[Cython] Bug in NULL handling introduced 0.14.1-1

Chris Colbert sccolbert at gmail.com
Sun Feb 13 21:37:26 CET 2011

The problem with delattr (and thus PyObject_DelAttr) arises when you define
a __delattr__ method on your class. There is not easy way to then call back
into the "normal" python delattr semantics, except by doing
object.__delattr__ (which is not optimized by Cython).

Futher, calling PyObject_GenericSetattr(obj, name, NULL) appears to be the
proper use, given that it properly follows the descriptor chain and will
call __delete__ if obj.name is a descriptor.

I would argue that there should be at least some way to pass a NULL pointer
in Cython where a PyObject* is expected.

On Sun, Feb 13, 2011 at 3:31 PM, Greg Ewing <greg.ewing at canterbury.ac.nz>wrote:

> Chris Colbert wrote:
>> I have cython file which is using PyObject_GenericSetAttr
>> Now in my script I am using that function to generically delete an
>> attribute by passing a NULL as the last value (this is proper way to trigger
>> a generic delattr in the Python c-api)
> I would have thought the proper way to do that was to use
> PyObject_DelAttr, which Pyrex exposes as delattr().
> I don't think PyObject_GenericSetAttr is even meant to be
> called directly -- it's intended for filling the tp_setattr
> slot of type objects.
>  This causes a segfault because the NULL is getting increfed via Py_INCREF
>> instead of Py_XINCREF.
> I would recommend against trying to "fix" this. You got
> away with it before because you happened to be passing the
> NULL value directly to a function which is expecting it.
> But casting NULL to an object reference is not something
> that should be encouraged, because in any other context it
> would quickly lead to disaster.
> --
> Greg
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