[Python-Dev] How to spell PyInstance_NewRaw in py3k? (original) (raw)

Alexander Belopolsky alexander.belopolsky at gmail.com
Thu Jul 15 01:24:28 CEST 2010

I am reposting the same question again because it seems to have gone unnoticed. Antoine Pitrou and I had a brief discussion on the tracker, but did not reach an agreement on whether a more elaborate code is needed to replace PyInstance_NewRaw than a simple type->tp_alloc() call.

I have reviewed the patch again and I am convinced that this issue comes into play only when 3.x loads 2.x pickles that contain instances of classic classes. (Specifically, this code is used to process INST and OBJ pickle opcodes that are not produced by 3.x.) This means that Antoine's concern that "tomorrow [object_new()] may entail additional operations" is not valid - there is no tomorrow for 2.x. :-) This also means that the type cannot inherit from anything other than object and thus cannot have funny tp_flags or tp_alloc that does not create a usable object.

I would like to commit the patch as presented. If a corner case is discovered later where type->tp_alloc() is not sufficient, we can deal with it then.

On Mon, Jun 28, 2010 at 3:59 PM, Alexander Belopolsky <alexander.belopolsky at gmail.com> wrote:

Issue #5180 [1] presented an interesting challenge: how to unpickle instances of old-style classes when a pickle created with 2.x is loaded in 3.x python?  The problem is that pickle protocol requires that unpickled instances be created without calling the init method.   This is necessary because pickle file may not contain information about how init method should be invoked.  Instead, implementations are required to bypass  init and populate instance's dict directly using data found in the pickle.

Pure python implementation uses the following trick that happens to work in 3.x: class Empty:  pass pickled = Empty() pickled.class = Pickled This of course, creates a new-style class in 3.x, but if 3.x version of Pickled behaves similarly to its 2.x predecessor, it should work. The cPickle implementation, on the other hand uses 2.x C API which is not available in 3.x.  Namely, the PyInstanceNewRaw function.  In order to fix the bug described in issue #5180, I had to emulate PyInstanceNewRaw using type->tpalloc.  I considered an rejected the idea to use tpnew instead. [2] Is this the right way to proceed?  The patch is attached to the issue. [3]

[1] http://bugs.python.org/issue5180 [2] http://bugs.python.org/issue5180#msg108846 [3] http://bugs.python.org/file17792/issue5180.diff

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