* Processing data:
    * For example, let's say you receive a file that must go through a slow process, you can return a response of "Accepted" (HTTP 202) and process the file in the background.

## Using `BackgroundTasks` { #using-backgroundtasks }

When you need `lifespan` to run in your tests, you can use the `TestClient` with a `with` statement:

{* ../../docs_src/app_testing/tutorial004_py310.py hl[9:15,18,27:28,30:32,41:43] *}


You can read more details about the ["Running lifespan in tests in the official Starlette documentation site."](https://www.starlette.dev/lifespan/#running-lifespan-in-tests)

For the deprecated `startup` and `shutdown` events, you can use the `TestClient` as follows:

Your API now has the power to control its own <dfn title="This is a joke, just in case. It has nothing to do with cookie consents, but it's funny that even the API can now reject the poor cookies. Have a cookie. 🍪">cookie consent</dfn>. 🤪🍪

You can use Pydantic's model configuration to `forbid` any `extra` fields:

{* ../../docs_src/cookie_param_models/tutorial002_an_py310.py hl[10] *}

### Modify the OpenAPI schema { #modify-the-openapi-schema }

Now you can add the ReDoc extension, adding a custom `x-logo` to the `info` "object" in the OpenAPI schema:

{* ../../docs_src/extending_openapi/tutorial001_py310.py hl[22:24] *}

### Cache the OpenAPI schema { #cache-the-openapi-schema }

You can use the property `.openapi_schema` as a "cache", to store your generated schema.

    /**
     * Gets the decrypted proxies.
     *
     * @return The decrypted proxy, can be empty but never {@code null}.
     */
    List<Proxy> getProxies();

    /**
     * Gets the problems that were encountered during the settings decryption.
     *
     * @return The problems that were encountered during the decryption, can be empty but never {@code null}.
     */
    List<SettingsProblem> getProblems();

# Background Tasks - `BackgroundTasks`

You can declare a parameter in a *path operation function* or dependency function with the type `BackgroundTasks`, and then you can use it to schedule the execution of background tasks after the response is sent.

You can import it directly from `fastapi`:

```python
from fastapi import BackgroundTasks
```

// Copyright 2015 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

/*
Asm, typically invoked as “go tool asm”, assembles the source file into an object
file named for the basename of the argument source file with a .o suffix. The
object file can then be combined with other objects into a package archive.

# Command Line

Usage:

	go tool asm [flags] file

# 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 }

Make sure you create a [virtual environment](../virtual-environments.md), activate it, and install `jinja2`:

memory or some of the containers will fail to start. You can tell that you
are short of memory if containers are exiting quickly after starting with
code 137 (128 + 9, where 9 means SIGKILL).

== Testing the REST layer

The REST layer is tested through specific tests that are executed against
a cluster that is configured and initialized via Gradle. The tests
themselves can be written in either Java or with a YAML based DSL.

# 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.