This can be very useful for setting up **resources** that you need to use for the whole app, and that are **shared** among requests, and/or that you need to **clean up** afterwards. For example, a database connection pool, or loading a shared machine learning model.

## Use Case { #use-case }

{* ../../docs_src/query_params_str_validations/tutorial002_py310.py hl[7] *}

As in this case (without using `Annotated`) we have to replace the default value `None` in the function with `Query()`, we now need to set the default value with the parameter `Query(default=None)`, it serves the same purpose of defining that default value (at least for FastAPI).

So:

```Python
q: str | None = Query(default=None)
```

]
```

These are **JavaScript** objects, not strings, so you can't pass them from Python code directly.

# Response - Change Status Code { #response-change-status-code }

You probably read before that you can set a default [Response Status Code](../tutorial/response-status-code.md){.internal-link target=_blank}.

But in some cases you need to return a different status code than the default.

## Use case { #use-case }

For example, imagine that you want to return an HTTP status code of "OK" `200` by default.

            // Mock send method
            public void send(final Object postcard) {
                // Mock implementation
            }
        };

        // Mock Postbox - no need to mock, the framework will handle it

        // Register components
        ComponentUtil.register(searchEngineClient, "searchEngineClient");
        ComponentUtil.register(systemHelper, "systemHelper");

# JSON Compatible Encoder { #json-compatible-encoder }

There are some cases where you might need to convert a data type (like a Pydantic model) to something compatible with JSON (like a `dict`, `list`, etc).

For example, if you need to store it in a database.

For that, **FastAPI** provides a `jsonable_encoder()` function.

## Using the `jsonable_encoder` { #using-the-jsonable-encoder }

- Temporary credentials do not need to be stored with the application but are generated dynamically and provided to the application when requested. When (or even before) the temporary credentials expire, the application can request new credentials.

Following are advantages for using temporary credentials:

- Eliminates the need to embed long-term credentials with an application.

<abbr title="A utility/system to store computed/generated values, to reuse them instead of computing them again.">"cache"</abbr> and pass it to all the "dependants" that need it in that specific request, instead of calling the dependency multiple times for the same request.

In an advanced scenario where you know you need the dependency to be called at every step (possibly multiple times) in the same request instead of using the "cached" value, you can set the parameter `use_cache=False` when...

# 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.

You don't necessarily need OAuth2 scopes, and you can handle authentication and authorization however you want.

But OAuth2 with scopes can be nicely integrated into your API (with OpenAPI) and your API docs.

Nevertheless, you still enforce those scopes, or any other security/authorization requirement, however you need, in your code.

In many cases, OAuth2 with scopes can be an overkill.