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Similarly to @WarKa's answer, with Pydantic v2:

from typing import Literal, Union, Annotated
from dataclasses import dataclass

from pydantic import RootModel, Field

@dataclass
class DeviceTokenGrant:
    grant_type: Literal["urn:ietf:params:oauth:grant-type:device_code"]
    client_id: str
    device_code: str

@dataclass
class RefreshTokenGrant:
    grant_type: Literal["refresh_token"]
    refresh_token: str


TokenGrant = RootModel[Annotated[Union[RefreshTokenGrant, DeviceTokenGrant], Field(discriminator="grant_type")]]


async def token(grant: Annotated[TokenGrant, Form()]):
    ...

Note the use of the discriminator attribute set on Field . See https://docs.pydantic.dev/latest/concepts/fields/#discriminator

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