SafeString coerced to str when Slots class uses Pydantic's BaseModel

[JuroOravec]

Problem

There's an error where Pydantic incorrectly coerces SafeString to str.

The end result is that Django template receives a plain string instead of SafeString, thus incorrectly escaping the HTML content:

This happens only when I:

1. Declare slots with Component.Slots
2. Use Pydantic's BaseModel to define the Slots class
3. Pass in the slots as SafeString

E.g.

from pydantic import BaseModel
from django_components import Component, SlotInput

class Table(Component):
    class Slots(BaseModel):
        project_nav: SlotInput

Table.render(
    slots={
        "project_nav": ProjectNav.render(...),
    }
)

I know that this is an issue with how Pydantic handles the SlotInput type, because the values are coerced to str only once Slots class is instantiated:

Solution

I have recently read through Pydantic docs page on union types and they have an unusual way of handling unions, so I believe this might the case.

Don't know the internals of Pydantic, so I think we should raise this with Pydantic project to ask for help.

For context, the definition of SlotInput is:

TSlotData = TypeVar("TSlotData", bound=Mapping)

SlotResult = Union[str, SafeString]

class SlotFunc(Protocol, Generic[TSlotData]):
    def __call__(self, ctx: SlotContext[TSlotData]) -> SlotResult:
        ...

@dataclass
class Slot(Generic[TSlotData]):
    ...

SlotInput = Union[SlotResult, SlotFunc[TSlotData], Slot[TSlotData]]

Moreover, definition of SafeString is

class SafeString(str, SafeData):
    ...

And our Component.render() returns SafeString instances. So I don't understand why Pydantic is coercing that to str instead of SafeString.

Based on their "exactness" logic in their docs, I would have expected that SafeString instance would match SafeString class over str