dataclass tries to generate `__init__()` even though the class already defines one

[tonyyuyiding]

Bug report

Bug description:

When running this code:

from dataclasses import dataclass

@dataclass
class A:
    a: int = 1

@dataclass
class B(A):
    b: int
    def __init__(self, *args, **kwargs): ...

I got TypeError: non-default argument 'b' follows default argument

However, if I set init=False for the second dataclass, like this:

from dataclasses import dataclass

@dataclass
class A:
    a: int = 1

@dataclass(init=False)
class B(A):
    b: int
    def __init__(self, *args, **kwargs): ...

There error disappeared.

I think the error comes from the process where dataclass tries to generate the default __init__() and checks the fields. But, if I understand correctly, this should not happen when the class already defines its own __init__().

Although I'm not sure whether the behavior should be defined as a bug, I do find the results inconsistant with the docs, which says

If the class already defines __init__(), this parameter (init) is ignored.

so, intuitively, adding init=False shouldn't have changed the result.

CPython versions tested on:

3.12, 3.13, 3.14

Operating systems tested on:

Windows

[tonyyuyiding]

btw I would be happy to try to fix it, if needed

[DavidCEllis]

Yes, this happens because _process_class only looks at the provided arguments and not the class attributes before adding the methods to the _FuncBuilder. The checks for methods that already exist are right at the end of the process.

This is true for all of the methods and not just __init__, but _init_fn does some early checks before adding anything to the _FuncBuilder which is why you're seeing this bug. Ideally I think we'd want to keep all of this logic together so all of the checks are still happening in the same place and we're not checking everything twice.