C-API proper initialization and deallocation of subclasses

[ptb]

While there doesn't appear to be too much interest in this question I thought I would post the solution.  I had to modify shoddy by adding the proper flags and clear/traverse methods such to ensure that cyclic garbage collection was properly handled.  I'm not quite sure why I had to do this since my shoddy instance did not have any cyclic references but I'm wondering if it was necessary since it's a subclass of list which has all the cyclic GC machinery in place.  I suspect I won't understand the answer but if someone with more knowledge can clear things up, that would great.

On Thursday, June 12, 2014 10:39:27 PM UTC-6, ptb wrote:
> Hello all,
> 
> 
> 
> I decided to play around with the C-API and have gotten stuck.  I went through the Shoddy example (https://docs.python.org/3/extending/newtypes.html#subclassing-other-types) in the docs and tried to extend it by adding a method which creates and returns a shoddy instance.  I dug around to find ways to allocate and initialize my shoddy instance and that seems to work well.  However, I get segfaults when I try to delete my instance.  The code is in the gist: https://gist.github.com/pbrady/f2daf50761e458bbe44a
> 
> 
> 
> The magic happens in the make_a_shoddy function.
> 
> 
> 
> Here's a sample session (Python 3.4.1)
> 
> 
> 
> >>> from shoddy import make_a_shoddy()
> 
> >>> shd = make_a_shoddy()
> 
> tup build
> 
> shd allocated
> 
> list style allocation successful
> 
> Py_SIZE(list) : 5
> 
> Py_SIZE(shoddy) : 5
> 
> >>> type(shd)
> 
> <class 'shoddy.Shoddy'>
> 
> >>> shd[:]
> 
> [1, 2, 3, 4, 5]
> 
> >>> shd.increment()
> 
> 1
> 
> >>> shd.increment()
> 
> 2
> 
> >>> del shd
> 
> Segmentation fault (core dumped)
> 
> 
> 
> This happens even if I don't set the destructor.  Any ideas on what I am doing wrong?
> 
> 
> 
> Thanks,
> 
> Peter.