[BUG] Union of a model with a Color raises error in creating `openAPI.json`.

To Reproduce

1. Create the following app (main.py):

from typing import Union

from fastapi import FastAPI
from pydantic import BaseModel
from pydantic.color import Color


class A(BaseModel):
    type: str


class B(BaseModel):
    bla: Color


C = Union[A, B]

app = FastAPI()


@app.get('/{var}', response_model=C)
def get(var: int):
    if var == 1:
        return {'type': 'a'}
    else:
        return {'bla': '#ff00ff'}

1. run uvicorn main:app

2. open http://127.0.0.1:8000/docs

3. See error (Failed to load API definition on the browser), mini stack trace:

  File ".../venv/lib/python3.7/site-packages/pydantic/schema.py", line 772, in field_singleton_schema
    raise ValueError(f'Value not declarable with JSON Schema, field: {field}')
ValueError: Value not declarable with JSON Schema, field: bla type=Color required

1. http://127.0.0.1:8000/1 and http://127.0.0.1:8000/2, no error in shown.

Expected behavior No error

Environment:

• OS: [macOS]
• FastAPI Version 0.31.0.
• Python version: python 3.7.3

Additional context No additional context, plain FastAPI. I am not sure if this is coming from pydantic or fastAPI: it may be a pydantic problem, or how fastAPI is using pydantic.

[varunchopra]

Not sure if you’re allowed to use var as it’s a keyword.

[tiangolo]

Thanks @euri10 for your help here and implementing it in Pydantic! 🍰 🚀

Thanks @jorgecarleitao for reporting back and closing the issue.

Just in case, this should be available in a recent version of FastAPI.