[Numpy-discussion] Re: getting data pointer to numpy object in Pyrex

[Robert Kern]

Brian Blais wrote:
> Hello,
> 
> I posted this on the Pyrex list, but I figured someone here may be able
> to answer it too.
> 
> Is here an easy way, in Pyrex, to get the data pointer to a numpy object
> which is *not* passed as a parameter?  For example, I know I can do:
> 
> #---------------
> cdef extern from "numpy/arrayobject.h":
> 
>   struct PyArray_Descr:
>     int type_num, elsize
>     char type
> 
>   ctypedef class numpy.ArrayType [object PyArrayObject]:
>     cdef char *data
>     cdef int nd
>     cdef int *dimensions, *strides
>     cdef object base
>     cdef PyArray_Descr *descr
>     cdef int flags
> 
> def test1(ArrayType w):  # pass a numpy object
> 
>     cdef double *w_p
> 
>     # get the pointers
>     w_p=<double *>w.data
> #---------------
> 
> ...but I would like to do something like...
> 
> #-------------------------------
> def test2(d):  # pass a python dict
> 
>     w=d['w']   # get the numpy object
> 
>     cdef double *w_p
> 
>     # get the pointers
>     w_p=<double *>w.data
> #-------------------------------
> 
> 
> Is there a way to do this?

You will probably want to use the c_numpy.pxd file in numpy/doc/pyrex/. It has
exposed a bit more of the API than you have. If you have any contributions, we
would like to fold them in.

In general, you would just make a cast like you would for any other C type.
However, you must be sure that the object actually is an array. I think the
following code would work as expected, including the case where d['w'] can't be
turned into an array, but I have not tested it.



cimport c_numpy
import numpy

c_numpy.import_array()

def test2(d):
    cdef c_numpy.ndarray w

    w = <c_numpy.ndarray>numpy.asarray(d['w'])
    # ...



-- 
Robert Kern
robert.kern at gmail.com

"I have come to believe that the whole world is an enigma, a harmless enigma
 that is made terrible by our own mad attempt to interpret it as though it had
 an underlying truth."
  -- Umberto Eco