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From: Peter Otten <__peter__@web.de>
Subject: Re: Pickling extension types
Date: Tue, 03 May 2011 19:39:27 +0200
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Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:4562

Stefan Kuzminski wrote:

> I have an extension type written in C, but I cannot get it to pickle, any
> insights would be greatly appreciated.
> 
> I see in the docs that I should define a __reduce__ method and that does
> get called, but I don't know specifically the type of the 'callable
> object' that should be the first thing in the tuple returned by
> __reduce__.

I haven't written a python extension in C, but I would approach the problem 
by looking for prior art in the stdlib, preferably a simple class:

>>> 1j.__reduce__()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.6/copy_reg.py", line 70, in _reduce_ex
    raise TypeError, "can't pickle %s objects" % base.__name__
TypeError: can't pickle complex objects

OK, complex does something else... *

One grep through the source later:

>>> slice(1,2,3).__reduce__()
(<type 'slice'>, (1, 2, 3))

And the implementation seems straightforward:

static PyObject *
slice_reduce(PySliceObject* self)
{
    return Py_BuildValue("O(OOO)", Py_TYPE(self), self->start, self->stop, 
self->step);
}



(*) it uses __getnewargs__