The process that happens when your API app calls the *external API* is named a "callback". Because the software that the external developer wrote sends a request to your API and then your API *calls back*, sending a request to an *external API* (that was probably created by the same developer).

Let's imagine that loading the model can **take quite some time**, because it has to read a lot of **data from disk**. So you don't want to do it for every request.

Or it might be the case that you just prefer to take other courses because they adapt better to your learning style.

When you need to use the `common_parameters()` dependency, you have to write the whole parameter with the type annotation and `Depends()`:

```Python
commons: Annotated[dict, Depends(common_parameters)]
```

But because we are using `Annotated`, we can store that `Annotated` value in a variable and use it in multiple places:

=== "Python 3.10+"

    ```Python hl_lines="12  16  21"

!!! info
    To receive uploaded files, first install <a href="https://github.com/Kludex/python-multipart" class="external-link" target="_blank">`python-multipart`</a>.

    E.g. `pip install python-multipart`.

    This is because uploaded files are sent as "form data".

## Import `File`

Import `File` and `UploadFile` from `fastapi`:

=== "Python 3.9+"

    ```Python hl_lines="3"

    yield request_state
    legacy_request_state_context_var.reset(contextvar_token)
```

...before this change it would raise an error when resetting the context variable, because the `contextvars` context was different, because of the way it was implemented.

!!! note
    FastAPI will know that the value of `q` is not required because of the default value `= None`.

    The `Union` in `Union[str, None]` will allow your editor to give you better support and detect errors.

## Additional validation

Import the `Response` class (sub-class) you want to use and declare it in the *path operation decorator*.

For large responses, returning a `Response` directly is much faster than returning a dictionary.

### Warning about replacing

That means that if you want to update the item `bar` using `PUT` with a body containing:

```Python
{
    "name": "Barz",
    "price": 3,
    "description": None,
}
```

because it doesn't include the already stored attribute `"tax": 20.2`, the input model would take the default value of `"tax": 10.5`.

And the data would be saved with that "new" `tax` of `10.5`.

## Partial updates with `PATCH`

```Python hl_lines="1"
{!../../../docs_src/handling_errors/tutorial001.py!}
```

### Raise an `HTTPException` in your code

`HTTPException` is a normal Python exception with additional data relevant for APIs.

Because it's a Python exception, you don't `return` it, you `raise` it.