Behaviour of Partial for objects with nullable types

[peterschutt]

TypedDict and dataclass have a slight difference in behavior when used with Partial. Omitted properties are excluded for TypedDict, while for dataclass they are set to None.

"""Minimal Starlite application."""
from dataclasses import dataclass
from typing import TypedDict

from starlite import Starlite, post, Partial


@dataclass
class DC:
    a: str
    b: int
    c: float | None


class TD(TypedDict):
    a: str
    b: int
    c: float | None


@post("/dataclass")
def hello_dataclass(data: Partial[DC]) -> Partial[DC]:
    return data


@post("/typeddict")
def hello_typeddict(data: Partial[TD]) -> Partial[TD]:
    """Route Handler that outputs hello world."""
    return data

endpoint	payload	response
/dataclass	{}	{"a":null,"b":null,"c":null}
/dataclass	{"c": null}	{"a":null,"b":null,"c":null}
/typeddict	{}	{}
/typeddict	{"c": null}	{"c": null}

The TypedDict behavior is advantageous as can discern when a nullable property has been explicitly set to null or not. However, a dataclass instance cannot be constructed without values for all attributes, so maybe some sort of sentinel default value is necessary for that case.

[peterschutt]

There is another side to the issue as well. As we make all the types Optional to get around the requirement for validation, but really we just need a requirement for optional presence in the payload. For example, the payload {"a": null} should be invalid for both the dataclass and typeddict endpoints in the example above, yet is accepted.

This is what I think we need for a proper Partial implementation:

1. A way to discern inside a handler if an objects value has been set by the request, or not.
2. Validation of data properties to be uninfluenced by the typing union with the sentinel value.
3. The default sentinel value would need to be ignored by openapi docs.

I think this can be solved by using a sentinel that is not a valid json type. It would be breaking, but I'm not sure how useful the current implementation is with these limitations anyway.

[peterschutt]

dto.factory.DTOConfig now has a partial flag that distinguishes between None and not set.