Pybind11: Create and return numpy array from C++ side

Question

How to create a numpy array from C++ side and give that to python?

I want Python to do the clean up when the returned array is no longer used by Python.

C++ side would not use delete ret; to free the memory allocated by new double[size];.

Is the following correct?

#include "pybind11/pybind11.h"
#include "pybind11/numpy.h"

namespace py = pybind11;

py::array_t<double> make_array(const py::ssize_t size) {
    double* ret = new double[size];
    return py::array(size, ret);
}

PYBIND11_MODULE(my_module, m) {
    .def("make_array", &make_array,
         py::return_value_policy::take_ownership);
}

I have to use the numpy instead of eigen interface of pybind11 because I use a structured numpy array. — R zu
– R zu, Commented Mar 8, 2018 at 18:25

3CEZVQ · Accepted Answer · 2018-03-08 18:10:07Z

16

Your are quite correct. A little better solution is below.

#include "pybind11/pybind11.h"
#include "pybind11/numpy.h"

namespace py = pybind11;

py::array_t<double> make_array(const py::ssize_t size) {
    // No pointer is passed, so NumPy will allocate the buffer
    return py::array_t<double>(size);
}

PYBIND11_MODULE(my_module, m) {
    .def("make_array", &make_array,
         py::return_value_policy::move); // Return policy can be left default, i.e. return_value_policy::automatic
}

edited Mar 8, 2018 at 18:10

answered Mar 8, 2018 at 18:00

3CEZVQ

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5 Comments

R zu Over a year ago

hmm. And this won't make a copy? I am checking if py::array_t has a move constructor.... It seems to have one here: github.com/pybind/pybind11/blob/…

3CEZVQ Over a year ago

Sure it will not make a copy due to return value optimization and copy elision.

R zu Over a year ago

The casting for py::return_value_policy::move is here: github.com/pybind/pybind11/blob/… Thanks.

R zu Over a year ago

Really really wish they talk about this in the official documentation...

John Over a year ago

Does py::array_t initialize the newly allocated buffer? Or should we initialize it ourselves?

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