[BUG] Pydantic __root__ model - incorrect handling

Describe the bug

https://pydantic-docs.helpmanual.io/usage/models/#custom-root-types Pydantic allows to create models with only __root__ field. In such scenario the model behaves as transparent wrapper for this single type.

When such model is used in response (request also?) fastapi does not treat it correctly and renders it as object with __root__ field.

Object is treated correctly by pydantic itself.

To Reproduce

from typing import List
from fastapi import FastAPI
from pydantic.main import BaseModel

app = FastAPI()


class RootTestClass(BaseModel):
    __root__: List[str]


@app.get("/")
async def root():
    response = RootTestClass(__root__=['a', 'b', 'c'])
    print(response.json())  # ["a", "b", "c"] so it's OK
    print(RootTestClass.schema())  # {'title': 'RootTestClass', 'type': 'array', 'items': {'type': 'string'}} this is also OK
    return response  # Wrong value in http response

Expected behavior

The response should be:

["a", "b", "c"]

but at the moment is:

{"__root__":["a","b","c"]}

Screenshots

N/A

Environment

• OS: Linux
• FastAPI Version: 0.47.1
• Python version: Python 3.7.5

Additional context

N/A

[davideasaf]

as @sidmani also mentioned, I’m running into wanting the ability to be able to say:

pydantic_list_as_root.dict()

and the above output a dict. Rather than having to manually loop through my List[pydantic_entity] and call dict() on each one.

However, I do appreciate what @tiangolo is trying to achieve by keeping things as pythonic as possible, but I would imagine that many if not all FastAPI implementations heavily rely on Pydantic for defining schemas. Therefore, I think it would be a great idea to embrace all/most of its capabilities.

[sidmani]

@tiangolo Supporting pydantic root types would allow a single validator (defined in the wrapper class) to be run on all objects of a certain type- otherwise, the validator must be specified in each object that has a child of that type (as far as I can tell- I’m new to fastAPI, please let me know if there’s a better way).