Each instance of the `SessionLocal` class will be a database session. The class itself is not a database session yet.

But once we create an instance of the `SessionLocal` class, this instance will be the actual database session.

We name it `SessionLocal` to distinguish it from the `Session` we are importing from SQLAlchemy.

We will use `Session` (the one imported from SQLAlchemy) later.

    If not provided, it's `None` by default, here we provide `"World"` as the default value to use.

Then you could call that Python program:

<div class="termy">

```console
// Here we don't set the env var yet
$ python main.py

// As we didn't set the env var, we get the default value

Hello World from Python

// But if we create an environment variable first
$ export MY_NAME="Wade Wilson"

In our code example above, we don't use it directly, but we pass it to FastAPI for it to use it.

The `lifespan` parameter of the `FastAPI` app takes an **async context manager**, so we can pass our new `lifespan` async context manager to it.

```Python hl_lines="22"
{!../../../docs_src/events/tutorial003.py!}
```

    ```Python hl_lines="11"
    {!> ../../../docs_src/dependencies/tutorial001.py!}
    ```

But then we get a `dict` in the parameter `commons` of the *path operation function*.

And we know that editors can't provide a lot of support (like completion) for `dict`s, because they can't know their keys and value types.

We can do better...

## What makes a dependency

Up to now you have seen dependencies declared as functions.

We are going to include this `APIRouter` in the main `FastAPI` app, but first, let's check the dependencies and another `APIRouter`.

## Dependencies

We see that we are going to need some dependencies used in several places of the application.

So we put them in their own `dependencies` module (`app/dependencies.py`).

```

## Create Pydantic models

As **Couchbase** "documents" are actually just "JSON objects", we can model them with Pydantic.

### `User` model

First, let's create a `User` model:

```Python hl_lines="24-28"
{!../../../docs_src/nosql_databases/tutorial001.py!}
```

We will use this model in our *path operation function*, so, we don't include in it the `hashed_password`.

### `UserInDB` model

We can use **OAuth2** to build that with **FastAPI**.

But let's save you the time of reading the full long specification just to find those little pieces of information you need.

Let's use the tools provided by **FastAPI** to handle security.

## How it looks

Let's first just use the code and see how it works, and then we'll come back to understand what's happening.

We can also use this same approach to access the request body in an exception handler.

All we need to do is handle the request inside a `try`/`except` block:

```Python hl_lines="13  15"
{!../../../docs_src/custom_request_and_route/tutorial002.py!}
```

If an exception occurs, the`Request` instance will still be in scope, so we can read and make use of the request body when handling the error:

    "completed": False,
}
```

but it doesn't have the `id` field.

So we create a new `dict`, that contains the key-value pairs from `note.dict()` with:

```Python
{**note.dict()}
```

`**note.dict()` "unpacks" the key value pairs directly, so, `{**note.dict()}` would be, more or less, a copy of `note.dict()`.

And then, we extend that copy `dict`, adding another key-value pair: `"id": last_record_id`:

```Python

### We Make Mistakes

We, as humans, make **mistakes**, all the time. Software almost *always* has **bugs** hidden in different places. 🐛

And we as developers keep improving the code as we find those bugs and as we implement new features (possibly adding new bugs too 😅).

### Small Errors Automatically Handled