/// danger

This is an "advanced" feature.

If you are just starting with **FastAPI** you might want to skip this section.

///

## Use cases { #use-cases }

Some use cases include:

* Converting non-JSON request bodies to JSON (e.g. <a href="https://msgpack.org/index.html" class="external-link" target="_blank">`msgpack`</a>).
* Decompressing gzip-compressed request bodies.

In this case, this `__call__` is what **FastAPI** will use to check for additional parameters and sub-dependencies, and this is what will be called to pass a value to the parameter in your *path operation function* later.

## Parameterize the instance { #parameterize-the-instance }

And now, we can use `__init__` to declare the parameters of the instance that we can use to "parameterize" the dependency:

/// tip

You could use any other SQL or NoSQL database library you want (in some cases called <abbr title="Object Relational Mapper: a fancy term for a library where some classes represent SQL tables and instances represent rows in those tables">"ORMs"</abbr>), FastAPI doesn't force you to use anything. 😎

///

Let's say that you want to use a different <abbr title="Content Delivery Network">CDN</abbr>, for example you want to use `https://unpkg.com/`.

This could be useful if for example you live in a country that restricts some URLs.

### Disable the automatic docs { #disable-the-automatic-docs }

The first step is to disable the automatic docs, as by default, those use the default CDN.

With it, you can use <a href="https://docs.pytest.org/" class="external-link" target="_blank">pytest</a> directly with **FastAPI**.

## Using `TestClient` { #using-testclient }

/// info

To use `TestClient`, first install <a href="https://www.python-httpx.org" class="external-link" target="_blank">`httpx`</a>.

* Never store plaintext passwords, only password hashes.
* Implement and use well-known cryptographic tools, like pwdlib and JWT tokens, etc.
* Add more granular permission controls with OAuth2 scopes where needed.
* ...etc.

/*
 * Copyright 2012-2025 CodeLibs Project and the Others.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,

So, for everything to work correctly, it's better to specify explicitly the allowed origins.

## Use `CORSMiddleware` { #use-corsmiddleware }

You can configure it in your **FastAPI** application using the `CORSMiddleware`.

* Import `CORSMiddleware`.
* Create a list of allowed origins (as strings).

### Target language

Translate to Ukrainian (українська).

Language code: uk.

### Grammar and tone

1) Use polite/formal address consistent with existing Ukrainian docs (use “ви/ваш”).
2) Keep the tone concise and technical.

### Headings

1) Follow existing Ukrainian heading style; keep headings short and instructional.
2) Do not add trailing punctuation to headings.

### Quotes

But there are specific cases where it's useful to get the `Request` object.

## Use the `Request` object directly { #use-the-request-object-directly }

Let's imagine you want to get the client's IP address/host inside of your *path operation function*.

For that you need to access the request directly.