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

Alexander Belopolsky alexander.belopolsky at gmail.com
Mon Jun 28 21:59:00 CEST 2010

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 PyInstance_NewRaw function. In order to fix the bug described in issue #5180, I had to emulate PyInstance_NewRaw using type->tp_alloc. I considered an rejected the idea to use tp_new 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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