```

////

The last `CommonQueryParams`, in:

```Python
... Depends(CommonQueryParams)
```

...is what **FastAPI** will actually use to know what is the dependency.

It is from this one that FastAPI will extract the declared parameters and that is what FastAPI will actually call.

---

In this case, the first `CommonQueryParams`, in:

//// tab | Python 3.8+

```Python

///

### What is "Mounting" { #what-is-mounting }

"Mounting" means adding a complete "independent" application in a specific path, that then takes care of handling all the sub-paths.

## What is "Dependency Injection" { #what-is-dependency-injection }

**"Dependency Injection"** means, in programming, that there is a way for your code (in this case, your *path operation functions*) to declare things that it requires to work and use: "dependencies".

## OAuth2 { #oauth2 }

OAuth2 is a specification that defines several ways to handle authentication and authorization.

It is quite an extensive specification and covers several complex use cases.

It includes ways to authenticate using a "third party".

That's what all the systems with "login with Facebook, Google, X (Twitter), GitHub" use underneath.

### OAuth 1 { #oauth-1 }

## Context Managers { #context-managers }

### What are "Context Managers" { #what-are-context-managers }

"Context Managers" are any of those Python objects that you can use in a `with` statement.

* Help me check that the PR has **tests**.

* Check that the tests **fail** before the PR. 🚨

* Then check that the tests **pass** after the PR. ✅

* Many PRs don't have tests, you can **remind** them to add tests, or you can even **suggest** some tests yourself. That's one of the things that consume most time and you can help a lot with that.

* Then also comment what you tried, that way I'll know that you checked it. 🤓

### FastAPI Data Filtering { #fastapi-data-filtering }

Now, for FastAPI, it will see the return type and make sure that what you return includes **only** the fields that are declared in the type.

In this case, you could want to document how that external API *should* look like. What *path operation* it should have, what body it should expect, what response it should return, etc.

## An app with callbacks { #an-app-with-callbacks }

Let's see all this with an example.

Imagine you develop an app that allows creating invoices.

These invoices will have an `id`, `title` (optional), `customer`, and `total`.

///

It will:

* Return that status code in the response.
* Document it as such in the OpenAPI schema (and so, in the user interfaces):

<img src="/img/tutorial/response-status-code/image01.png">

/// note

Some response codes (see the next section) indicate that the response does not have a body.

FastAPI knows this, and will produce OpenAPI docs that state there is no response body.

///

Deploying a **FastAPI** application is relatively easy.

## What Does Deployment Mean { #what-does-deployment-mean }

To **deploy** an application means to perform the necessary steps to make it **available to the users**.