* 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.

                )
            else:
                return None
        if scheme.lower() != "bearer":
            if self.auto_error:
                raise HTTPException(
                    status_code=HTTP_403_FORBIDDEN,
                    detail="Invalid authentication credentials",
                )
            else:
                return None
        return HTTPAuthorizationCredentials(scheme=scheme, credentials=credentials)

def by_alias_dict():
    return {"alias": "Foo"}


@app.get("/by-alias/model", response_model=Model)
def by_alias_model():
    return Model(alias="Foo")


@app.get("/by-alias/list", response_model=List[Model])
def by_alias_list():
    return [{"alias": "Foo"}, {"alias": "Bar"}]


@app.get("/no-alias/dict", response_model=ModelNoAlias)
def no_alias_dict():
    return {"name": "Foo"}

            if self.auto_error:
                raise HTTPException(
                    status_code=HTTP_403_FORBIDDEN, detail="Not authenticated"
                )
            else:
                return None
        return authorization


class OAuth2PasswordBearer(OAuth2):
    """
    OAuth2 flow for authentication using a bearer token obtained with a password.
    An instance of it would be used as a dependency.

    is_offer: Union[bool, None] = None


@app.get("/")
def read_root():
    return {"Hello": "World"}


@app.get("/items/{item_id}")
def read_item(item_id: int, q: Union[str, None] = None):
    return {"item_id": item_id, "q": q}


@app.put("/items/{item_id}")
def update_item(item_id: int, item: Item):
    return {"item_name": item.name, "item_id": item_id}
```

    is_offer: Union[bool, None] = None


@app.get("/")
def read_root():
    return {"Hello": "World"}


@app.get("/items/{item_id}")
def read_item(item_id: int, q: Union[str, None] = None):
    return {"item_id": item_id, "q": q}


@app.put("/items/{item_id}")
def update_item(item_id: int, item: Item):
    return {"item_name": item.name, "item_id": item_id}
```

    is_offer: bool = Optional[None]


@app.get("/")
def read_root():
    return {"Hello": "World"}


@app.get("/items/{item_id}")
def read_item(item_id: int, q: Optional[str] = None):
    return {"item_id": item_id, "q": q}


@app.put("/items/{item_id}")
def update_item(item_id: int, item: Item):
    return {"item_name": item.name, "item_id": item_id}
```

    is_offer: Union[bool, None] = None


@app.get("/")
def read_root():
    return {"Hello": "World"}


@app.get("/items/{item_id}")
def read_item(item_id: int, q: Union[str, None] = None):
    return {"item_id": item_id, "q": q}


@app.put("/items/{item_id}")
def update_item(item_id: int, item: Item):
    return {"item_name": item.name, "item_id": item_id}
```

### Step 5: return the content

```Python hl_lines="8"
{!../../../docs_src/first_steps/tutorial001.py!}
```

You can return a `dict`, `list`, singular values as `str`, `int`, etc.

You can also return Pydantic models (you'll see more about that later).

            kwargs.update(**current_json_schema_extra)
        use_kwargs = {k: v for k, v in kwargs.items() if v is not _Unset}

        super().__init__(**use_kwargs)

    def __repr__(self) -> str:
        return f"{self.__class__.__name__}({self.default})"


class Path(Param):
    in_ = ParamTypes.path

    def __init__(
        self,
        default: Any = ...,
        *,