The fact that it's never been needed in the years that dataclasses and attrs have existed tell me it's kind of a niche requirement.

This does not seem like the ugliest code I've ever seen:

        if hasattr(super(), "__post_init__"):
            super().__post_init__()

or the probably more efficient, but more lines of code:

        try:
            post_init = super().__post_init__
        except AttributeError:
            pass
        else:
            post_init()

As always with calling super functions, the whole hierarchy needs to cooperate. Say you had a dataclass:

@dataclass
class Base:
    def __post_init__(self, some_arg):
        pass

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". I don't think adding ways to make the "always call super" pattern easier is a good idea.

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.