how to return numpy.array from boost::python?

Question

I would like to return some data from c++ code as a numpy.array object. I had a look at boost::python::numeric, but its documentation is very terse

Answer 1

It's a bit late, but after many unsuccessful tries I found a way to expose c++ arrays as numpy arrays directly. Here is a short C++11 example using boost::python and Eigen:

#include 
#include 

#include 

// c++ type
struct my_type {
  Eigen::Vector3d position;
};


// wrap c++ array as numpy array
static boost::python::object wrap(double* data, npy_intp size) {
  using namespace boost::python;

  npy_intp shape[1] = { size }; // array size
  PyObject* obj = PyArray_New(&PyArray_Type, 1, shape, NPY_DOUBLE, // data type
                              NULL, data, // data pointer
                              0, NPY_ARRAY_CARRAY, // NPY_ARRAY_CARRAY_RO for readonly
                              NULL);
  handle<> array( obj );
  return object(array);
}



// module definition
BOOST_PYTHON_MODULE(test)
{
  // numpy requires this
  import_array();

  using namespace boost::python;

  // wrapper for my_type
  class_< my_type >("my_type")
    .add_property("position", +[](my_type& self) -> object {
        return wrap(self.position.data(), self.position.size());
      });

}

The example describes a "getter" for the property. For the "setter", the easiest way is to assign the array elements manually from a boost::python::object using a boost::python::stl_input_iterator.