[FEATURE] Add support for Pydantic's dataclasses

Describe the question I am using a pydantic.dataclasses.dataclass as a response_model of my fast api route.

from fastapi import FastAPI
from pydantic.dataclasses import dataclass


@dataclass
class Foo:
    bar: int

app = FastAPI()

@app.get('/foo', response_model=Foo)
def get_bar() -> Foo:
    return Foo(bar=1)

To Reproduce Steps to reproduce the behavior:

1. Create a file app.py with the above code.
2. Start the app: uvicorn app:app
3. Open the browser and go to localhost:8000/foo
4. See error:

pydantic.error_wrappers.ValidationError: 1 validation error
response
  __init__() got an unexpected keyword argument '__initialised__' (type=type_error)

Expected behavior No validation error and serialized json does not contain any keys __initialised__.

Environment:

• OS: Linux
• FastAPI Version 0.25.0
• Python version 3.7.3

[dmontagu]

@justinttl It looks like pydantic dataclasses work as long as you provide a config with from_orm=True:

from fastapi import FastAPI
from pydantic.dataclasses import dataclass
from starlette.testclient import TestClient


class ORMConfig:
    orm_mode = True


@dataclass(config=ORMConfig)
class XYZResponse:
    x: str


app = FastAPI()


@app.get(
    '/xyz',
    response_model=XYZResponse,
)
async def calculate_xyz():
    return XYZResponse("hello")


print(TestClient(app).get("/xyz").content)
# b'{"x":"hello"}'

---

@tiangolo I feel like there is room for improvement around the handling and/or error messages here. Maybe we should require orm_mode for any model used by FastAPI? Or turn it on automatically?

@samuelcolvin Any ideas about how we might more clearly indicate to users that the problem is coming from orm_mode=False?

[frejsoya]

This does not work with nested dataclassed. The inner class record will have ...,__intialized__ : true,...