> How would an arbitrary derived class know how to call this? It can't. There has to be knowledge of the base class's requirements already. Surely knowing "__post_init__ must be called with some_arg" isn't too different from "I know __post_init__ doesn't exist".

This is exactly the same problem you have with all other "augmenting methods" that have arbitrary parameters (e.g., __init__).  When calling super on a non-final class you could simply forward keyword arguments.


@dataclass
class X:
    def __post_init__(self, **kwargs):
        super().__post_init__(**kwargs)
        ...

@dataclass
class Y(X):
    def __post_init__(self, **kwargs):
        super().__post_init__(**kwargs)
        ...

> I'm still unconvinced, but I'll hold off on making a decision to see if there's more support. Maybe taking it to python-ideas would be worthwhile.

Sounds good, done:  https://groups.google.com/g/python-ideas/c/-gctNaSqgew