Derived classes with @dataclass(slots=True) generate empty __slots__ about dataclassy HOT 2 CLOSED

DataClassMeta.new is invoked once by the dataclass decorator, and then once again going up the chain from Derived -> Base -> DataClassMeta (-> type), which sees the result of the first invocation and decides it doesn't need to add any new slots, only to have the result of the first invocation lost in the final result. Does that sound like a reasonable version of events?

Yes, if I understand your explanation correctly, this is indeed what happens. Using a metaclass means that DataClassMeta.__new__ is run for each new subclass, then applying the decorator means the subclass is replaced with a new version (this happens here if you are interested). It can cause issues.

If you're wondering, the way my fix works is by exploiting the fact that the dict_ passed in looks different between these two invocations. In the first instance (subclassing), the metaclass is called before Python has done any of its inheritance magic, meaning __slots__ is empty. In the second instance (decorator usage), the class is already fully formed, meaning __slots__ has been set, so it's not replaced.

This change also means that a user-defined __slots__ will not be wiped out which I think is good anyway.

from dataclassy.

Thanks for the detailed explanation and quick fix. I'm not sure I had it 100% in my original explanation, but it's clear what was going on now.

from dataclassy.