Python: Get cv::Mat from c++ dll

Question

I created a C++ class, which uses OpenCV. One method returns a cv::Mat which I want to "get" from inside a python file.

#include <iostream>
#include <opencv2/highgui/highgui.hpp>
//#include <opencv2/imgproc/imgproc.hpp>
using namespace cv;
using namespace std;

class webcam{
    private:
        cv::VideoCapture webcamframe;
        cv::Mat frame;
    public:
        webcam();
        cv::Mat nextFrame();
};

webcam::webcam() {
    std::cout << "Init of webcam module \n";
}

cv::Mat webcam::nextFrame() {
    std::cout << "Asking for a new camera frame. \n";
    cv::Mat image = imread("1.jpg", CV_LOAD_IMAGE_COLOR);

    if (!image.data)
        std::cout << "ERROR LOADING!";
    else
        return image;
}

extern "C" {
    webcam* webcam_new() { return new webcam(); }
    cv::Mat frame(webcam* wc) { wc->nextFrame(); }
}

I wrapped the C++ file so that it is possible to use from python. But I can't figure out how to get the cv::Mat?

from cv2 import cv
from ctypes import cdll
import numpy
import ctypes

lib = cdll.LoadLibrary('./webcam.so')

class camCon(object):
    def __init__(self):
        self.obj = lib.webcam_new()

    def frame(self):
        lib.frame(self.obj)

cameraConnection = camCon()
pyMat = cv.CreateMat(640, 480, 0)
pyMat = cameraConnection.frame()
pyMat.Set()
a = numpy.asarray(pyMat)
print("data", a)

The result is a = None.
I verified whether the file is readable from C++ (I checked with imshow), and it is.

What am I doing wrong?