[Numpy-discussion] numpy, Py_ssize_t, cython and 64 bits python 2.4

Charles R Harris charlesr.harris@gmail....
Sun Nov 9 22:23:28 CST 2008


On Sun, Nov 9, 2008 at 8:38 PM, David Cournapeau <cournape@gmail.com> wrote:

> Hi Dag,
>
> On Mon, Nov 10, 2008 at 8:29 AM, Dag Sverre Seljebotn
> <dagss@student.matnat.uio.no> wrote:
>
> >
> > Robert is right, it could just as well say "ctypedef int npy_intp".
> > Perhaps it should (but it would not fix the problem). I didn't think too
> > much about it, just copied the definition I found in the particular
> > NumPy headers on my drive, knowing it wouldn't make a difference.
> >
> > Some comments on the real problem:
> >
> > What the Cython numpy.pxd file does is implementing PEP 3118 [1], which
> > is supported by Cython in all Python versions (ie backported, not
> > following any standard). And, in Py_buffer, the strides and shapes are
> > Py_ssize_t* (which is also backported as David mentions). So, in order
> > to cast the shape and stride arrays and return them in the Py_buffer
> > struct, they need to have the datatype defined by the backported PEP
> > 3118, i.e. the backported Py_ssize_t, i.e. int.
> >
>
> Thanks for those explanation.
>
> > a) Rather than raise an exception on line 56, one can instead create new
> > arrays (using malloc) and copy the contents of shape and strides to
> > arrays with elements of the right size.
> > b) These must then be freed in __releasebuffer__ (under the same
> > conditions).
> > c) Also "info.obj" must then be set to "self". Note that it is set to
> > None on line 91, that line should then be moved.
>
> Ok, sounds easy enough.
>
> > OTOH, one could also opt for changing how PEP 3118 is backported and say
> > that "for Python 2.4 we say that Py_buffer has Py_intptr_t* fields
> > instead". This would be more work to get exactly right, and would be
> > more contrived as well, but is doable if one really wants to get rid of
> > the extra mallocs.
>
> This sounds like the right solution, but OTOH, since I am not familiar
> with the cython codebase at all, I will prepare a patch following the
> first solution, and I guess you can always improve it later.
>

I think this is the way to go. However, I think the relevant variables in
Py_buffer (there are three)  should have their own type that is set earlier
when Cython does all it's Python version voodoo. This makes it easy to add
various exceptions if needed. I can't seem to find that spot, though.

Chuck
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