Cython: can't convert Python object to 'double *'

Question

I\'m writting a Cython wrapper to a C function. I have a pxd file with the following signature:

double contr_hrr(int lena, double xa, double ya, double za, doubl         


        

Answer 1

I think that you cannot do otherwise, but convert it by yourself :

cimport cython
from libc.stdlib cimport malloc, free

...
cdef double *anorms
cdef unsigned int i;

anorms = malloc(len(anorms2)*cython.sizeof(double))
if anorms is NULL:
    raise MemoryError()

for i in xrange(len(anorms2)):
    anorms[i] = anorms2[i]

return contr_hrr(len(acoefs),a.origin[0],a.origin[1],a.origin[2],anorms)

If you had been in C++, this would have been different because

The following coercions are available:
Python type   =>   C++ type             => Python type
bytes              std::string             bytes
iterable           std::vector             list
iterable           std::list               list
iterable           std::set                set
iterable (len 2)   std::pair               tuple (len 2)

If you could switch to C++, you would have a direct translation from List[float] to vector :

from libcpp.vector cimport vector

def py_contr_hrr(vector[double] anorms2, ...):
    ...
    return contr_hrr(len(acoefs),a.origin[0],a.origin[1],a.origin[2],anorms2)

And calling directly from Python side :

anorms2 = [12.0, 0.5, ...]
py_contr_hrr(anorms2, ....)

Source : http://docs.cython.org/src/userguide/wrapping_CPlusPlus.html#standard-library

But I don't know if it is an option that you can consider... It depends on your project's constraints, of course.

EDIT : I didn't know about Nikita's way of doing (which is an elegant one, by the way), and I don't know ether which way is the best suitable concerning performances on big arrays.