Cython PXD does not seem to be used for PYX file

Question

I am trying to learn cython by implementing a linear interpolator from C++ to Python. I am trying to use PXD header files for the final Intepolator object so that it can be reused in other methods / classes down the line, so I want to have the PXD header file available.

I have a cpp_linear_interpolation.cpp and cpp_linear_interpolation.h that work fine, the interpolator gets instantiated with two vectors of double (x and y) as input.

There are my files

cy_linear_interpolation.pxd

# distutils: language = c++

import cython
import numpy as np
cimport numpy as np
from libcpp.vector cimport vector

cdef extern from "cpp_linear_interpolation.h":
    cdef cppclass cppLinearInterpolation:
        cppLinearInterpolation(vector[double], vector[double]) except +
        vector[double] interp(vector[double]) except +

        vector[double] m_x
        vector[double] m_y
        int m_len
        double m_x_min
        double m_x_max

py_linear_interpolation.pxd

from cy_linear_interpolation cimport cppLinearInterpolation

cdef class LinearInterpolation:
     cdef cppLinearInterpolation * thisptr

py_linear_interpolation.pyx

import cython
import numpy as np
cimport numpy as np
from libcpp.vector cimport vector
from cy_linear_interpolation cimport cppLinearInterpolation


cdef class LinearInterpolation:
    # cdef cppLinearInterpolation *thisptr

    def __cinit__(self,vector[double] x,vector[double] y):
        self.thisptr = new cppLinearInterpolation(x, y)

    def __dealloc__(self):
        del self.thisptr

    def interpolate(self,vector[double] x_new):
        return self.thisptr.interp(x_new)

setup.py

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


setup( name = 'Cython_LinInt',
       ext_modules=[Extension("cython_linear_interpolation",
                              sources=["py_linear_interpolation.pyx", "cpp_linear_interpolation.cpp",],
                              language="c++",
                              include_dirs=[numpy.get_include()]) 
       ],
       cmdclass = {'build_ext': build_ext},
)    

compiling with Microsoft (R) C/C++ Optimizing Compiler Version 15.00.30729.01 for x64

I get the error message

Cannot convert 'cppLinearInterpolation *' to Python object

If I move

cdef cppLinearInterpolation * _thisptr

to the pyx file (the commented out line in py_linear_interpolation.pyx), it compiles and run, but then I cannot access the pointer from another cython file. Ideally I would be able to instantiate the interpolator from python, and use it as argument for other python / cython functions. I am sure I must be doing something stupid, but I have been blocked on that problem for a little while now and have not found yet the solution...

EDIT: there was a typo in py_linear_interpolation.pyx, now corrected EDIT 2: the same type was in py_linear_interpolation.pyd, the member name is thisptr, the code still isn t compiling and I get the same error. It does not seem that the cython compiler recognizes that self.thisptr is not a python object and should be a pointer to a cppLinearInterpolation

Answer 1

Change this:

self.thisptr = new cppLinearInterpolation(x, y)

To:

self._thisptr = new cppLinearInterpolation(x, y)