But in this case, the same **FastAPI** application will handle the API and the authentication.

So, let's review it from that simplified point of view:

* The user types the `username` and `password` in the frontend, and hits `Enter`.

!!! check "vérifier"
    Donc, avec ces mêmes déclarations de type Python, **FastAPI** vous fournit de la validation de données.

    Notez que l'erreur mentionne le point exact où la validation n'a pas réussi.

    Ce qui est incroyablement utile au moment de développer et débugger du code qui interagit avec votre API.

## Documentation

You would probably get a cloud server (a virtual machine) or something similar, and it would have a <abbr title="That doesn't change">fixed</abbr> **public IP address**.

In the DNS server(s) you would configure a record (an "`A record`") to point **your domain** to the public **IP address of your server**.

You would probably do this just once, the first time, when setting everything up.

!!! tip

```Python hl_lines="2"
{!../../../docs_src/python_types/tutorial001.py!}
```

### Edit it

It's a very simple program.

But now imagine that you were writing it from scratch.

At some point you would have started the definition of the function, you had the parameters ready...

But then you have to call "that method that converts the first letter to upper case".

```Python hl_lines="3"
{!../../../docs_src/first_steps/tutorial001.py!}
```

Here the `app` variable will be an "instance" of the class `FastAPI`.

This will be the main point of interaction to create all your API.

### Step 3: create a *path operation*

#### Path

"Path" here refers to the last part of the URL starting from the first `/`.

So, in a URL like:

```

I have been avoiding the creation of a new framework for several years. First I tried to solve all the features covered by **FastAPI** using many different frameworks, plug-ins, and tools.

            },
            {
                "name": "Bar",
                "price": "35.4",
            },
            {
                "name": "Baz",
                "price": "thirty five point four",
            },
        ],
    ),
):
    results = {"item_id": item_id, "item": item}

                "invalid": {
                    "summary": "Invalid data is rejected with an error",
                    "value": {
                        "name": "Baz",
                        "price": "thirty five point four",
                    },
                },
            },
        ),
    ],
):
    results = {"item_id": item_id, "item": item}

                "invalid": {
                    "summary": "Invalid data is rejected with an error",
                    "value": {
                        "name": "Baz",
                        "price": "thirty five point four",
                    },
                },
            },
        ),
    ],
):
    results = {"item_id": item_id, "item": item}

!!! tip
    When writing the code to document a callback, it might be useful to imagine that you are that *external developer*. And that you are currently implementing the *external API*, not *your API*.

    Temporarily adopting this point of view (of the *external developer*) can help you feel like it's more obvious where to put the parameters, the Pydantic model for the body, for the response, etc. for that *external API*.

### Create a callback `APIRouter`