@DataClass
class Foo:
x: dict = field(default_factory=dict)

@DataClass
class Bar(Foo):
y: int = 1

@DataClass
class Baz(Foo):
def blorf(self):
print('hello')

Foo().x
{}

Bar().x
{}

Baz().x

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: __init__() missing 1 required positional argument: 'x'

I understand that this is desired behavior when the subclass contains non-default attributes. But subclasses that define no additional attributes should work just the same as those that define only additional default attributes.

A similar issue was raised and dismissed when dataclasses was in development on GitHub: ericvsmith/dataclasses#112, but that only concerned the case of subclasses defining non-default attributes.

That's a great bug report. Thanks for the tiny code to replicate it.

It turns out the code isn't quite doing what I thought. I'll have to give some thought to exactly how I'm going to handle this without breaking other cases, but I should have it fixed soon.

Thanks again.

New changeset 8f6eccd by Eric V. Smith in branch 'master':
bpo-32896: Fix error when subclassing a dataclass with a field that uses a default_factory (GH-6170)
8f6eccd

New changeset 22136c9 by Eric V. Smith (Miss Islington (bot)) in branch '3.7':
bpo-32896: Fix error when subclassing a dataclass with a field that uses a default_factory (GH-6170) (GH-6171)
22136c9

Thanks again for the bug report. This ended up being a simple fix, but an important one.