/// 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.

///

## About HTTP status codes { #about-http-status-codes }

/// note

If you already know what HTTP status codes are, skip to the next section.

///

## FastAPI App with Tags { #fastapi-app-with-tags }

In many cases, your FastAPI app will be bigger, and you will probably use tags to separate different groups of *path operations*.

For example, you could have a section for **items** and another section for **users**, and they could be separated by tags:

{* ../../docs_src/generate_clients/tutorial002_py39.py hl[21,26,34] *}

There's a mini <a href="https://sqlmodel.tiangolo.com/tutorial/fastapi/" class="external-link" target="_blank">tutorial on using SQLModel with FastAPI</a>. ✨

If you have dependencies with `yield`, the exit code will run *after* the middleware.

If there were any background tasks (covered in the [Background Tasks](background-tasks.md){.internal-link target=_blank} section, you will see it later), they will run *after* all the middleware.

///

## Create a middleware { #create-a-middleware }

To create a middleware you use the decorator `@app.middleware("http")` on top of a function.

In that case, they will be applied to all the *path operations* in the application:

{* ../../docs_src/dependencies/tutorial012_an_py39.py hl[16] *}


And all the ideas in the section about [adding `dependencies` to the *path operation decorators*](dependencies-in-path-operation-decorators.md){.internal-link target=_blank} still apply, but in this case, to all of the *path operations* in the app.

<div class="termy">

```console
$ python myapp.py
```

</div>

then the internal variable `__name__` in your file, created automatically by Python, will have as value the string `"__main__"`.

So, the section:

```Python
    uvicorn.run(app, host="0.0.0.0", port=8000)
```

will run.

---

This won't happen if you import that module (file).

So, if you have another file `importer.py` with:

sub-application could also have its own mounted sub-applications and everything would work correctly, because FastAPI handles all these `root_path`s automatically.

You will learn more about the `root_path` and how to use it explicitly in the section about [Behind a Proxy](behind-a-proxy.md){.internal-link target=_blank}....

# Tutorial - User Guide { #tutorial-user-guide }

This tutorial shows you how to use **FastAPI** with most of its features, step by step.

Each section gradually builds on the previous ones, but it's structured to separate topics, so that you can go directly to any specific one to solve your specific API needs.

It is also built to work as a future reference so you can come back and see exactly what you need.

## Run the code { #run-the-code }

# Extending OpenAPI { #extending-openapi }

There are some cases where you might need to modify the generated OpenAPI schema.

In this section you will see how.

## The normal process { #the-normal-process }

The normal (default) process, is as follows.

A `FastAPI` application (instance) has an `.openapi()` method that is expected to return the OpenAPI schema.

applications **FastAPI** s'exécutant sous Uvicorn sont <a href="https://www.techempower.com/benchmarks/#section=test&runid=7464e520-0dc2-473d-bd34-dbdfd7e85911&hw=ph&test=query&l=zijzen-7" class="external-link" target="_blank"> parmi les frameworks existants en Python les plus rapides </a>, juste derrière Starlette et Uvicorn (utilisés en interne par FastAPI). (*)

Pour en savoir plus, consultez la section <a href="https://fastapi.tiangolo.com/fr/benchmarks/" class="internal-link" target="_blank">Benchmarks</a>....