How to create a numpy array of boost::python::object types

Question

I'm trying to create a 2x2 numpy array of python objects:

#include <boost/python.hpp>
#include <boost/python/numpy.hpp>
int main()
{
    Py_Initialize();
    boost::python::numpy::initialize();
    boost::python::tuple shape = boost::python::make_tuple(2, 2);
    boost::python::object obj;
    boost::python::numpy::dtype dt = boost::python::numpy::dtype(obj);
    boost::python::numpy::ndarray array = boost::python::numpy::empty(shape, dt);
    std::cout << "Datatype is: " << boost::python::extract<char const *> boost::python::str(array.get_dtype())) << std::endl;
}

But the output is "Datatype is: float64" rather than a python object type.

What am I doing wrong?

I suspect I'm misusing the dtype constructor.

Bathsheba · Accepted Answer · 2020-08-07 10:26:36Z

2

+200

You're using the dtype constructor correctly; it's obj that's causing the trouble.

The default construction boost::python::object obj; sets obj as the 'None' Python object. The dtype associated with 'None' is a NPY_DEFAULT array descriptor type. And that maps to a double when creating the numpy array, which explains your output. (That makes sense from a Python perspective - the default numpy array type is a double precision floating point type.)

You can constuct a dtype with an object type (NPY_OBJECT) using

boost::python::numpy::dtype dt = boost::python::numpy::dtype(boost::python::object("O"));

which, in your case, is the fix. I've also taken the liberty of using an anonymous temporary which is how it's done in the Boost documentation. The "O" denotes the object type.

answered Aug 7, 2020 at 10:26

Bathsheba

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