feat: allow keep not nullable fields not nullable

[ADR-007]

Currently, pydantic-partial allows to set None for not nullable fields, which brakes the validate logic on the FastAPI endpoint.

This PR provides a new parameter use_undefined, to use UNDEFINED value as default instead of None, and doesn't make fields nullable.

Please take a look on the test cases.

Use case:

class UserUpdateSchema(PartialModelMixin, pydantic.BaseModel):
    first_name: str
    second_name: str | None
  

@router.put()
def update_me(user: AuthUser, payload: UserUpdateSchema):
    user.set_values(payload.model_dump())

@router.patch()
def partial_update_me(user: AuthUser, payload: UserUpdateSchema.model_as_partial(
    use_undefined=True, # without this flag `first_name` can be set to `None` which is wrong
)):
    user.set_values(payload.model_dump(exclude_unset=True))

[ADR-007]

Actually, I can have everything I want to using this code snippet:

T = TypeVar("T", bound=BaseModel)


def create_partial_model(
    model: type[T],
    optional_fields: Iterable[str] | None = (),
    default_value: Any = None,
) -> type[T]:
    fields = {}
    for field in optional_fields or model.model_fields.keys():
        field_info = model.model_fields[field]

        fields[field] = (
            field_info.rebuild_annotation(),
            FieldInfo.merge_field_infos(field_info, default=default_value),
        )

    return create_model(
        model.__name__ + "Partial",
        __base__=model,
        **fields,
    )

So, I'll, probably, just create another library.

[ADR-007]

Here is the solution

[ddanier]

Not sure I like the proposed change. It just moves the issue to another value. Also I normally just use pydantic-apply and then apply only the set fields, so you don't need to care about fields being None - as those are still unset.

I will close this PR, as you created a solution in your own package ;-)

[ddanier]

Still many thanks for putting effort into this!