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

    * Suivez-moi pour lire quand je publie quelque chose de nouveau.

## Tweeter sur **FastAPI**

<a href="https://twitter.com/compose/tweet?text=I'm loving FastAPI because... https://github.com/tiangolo/fastapi cc @tiangolo" class="external-link" target="_blank">Tweetez à propos de **FastAPI**</a> et faites-moi savoir, ainsi qu'aux autres, pourquoi vous aimez ça. 🎉

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
if "johndoe" == "stanleyjobson" and "love123" == "swordfish":
    ...
```

But right at the moment Python compares the first `j` in `johndoe` to the first `s` in `stanleyjobson`, it will return `False`, because it already knows that those two strings are not the same, thinking that "there's no need to waste more computation comparing the rest of the letters". And your application will say "Incorrect username or password".

```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.

* Plays nicely with your **<abbr title="Integrated Development Environment, similar to a code editor">IDE</abbr>/<abbr title="A program that checks for code errors">linter</abbr>/brain**:
    * Because pydantic data structures are just instances of classes you define; auto-completion, linting, mypy and your intuition should all work properly with your validated data.
* Validate **complex structures**:

## OpenAPI and API docs

If you return additional status codes and responses directly, they won't be included in the OpenAPI schema (the API docs), because FastAPI doesn't have a way to know beforehand what you are going to return.

!!! info
    For more information, check <a href="https://docs.python.org/3/library/__main__.html" class="external-link" target="_blank">the official Python docs</a>.

## Run your code with your debugger

Because you are running the Uvicorn server directly from your code, you can call your Python program (your FastAPI application) directly from the debugger.

---

For example, in Visual Studio Code, you can: