Cannot convert 'vector<unsigned long>' to Python object

Question

I am trying to wrap a c++ function with signature

vector < unsigned long > Optimized_Eratosthenes_sieve(unsigned long max)

using Cython. I have a file sieve.h containing the function, a static library sieve.a and my setup.py is as follows:

from distutils.core import setup
from distutils.extension import Extension
from Cython.Distutils import build_ext

ext_modules = [Extension("sieve",
                     ["sieve.pyx"],
                     language='c++',
                     extra_objects=["sieve.a"],
                     )]

setup(
  name = 'sieve',
  cmdclass = {'build_ext': build_ext},
  ext_modules = ext_modules
)

In my sieve.pyx I am trying:

from libcpp.vector cimport vector

cdef extern from "sieve.h":
    vector[unsigned long] Optimized_Eratosthenes_sieve(unsigned long max)

def OES(unsigned long a):
    return Optimized_Eratosthenes_sieve(a) # this is were the error occurs

but I am getting this "Cannot convert 'vector' to Python object" error. Am I missing something?

SOLUTION: I have to return a python object from my OES function:

def OES(unsigned long a):
    cdef vector[unsigned long] aa
    cdef int N
    b = []
    aa = Optimized_Eratosthenes_sieve(a)
    N=aa.size()
    for i in range(N):
        b.append(aa[i]) # creates the list from the vector
    return b

Answer 1

If you only need to call you function for C++, declare it with cdef instead of def.

On the other hand, if you need to call it from Python, your function has to return a Python object. In this case, you'll probably make it return a Python list of integers.