When the method __post_init__ doesnt called?

Question

I found the issue where was conversation about an explict call of parent's __post_init__ method using super(), but if I try this:

from dataclasses import dataclass


@dataclass
class A:
    def __post_init__(self):
        print("A")


@dataclass
class B(A):
    pass


b = B()

it will output:

A

So, parents method works without explict call.

1. I don't understand what talk about was in this issue or problem with inheritence was solved?
2. In which case __post_init__ method of dataclass doesnt called?

Answer 1

The issue under discussion is in how __init__ and __post_init__ differ.

object defines __init__; it doesn't do anything, but that means it's always safe to call super().__init__ in your own definitions of __init__. You have to take care what arguments you pass to it, but the attribute lookup itself will always succeed.

object does not define __post_init__, so calling super().__post_init__ requires you to know whether at least one class in your MRO will define it, or that you check for it first, or you'll get an AttributeError.

So __post_init__, if defined, will always be called, but it's not safe to assume for explicit super().__post_init__ calls that the method exists.