Definitely looks like a bug. Seems like the following code is at play:

https://github.com/pydantic/pydantic-core/blob/a65f3272f002c7663c368aa4708ca706547e3bdb/src/validators/model.rs#L209-L215

You can however omit the = {} assignment to __pydantic_extra__, it will fix your issue.

Thank you.

I'm really not familiar with pydantic's internals so this all seems very strange to me!

May be worth pointing out that the error persists if one uses the suggested form of __pydantic_extra__ from the docs:

https://docs.pydantic.dev/latest/concepts/models/#extra-fields

i.e.

class Model(BaseModel):
    __pydantic_extra__: Dict[str, int] = Field(init=False)
    model_config = ConfigDict(extra="allow", 
                              validate_assignment=True)

m = Model()
m.a = 1

assert m.__pydantic_extra__ == {'a': 1}
# also fails

Hmm, I'm not so convinced this is a bug. This is the behavior I'd expect - a is not defined on the original model, so shouldn't we consider it an extra attribute?

@sydney-runkle the issue is regarding __pydantic_extra__ being set inside __pydantic_extra__ itself

Ah yes, that's not right...

I presume we can just add an exclusionary rule for that case

Just a side note, this is semi related: #9672. We could potentially tackle these at the same time.