将一个 numpy 数组从 Python 传递给 C

Question

I'm having successfully embedded a Python script into a C module. The Python script produces a multi-dimensional Numpy array. Whereas the entire calculation in python takes 9 ms, the final tolist() conversion in order to return it to C takes 4 ms alone. I would like to change that by passing the Numpy array as reference and do the iterations in C again. But I can't currently figure out, how this can be done.

There are a lot of samples around, which use the other way around: Passing a Numpy array to a C function which is called from Python, but this is not my use case.

Any pointer welcome.

Answer 1

Ok, it's a while ago but I solved it like so:

My python process delivers an array, containing one array, containing one array, containing N arrays of M floats each. The input is a JPEG image.

Unwrapping it like so:

int predict(PyObject *pyFunction, unsigned char *image_pointer, unsigned long image_len) {

    int result = -1;

    PyObject *pImage = NULL;
    PyObject *pList = NULL;

    pImage = PyBytes_FromStringAndSize((const char *)image_pointer, image_len);
    if (!pImage) {
        fprintf(stderr, "Cannot provide image to python 'predict'\n");
        return result;
    }

    pList = PyObject_CallFunctionObjArgs(pyFunction, pImage, NULL);
    Py_DECREF(pImage);

    PyArrayObject *pPrediction = reinterpret_cast<PyArrayObject *>(pList);
    if (!pPrediction) {
        fprintf(stderr, "Cannot predict, for whatever reason\n");
        return result;
    }

    if (PyArray_NDIM(pPrediction) != 4) {
        fprintf(stderr, "Prediction failed, returned array with wrong dimensions\n");
    } else {
        RESULTPTR pResult = reinterpret_cast<RESULTPTR>(PyArray_DATA(pPrediction));

        int len0 = PyArray_SHAPE(pPrediction)[0];
        int len1 = PyArray_SHAPE(pPrediction)[1];
        int len2 = PyArray_SHAPE(pPrediction)[2];
        int len3 = PyArray_SHAPE(pPrediction)[3];

        for (int i = 0; i < len0; i++) {
            int offs1 = i * len1;
            for (int j = 0; j < len1; j++) {
                int offs2 = j * len2;
                for (int k = 0; k < len2; k++) {
                    int offs3 = k * len3;
                    for (int l = 0; l < len3; l++) {
                        float f = (*pResult)[offs1 + offs2 + offs3 + l];
                        //printf("data: %.8f\n", f);
                    }
                }
            }
        }
        result = 0;
    }
    Py_XDECREF(pList);
    return result;
}

HTH