## 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.

## What is a Container Image { #what-is-a-container-image }

A **container** is run from a **container image**.

////

/// tip

Every time you install a **new package** in that environment, **activate** the environment again.

This makes sure that if you use a **terminal (<abbr title="command line interface">CLI</abbr>) program** installed by that package, you use the one from your virtual environment and not any other that could be installed globally, probably with a different version than what you need.

///

     * extra state from AbstractFuture), our options are limited: A call to setException() would be
     * a no-op. We could log, but if that's what we really want, we should modify
     * AbstractFuture.executeListener to do so, since that method would have the ability to continue
     * to execute other listeners.
     *

     * extra state from AbstractFuture), our options are limited: A call to setException() would be
     * a no-op. We could log, but if that's what we really want, we should modify
     * AbstractFuture.executeListener to do so, since that method would have the ability to continue
     * to execute other listeners.
     *

We will talk a lot about the running "**process**", so it's useful to have clarity about what it means, and what's the difference with the word "**program**".

### What is a Program { #what-is-a-program }

The word **program** is commonly used to describe many things:

* The **code** that you write, the **Python files**.
* The **file** that can be **executed** by the operating system, for example: `python`, `python.exe` or `uvicorn`.

After this, the client and the server have an **encrypted TCP connection**, this is what TLS provides. And then they can use that connection to start the actual **HTTP communication**.

And that's what **HTTPS** is, it's just plain **HTTP** inside a **secure TLS connection** instead of a pure (unencrypted) TCP connection.

/// tip

Notice that the encryption of the communication happens at the **TCP level**, not at the HTTP level.

///

...
```

#### What is OpenAPI for { #what-is-openapi-for }

The OpenAPI schema is what powers the two interactive documentation systems included.

And there are dozens of alternatives, all based on OpenAPI. You could easily add any of those alternatives to your application built with **FastAPI**.