```JSON hl_lines="4-8"
{
    "openapi": "3.1.0",
    // More stuff here
    "servers": [
        {
            "url": "/api/v1"
        }
    ],
    "paths": {
            // More stuff here
    }
}
```

```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:

```

    In this case, it's a list of `Item` dataclasses.

6. Here we are returning a dictionary that contains `items` which is a list of dataclasses.

    FastAPI is still capable of <abbr title="converting the data to a format that can be transmitted">serializing</abbr> the data to JSON.

7. Here the `response_model` is using a type annotation of a list of `Author` dataclasses.

* It will **limit and filter** the output data to what is defined in the return type.
    * This is particularly important for **security**, we'll see more of that below.

## `response_model` Parameter

There are some cases where you need or want to return some data that is not exactly what the type declares.

The data is calculated each month, you can read the <a href="https://github.com/tiangolo/fastapi/blob/master/.github/actions/people/app/main.py" class="external-link" target="_blank">source code here</a>.

Here I'm also highlighting contributions from sponsors.

    Notice that all this code is pure SQLAlchemy Core.

    `databases` is not doing anything here yet.

## Import and set up `databases`

* Import `databases`.
* Create a `DATABASE_URL`.
* Create a `database` object.

```Python hl_lines="3  9  12"
{!../../../docs_src/async_sql_databases/tutorial001.py!}
```

!!! tip
    If you were connecting to a different database (e.g. PostgreSQL), you would need to change the `DATABASE_URL`.

You can customize that function. It takes an `APIRoute` and outputs a string.

For example, here it is using the first tag (you will probably have only one tag) and the *path operation* name (the function name).

You can then pass that custom function to **FastAPI** as the `generate_unique_id_function` parameter:

Of course, there are some cases where there's no problem in running the previous steps multiple times, in that case, it's a lot easier to handle.

In Python 3.10 there's also a **new syntax** where you can put the possible types separated by a <abbr title='also called "bitwise or operator", but that meaning is not relevant here'>vertical bar (`|`)</abbr>.

=== "Python 3.10+"

    ```Python hl_lines="1"
    {!> ../../../docs_src/python_types/tutorial008b_py310.py!}

This time, it will overwrite the method `APIRoute.get_route_handler()`.

This method returns a function. And that function is what will receive a request and return a response.

Here we use it to create a `GzipRequest` from the original request.

```Python hl_lines="18-26"
{!../../../docs_src/custom_request_and_route/tutorial001.py!}
```

!!! note "Technical Details"