[Cython] Bug in NULL handling introduced 0.14.1-1

Chris Colbert sccolbert at gmail.com
Sun Feb 13 21:39:39 CET 2011

I should mention, that these PyObject_Generic* functions are being used from
with __getattribute__, __setattr__, and __delattr__ methods. I should have
added that context in the begining.

On Sun, Feb 13, 2011 at 3:37 PM, Chris Colbert <sccolbert at gmail.com> wrote:

> 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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