[Numpy-discussion] subclassing array in c

[mark florisson]

On 30 March 2012 21:38, mark florisson <markflorisson88 at gmail.com> wrote:
> On 30 March 2012 19:53, Chris Barker <chris.barker at noaa.gov> wrote:
>> On Fri, Mar 30, 2012 at 10:57 AM, mark florisson
>> <markflorisson88 at gmail.com> wrote:
>>> Although the segfault was caused by a bug in NumPy, you should
>>> probably also consider using Cython, which can make a lot of this pain
>>> and boring stuff go away.
>>
>> Is there a good demo/sample somewhere of an ndarray subclass in Cython?
>>
>> Some quick googling turned up a number of people asking about it, but
>> I didn't find (quickly) a wiki page or demo about it.
>>
>> -Chris
>>
>> --
>>
>> Christopher Barker, Ph.D.
>> Oceanographer
>>
>> Emergency Response Division
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>>
>> Chris.Barker at noaa.gov
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>
> It's not common to do, I tried the following:
>
> cimport numpy
>
> cdef extern from "Python.h":
>    ctypedef struct PyTypeObject:
>        void *tp_alloc
>
>    object PyType_GenericAlloc(PyTypeObject *type, Py_ssize_t nitems)
>
> cdef myalloc(PyTypeObject *type, Py_ssize_t nitems):
>    print "allocating"
>    return PyType_GenericAlloc(type, nitems)
>
> cdef class MyClass(numpy.ndarray) :
>    cdef int array[10000000]
>
> (<PyTypeObject *> MyClass).tp_alloc = <void *> myalloc # This works
> around the NumPy bug
> cdef MyClass obj = MyClass((10,))
> obj.array[999999] = 20
>
> The array attribute is quite large here to cause a segfault if our
> trick to replace the tp_alloc isn't working. It's kind of a hack, but
> the only alternative is to use composition instead.

(So remove the array attribute, it's just for demonstration :)