `WSGIMiddleware` and use it to wrap your WSGI application, for example, Flask, Django, etc.

## Using `WSGIMiddleware`

You need to import `WSGIMiddleware`.

Then wrap the WSGI (e.g. Flask) app with the middleware.

And then mount that under a path.

```Python hl_lines="2-3  23"
{!../../../docs_src/wsgi/tutorial001.py!}
```

It is also built to work as a future reference.

So you can come back and see exactly what you need.

## Run the code

All the code blocks can be copied and used directly (they are actually tested Python files).

To run any of the examples, copy the code to a file `main.py`, and start `fastapi dev` with:

# Security Tools

When you need to declare dependencies with OAuth2 scopes you use `Security()`.

But you still need to define what is the dependable, the callable that you pass as a parameter to `Depends()` or `Security()`.

There are multiple tools that you can use to create those dependables, and they get integrated into OpenAPI so they are shown in the automatic docs UI, they can be used by automatically generated clients and SDKs, etc.

# 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"

# Request Body

When you need to send data from a client (let's say, a browser) to your API, you send it as a **request body**.

A **request** body is data sent by the client to your API. A **response** body is the data your API sends to the client.

Your API almost always has to send a **response** body. But clients don't necessarily need to send **request** bodies all the time.

!!! note
    Notice that the input model is still validated.

    So, if you want to receive partial updates that can omit all the attributes, you need to have a model with all the attributes marked as optional (with default values or `None`).

## Fortgeschrittene Beschreibung mittels Docstring

Sie können die verwendeten Zeilen aus dem Docstring einer *Pfadoperation-Funktion* einschränken, die für OpenAPI verwendet werden.

Das Hinzufügen eines `\f` (ein maskiertes „Form Feed“-Zeichen) führt dazu, dass **FastAPI** die für OpenAPI verwendete Ausgabe an dieser Stelle abschneidet.

Sie wird nicht in der Dokumentation angezeigt, aber andere Tools (z. B. Sphinx) können den Rest verwenden.

From the list of deployment concepts from above, using workers would mainly help with the **replication** part, and a little bit with the **restarts**, but you still need to take care of the others:

* **Security - HTTPS**
* **Running on startup**
* ***Restarts***
* Replication (the number of processes running)
* **Memory**
* **Previous steps before starting**