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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