This would actually be useful for implementing my proposal of abstract attributes: [abc] Add abstract attributes via `abstract` type-hint
In essence, __post__ could be used by libraries to validate class instances, after they have been created. (abstract base classes and pydantic come to mind)
The issues with the solutions proposed so far in this thread is that they require the user to call super()___init__ last in their own __init__. However, sometimes it is necessary to call super().__init__ early. The huge advantage of __post__ would be that it will be guaranteed that it is only called after __init__ and potential super().__init__ are resolved.