Extending Python: process numpy arrays

Question

I wrote an extension for Python 3 in C++

My module is capable of handling arrays like [1.0, 0.0, 0.0]. I want to add support for numpy arrays as well.

I process arrays with the following code:

PyObject * MyFunction(PyObject * self, PyObject * args) {
    PyObject * list;

    if (!PyArg_ParseTuple(args, "O!:MyFunction", PyList_Type, &list)) {
        return 0;
    }

    int count = (int)PyList_Size(list);
    for (int i = 0; i < count; ++i) {
        double value = PyFloat_AsDouble(PyList_GET_ITEM(list, i));

        // ...
    }
}

I want a function that can iterate through this: np.array([2,3,1,0])

TL;DR:

Numpy equivalent for:

PyList_Type
PyList_Size
PyList_GET_ITEM or PyList_GetItem

No real answer and maybe you got good reasons to use this native way of interfacing c++, but did you consider cython. This makes all that stuff much easier/nicer and is used by some very cool projects (e.g. scikit-learn). — sascha
– sascha, Commented Jul 31, 2016 at 18:03
normally, it's the other way round: you write a numpy-function, which also can handle lists. — Daniel
– Daniel, Commented Jul 31, 2016 at 18:04
I just wan't to be able to iterate through a numpy array in C++ — Szabolcs Dombi
– Szabolcs Dombi, Commented Jul 31, 2016 at 18:11
I'm not an expert, but what's the problem there? Cython surely needs a C++ compiler (which could be awkward for windows-users) if you are interfacing C++ code, but the same should apply to the native-bindings. A lot of projects use cython and are available on PyPI. — sascha
– sascha, Commented Jul 31, 2016 at 18:18

Szabolcs Dombi · Accepted Answer · 2016-09-13 19:50:41Z

First of all there is no numpy equivalent for:

PyList_Type
PyList_Size
PyList_GET_ITEM or PyList_GetItem

The numpy.array implements the buffer interface, so one can write:

const char * data;
int size;

PyArg_ParseTuple(args, "y#:MyFunction", &data, &size);

The numpy.array([1.0, 0.0, 0.0]) uses double precision:

double * array = (double *)data;
int length = size / sizeof(double);

The full example:

C++

PyObject * MyFunction(PyObject * self, PyObject * args) {
    const char * data;
    int size;

    if (!PyArg_ParseTuple(args, "y#:MyFunction", &data, &size)) {
        return 0;
    }

    double * content = (double *)data;
    int length = size / sizeof(double);

    for (int i = 0; i < length; ++i) {
        double value = content[i];

        // ...
    }

    Py_RETURN_NONE;
}

Python

MyFunction(numpy.array([1.0, 2.0, 3.0]))

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