Use the original value when validating an assignment to a pydantic object

Question

Is it possible to have pydantic look at the original value of an attribute when validating an assignment? I want to create a schema where a field can start off None, and can be set to a string, but can then never be modified again. Something like

class M(pydantic.BaseModel):
    maybe_str: Optional[str] = None
    
    @non_existent_validate_on_assignment_decorator("maybe_str")
    def permit_assignment_unless_maybe_str_is_already_set(self, v):
        assert self.maybe_str is None
        return v

In my dream, m = M(); m.maybe_str = "x" would be ok but m = M(maybe_str="y"); m.maybe_str = "x" would raise an AssertionError.