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I am trying to use Literal to create a discriminated union with Pydantic. There are events about a Job resource, and I want to distinguish them by event_name. For JobPublishedEvents I want to ensure, that some extra_field is present.

class GenericJobEvent(BaseModel):
    event_name: str
    id: int


class JobPublishedEvent(GenericJobEvent):
    event_name: Literal['job.published']
    extra_field: str


class Wrapper(BaseModel):
    wrapped: Union[JobPublishedEvent, GenericJobEvent]

print(type(Wrapper(wrapped={'event_name': 'some.event', 'id': 1}).wrapped))  # GenericJobEvent
print(type(Wrapper(wrapped={'event_name': 'job.published', 'id': 1, 'extra_field': 'extra'}).wrapped))  # JobPublishedEvent
print(type(Wrapper(wrapped={'event_name': 'job.published', 'id': 1}).wrapped))  # GenericJobEvent

The first 2 cases behave as expected, for the third I would like a validation error since the literal matches, but the schema is not fulfilled. I get why the fallback to the GenericJobEvent is valid, though.

Does anybody have an idea on how to achieve this?

elactic
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1 Answers1

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The problem is due to the fact that while JobPublishedEvent fails, GenericJobEvent does not. Think about it, the event_name attribute is a string, the id is an int. Everything matches.

So, what you could do, is to validate also the event_name in GenericJobEvent and reject the event name job.published

You can validate it as explained in the docs

@validator('event_name')
    def event_name_filter(cls, v):
        if 'job.published' == v:
            raise ValueError('GenericJobEvent cannot have event_name equal to job.published')
        return v.title()
lsabi
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  • Yeah, that's what I meant with "I get why the fallback to the GenericJobEvent is valid". What I would need is for the GenericEvent to accept all strings except the ones defined in subclasses... Thank you for your solution! – elactic Jun 23 '20 at 09:27
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    Sorry, too much multitasking led to distracting me. Glad I've helped – lsabi Jun 23 '20 at 15:14