How to solve multiple and nested discriminators with Pydantic v2?

Question

I am trying to validate Slack interaction payloads, that look like these:

type: block_actions
container:
  type: view
...

type: block_actions
container:
  type: message
...

type: view_submission
...

I use 3 different models for payloads coming to the same interaction endpoint:

class MessageContainer(BaseModel):
    type: Literal["message"]
    ...

class ViewContainer(BaseModel):
    type: Literal["view"]
    ...

class MessageActions(ActionsBase):
    type: Literal["block_actions"]
    container: MessageContainer
    ...

class ViewActions(ActionsBase):
    type: Literal["block_actions"]
    container: ViewContainer
    ...

class ViewSubmission(BaseModel):
    type: Literal["view_submission"]
    ...

and I was planning to use

BlockActions = Annotated[
    MessageActions | ViewActions,
    Field(discriminator="container.type"),
]
SlackInteraction = Annotated[
    ViewSubmission | BlockActions,
    Field(discriminator="type"),
]
SlackInteractionAdapter = TypeAdapter(SlackInteraction)

but cannot make it work with v2.10.4.

Do I have to dispatch them manually or there is a way to solve it with Pydantic?

Answer 1

Not sure it's possible to use 2 discriminators to resolve one type (as you are trying to do).

I can suggest you 3 options:

1. Split block_actions into block_message_actions and block_view_actions:

from typing import Annotated, Literal

from pydantic import BaseModel, Field, TypeAdapter


class MessageContainer(BaseModel):
    pass

class ViewContainer(BaseModel):
    pass


class ActionsBase(BaseModel):
    pass

class MessageActions(ActionsBase):
    type: Literal["block_message_actions"]
    container: MessageContainer


class ViewActions(ActionsBase):
    type: Literal["block_view_actions"]
    container: ViewContainer


class ViewSubmission(BaseModel):
    type: Literal["view_submission"]


SlackInteraction = Annotated[
    ViewSubmission | ViewActions | MessageActions,
    Field(discriminator="type"),
]

SlackInteractionAdapter = TypeAdapter(SlackInteraction)



a = SlackInteractionAdapter.validate_python({"type": "view_submission"})
assert isinstance(a, ViewSubmission)


b = SlackInteractionAdapter.validate_python(
    {"type": "block_message_actions", "container": {}},
)
assert isinstance(b, MessageActions)
assert isinstance(b.container, MessageContainer)


c = SlackInteractionAdapter.validate_python(
    {"type": "block_view_actions", "container": {}},
)
assert isinstance(c, ViewActions)
assert isinstance(c.container, ViewContainer)

2. Use Discriminated Unions with callable Discriminator:

def get_discriminator_value(v: Any) -> str:
    if isinstance(v, dict):
        if v["type"] == "view_submission":
            return "view_submission"
        return "message_action" if v["container"]["type"] == "message" else "view_action"
    if v.type == "view_submission":
        return "view_submission"
    return "message_action" if v.container.type == "message" else "view_action"


SlackInteraction = Annotated[
    Union[
        Annotated[ViewSubmission, Tag("view_submission")],
        Annotated[MessageActions, Tag("message_action")],
        Annotated[ViewActions, Tag("view_action")],
    ],
    Discriminator(get_discriminator_value),
]
SlackInteractionAdapter = TypeAdapter(SlackInteraction)

3. Use nested discriminated unions:

from typing import Annotated, Literal

from pydantic import BaseModel, Field, TypeAdapter


class MessageContainer(BaseModel):
    type: Literal["message"]

class ViewContainer(BaseModel):
    type: Literal["view"]


ActionContainer = Annotated[
    MessageContainer | ViewContainer,
    Field(discriminator="type"),
]

class BlockActions(BaseModel):
    type: Literal["block_actions"]
    container: ActionContainer



class ViewSubmission(BaseModel):
    type: Literal["view_submission"]



SlackInteraction = Annotated[
    ViewSubmission | BlockActions,
    Field(discriminator="type"),
]

SlackInteractionAdapter = TypeAdapter(SlackInteraction)


a = SlackInteractionAdapter.validate_python({"type": "view_submission"})
assert isinstance(a, ViewSubmission)


b = SlackInteractionAdapter.validate_python(
    {"type": "block_actions", "container": {"type": "message"}},
)
assert isinstance(b, BlockActions)
assert isinstance(b.container, MessageContainer)


c = SlackInteractionAdapter.validate_python(
    {"type": "block_actions", "container": {"type": "view"}},
)
assert isinstance(c, BlockActions)
assert isinstance(c.container, ViewContainer)