[FEATURE] Inheritance and Polymorphism support

Actually, there is no support for Inheritance and Polymorphism as I know in fastapi : https://swagger.io/docs/specification/data-models/inheritance-and-polymorphism/

Maybe annotations like that will do the job :

class Item(BaseModel):
    type: str

class CarItem(Item):
    type: str

class PlaneItem(Item):
    type: str
    size: str


@app.post("/items/", response_model=Item, one_of=[CarItem, PlaneItem], discriminator="type")
async def create_item(*, item: Item):
    return item

For now, we need to define an any response_model since defining an Item response_model will remove the extra attributes during serialization.

I’ll try to hack using the based starlette api : https://www.starlette.io/schemas/ but comment are unused by fastapi (except for description)

[rapiz1]

I think real inheritance and polymorphism support would be very useful. Comments above show that using typing.Union forces users to carefully put the class in order and is not a generic solution.

[sm-Fifteen]

@mcauto: Ah, that’s a common mistake people make. Pydantic can’t tell what data structure your JSON is supposed to be aside from matching it against one of your possible models. As it happens, since Woman adds no new fields to Human and Man does, there’s no way for Pydantic to tell whether the payload you sent it just a Woman object (essentially a Human object) with extra fields or a Man object, since both match.

If you really want the absence of something to define whether or not your Human subtype is Man or Woman, I believe you can just have Woman forbid the presence of extra fields, like so:

class Woman(Human):
    """ woman """
    class Config:
        extra = 'forbid'

(I have not tested it myself, though, so apologies if this doesn’t work)