[SOLVED] Stack Overflow question on FastAPI

fastapi_error_example.py

from typing import Optional, List, Union
from typing_extensions import Literal

import uvicorn
from pydantic import BaseModel, Field
from fastapi import FastAPI, Body


class Payload(BaseModel):
    start: int
    end: int


class Link(BaseModel):
    source: int
    target: int
    type: str


class SchoolNode(BaseModel):
    id: int
    label: Literal['school']
    code: Optional[str]


class PersonNode(BaseModel):
    id: int
    label: Literal['person']
    name: Optional[str]
    surname: Optional[str]


class PetNode(BaseModel):
    id: int
    label: Literal['pet']
    name: Optional[str]
    surname: Optional[str]


class UnionNode(BaseModel):
    __root__: Union[SchoolNode, PersonNode, PetNode] = Field(..., discriminator='label')


class Response(BaseModel):
    links: List[Link]
    nodes: List[UnionNode]
    start: int
    end: int


app = FastAPI()


@app.post("/shortest-path", response_model=Response)
def compute_shortest_path(payload: Payload = Body(
    ...
)):
    # do_stuff
    graph = {
        'links': [
            {'source': payload.start, 'target': 123, 'type': 'GO_TO_SCHOOL'},
            {'source': 123, 'target': payload.end, 'type': 'GO_TO_SCHOOL'},
        ],
        'nodes': [
            {'id': payload.start, 'label': 'person', 'name': 'Bob', 'surname': 'Foo'},
            {'id': 123, 'label': 'school', 'code': 'ABCD'},
            {'id': payload.end, 'label': 'person', 'name': 'Alice', 'surname': 'Bar'}
        ],
        'start': payload.start,
        'end': payload.end
    }
    return Response(**graph)


if __name__ == '__main__':
    uvicorn.run("fastapi_error_example:app")

The solution was creating a unique model UnionNode with a __root__ property with Field(..., discriminator='label').
Moreover, label property in nodes must have a Literal typing, here imported from typing_extensions for Python version < 3.8 compatibility.