When subclassing the ctypes.Structure class, __new__ and __init__ are not invoked when using the inherited classmethod from_buffer_copy to create the object.

I think this is because tp_alloc is ultimately used by GenericPyCData_new when creating the object using the from_buffer_copy classmethod inherited from _CData.

https://github.com/python/cpython/blob/be143ec99674ba38c5811f34cdb85ef39c2dc8f8/Modules/_ctypes/_ctypes.c#L3202

Expected behavior: creation of Structure subclass object would invoke __new__ and possibly __init__.

Hi there, I’m a newcomer would like to take a look at this. Planning to investigate over the next few days.

Looking at the test case 'test_from_buffer_copy' here:
https://github.com/python/cpython/blob/master/Lib/ctypes/test/test_frombuffer.py

This suggests the failure to call init in this instance is expected behavior (similarly for from_buffer), unlike e.g. the classmethod fromkeys() with a dictionary subclass which behaves as expected.

Are there any required changes here, perhaps if not code then documentation?

It might make sense that CSimpleData types do not call init, which makes sense for _fields_ but it doesn't make sense to offer subclassing (of Structure) and not use the subclass, instead creating a type of the same name.

That test case is bugged, missing an underscore... The git blame wasn't "available" for that particular test but I would argue that it's testing for undocumented behavior :p

I wanted to be able to do assertions/initialization, I'm sure other people have expected from_buffer(_copy) et al would act as a class method, returning an instance of the subclass.

More generally, I think that GenericPyCData_new should not "tp_alloc"... What's _new about that?

My guess is there was at some level desire to "optimize" ctypes. If that does turn out to be the case I'd hope for a "unsimple" CData and ask that Structure be an instance of that.

Either way docs reflecting the behavior to Structure would be nice, I just hope it includes "prior to version" :p thanks for taking a look

Becca were you still interested in this issue? 

Anyone agree the unit test seems to have a bug if that is intended behavior?

Is there a better forum for discussion about design/behavior, maybe the more generic issue of GenericPyCData_new using tp_alloc instead of tp_new

I would still be interested in this, with some clarity on the testing/expected behavior.

Justin Capella wrote:
> Is there a better forum for discussion about design/behavior, maybe the more generic issue of GenericPyCData_new using tp_alloc instead of tp_new

Rebecca Morgan wrote:
> I would still be interested in this, with some clarity on the testing/expected behavior.

I'm not personally active on the capi-sig mailing list, but it seems like it would be a suitable location for discussing the behavior of GenericPyCData_new. If you don't receive a response in capi-sig, you could also try python-dev to bring some attention to the issue since most of the core developers actively check there.