So, even with the code above that doesn't use Pydantic explicitly, FastAPI is using Pydantic to convert those standard dataclasses to Pydantic's own flavor of dataclasses.

And of course, it supports the same:

* data validation
* data serialization
* data documentation, etc.

    "tax": 10.5,
    "tags": []
}
```

FastAPI is smart enough (actually, Pydantic is smart enough) to realize that, even though `description`, `tax`, and `tags` have the same values as the defaults, they were set explicitly (instead of taken from the defaults).

So, they will be included in the JSON response.

!!! tip
    Notice that the default values can be anything, not only `None`.

And the sub-application could also have its own mounted sub-applications and everything would work correctly, because FastAPI handles all these `root_path`s automatically.

Of course, you can also declare additional query parameters whenever you need, additional to any body parameters.

As, by default, singular values are interpreted as query parameters, you don't have to explicitly add a `Query`, you can just do:

```Python
q: Union[str, None] = None
```

Or in Python 3.10 and above:

```Python
q: str | None = None
```

For example:

=== "Python 3.10+"

    'ignore:datetime\.datetime\.utcfromtimestamp\(\) is deprecated and scheduled for removal in a future version\..*:DeprecationWarning:sqlalchemy',
    # TODO: remove after upgrading python-jose to a version that explicitly supports Python 3.12
    # also, if it won't receive an update, consider replacing python-jose with some alternative
    # related issues:
    #   - https://github.com/mpdavis/python-jose/issues/332

But that will only allow certain types of communication, excluding everything that involves credentials: Cookies, Authorization headers like those used with Bearer Tokens, etc.

So, for everything to work correctly, it's better to specify explicitly the allowed origins.

## Use `CORSMiddleware`

You can configure it in your **FastAPI** application using the `CORSMiddleware`.

* Import `CORSMiddleware`.
* Create a list of allowed origins (as strings).

```Python
q: str | None = Query(default=None)
```

...makes the parameter optional, with a default value of `None`, the same as:

```Python
q: str | None = None
```

But it declares it explicitly as being a query parameter.

!!! info
    Keep in mind that the most important part to make a parameter optional is the part:

    ```Python
    = None
    ```

    or the:

    ```Python

        bool,
        Doc(
            """
            Pydantic's `exclude_unset` parameter, passed to Pydantic models to define
            if it should exclude from the output the fields that were not explicitly
            set (and that only had their default values).
            """
        ),
    ] = False,
    exclude_defaults: Annotated[
        bool,
        Doc(
            """

                    "msg": "value is not a valid dict",
                    "type": "type_error.dict",
                }
            ]
        }
    )


@needs_py310
def test_explicit_content_type(client: TestClient):
    response = client.post(
        "/items/",
        content='{"name": "Foo", "price": 50.5}',
        headers={"Content-Type": "application/json"},
    )

                    "loc": ["body"],
                    "msg": "value is not a valid dict",
                    "type": "type_error.dict",
                }
            ]
        }
    )


def test_explicit_content_type(client: TestClient):
    response = client.post(
        "/items/",
        content='{"name": "Foo", "price": 50.5}',
        headers={"Content-Type": "application/json"},
    )