Using transitions state machine in Pydantic model

Question

Originally asked here but I've narrowed down the problem further.

Python's transitions library's Machine doesn't appear to work with Pydantic BaseModel classes. Even if I allow for arbitrarily adding attributes in the Config subclass, the code gets stuck at the add_model(self) step.

Example:

class TestModel(BaseModel):
    id: Optional[int]
    _fsm: "Machine" = PrivateAttr()

    def __init__(self, **kwargs):
        super().__init__(**kwargs)

        self._fsm = fsm.get_fsm(default_status)
        self._fsm.add_model(self)  # gets stuck here

    class Config:
        extra = Extra.allow