But you can configure the **application server** to trust the *forwarded* headers sent by the **proxy**. If you are using FastAPI CLI, you can use the *CLI Option* `--forwarded-allow-ips` to tell it from which IPs it should trust those *forwarded* headers.

If you use any other tool to manage your installations, like `uv`, Poetry, Pipenv, or others, they all have a way that you can use to define specific versions for your packages.

## Available versions { #available-versions }

You can see the available versions (e.g. to check what is the current latest) in the [Release Notes](../release-notes.md){.internal-link target=_blank}.

`@lru_cache` is part of `functools` which is part of Python's standard library, you can read more about it in the <a href="https://docs.python.org/3/library/functools.html#functools.lru_cache" class="external-link" target="_blank">Python docs for `@lru_cache`</a>.

## Recap { #recap }

# Body - Fields { #body-fields }

The same way you can declare additional validation and metadata in *path operation function* parameters with `Query`, `Path` and `Body`, you can declare validation and metadata inside of Pydantic models using Pydantic's `Field`.

## Import `Field` { #import-field }

First, you have to import it:

{* ../../docs_src/body_fields/tutorial001_an_py310.py hl[4] *}


/// warning

# Templates { #templates }

You can use any template engine you want with **FastAPI**.

A common choice is Jinja2, the same one used by Flask and other tools.

There are utilities to configure it easily that you can use directly in your **FastAPI** application (provided by Starlette).

## Install dependencies { #install-dependencies }

As all these methods are `async` methods, you need to "await" them.

For example, inside of an `async` *path operation function* you can get the contents with:

```Python
contents = await myfile.read()
```

If you are inside of a normal `def` *path operation function*, you can access the `UploadFile.file` directly, for example:

```Python
contents = myfile.file.read()
```

/// note | `async` Technical Details

But you can return a `JSONResponse` directly from your *path operations*.

It might be useful, for example, to return custom headers or cookies.

## Return a `Response` { #return-a-response }

In fact, you can return any `Response` or any sub-class of it.

/// tip

`JSONResponse` itself is a sub-class of `Response`.

///

The `directory="static"` refers to the name of the directory that contains your static files.

The `name="static"` gives it a name that can be used internally by **FastAPI**.

All these parameters can be different than "`static`", adjust them with the needs and specific details of your own application.

## More info { #more-info }

# Body - Updates { #body-updates }

## Update replacing with `PUT` { #update-replacing-with-put }

To update an item you can use the <a href="https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods/PUT" class="external-link" target="_blank">HTTP `PUT`</a> operation.

You can use the `jsonable_encoder` to convert the input data to data that can be stored as JSON (e.g. with a NoSQL database). For example, converting `datetime` to `str`.

# Middleware { #middleware }

You can add middleware to **FastAPI** applications.

A "middleware" is a function that works with every **request** before it is processed by any specific *path operation*. And also with every **response** before returning it.

* It takes each **request** that comes to your application.
* It can then do something to that **request** or run any needed code.