Accessing the data

Sometimes, it doesn’t make sense for data to be immediately parsed into an instance of the target class. We just saw an example of this in the previous section, Read only fields. When required fields are excluded from, or do not exist in the client submitted data we will get an error upon instantiation of the class.

The solution to this is the DTOData type.

from __future__ import annotations

from dataclasses import dataclass

from litestar import Litestar, post
from litestar.dto import DataclassDTO, DTOConfig, DTOData
from litestar.params import JSONBody


@dataclass
class Person:
    name: str
    age: int
    email: str
    id: int


class ReadDTO(DataclassDTO[Person]):
    config = DTOConfig(exclude={"email"})


class WriteDTO(DataclassDTO[Person]):
    config = DTOConfig(exclude={"id"})


@post("/person", dto=WriteDTO, return_dto=ReadDTO, sync_to_thread=False)
def create_person(data: JSONBody[DTOData[Person]]) -> Person:
    # Logic for persisting the person goes here
    return data.create_instance(id=1)


app = Litestar(route_handlers=[create_person])

The DTOData type is a container for data that can be used to create instances, and access the underlying parsed and validated data. In our latest adjustments, we import that from litestar.dto.factory.

The handler function’s data parameter type is changed to DTOData[Person] instead of Person, and accordingly, the value injected to represent the inbound client data will be an instance of DTOData.

In the handler, we produce a value for the id field, and create an instance of Person using the create_instance method of the DTOData instance.

And our app is back to a working state:

Tip

To provide values for nested attributes you can use the “double-underscore” syntax as a keyword argument to the create_instance() method. For example, address__id=1 will set the id attribute of the address attribute of the created instance.

See Providing values for nested data for more information.

The DTOData type has some other useful methods, and we’ll take a look at those in the next section: Updating instances.