Passing a C++ std::Vector to numpy array in Python

Question

I am trying a pass a vector of doubles that I generate in my C++ code to a python numpy array. I am looking to do some downstream processing in P

Answer 1

Since there is no answer to this that is actually helpful for people that might be looking for this sort of thing I figured I'd put an easy solution.

First you will need to create a python extension module in C++, this is easy enough to do and is all in the python c-api documentation so i'm not going to go into that.

Now to convert a c++ std::vector to a numpy array is extremely simple. You first need to import the numpy array header

#include 

and in your intialising function you need to import_array()

PyModINIT_FUNC
inittestFunction(void){
   (void) Py_InitModule("testFunction". testFunctionMethods);
   import_array();
}

now you can use the numpy array functions that are provided. The one that you will want for this is as the OP said a few years back PyArray_SimpleNewFromData, it's stupidly simple to use. All you need is an array of type npy_intp, this is the shape of the array to be created. make sure it is the same as your vector using testVector.size(), (and for multiple dimensions do testVector[0].size(), testVector[0][0].size() ect. vectors are guaranteed to be continuous in c++11 unless it's a bool).

//create testVector with data initialised to 0
std::vector> testVector;
testVector.resize(width, std::vector(height, 0);
//create shape for numpy array
npy_intp dims[2] = {width, height}
//convert testVector to a numpy array
PyArrayObject* numpyArray = (PyArrayObject*)PyArray_SimpleNewFromData(2, dims, NPY_UINT16, (uint16_t*)testVector.data());

To go through the paramaters. First you need to cast it to a PyArrayObject, otherwise it will be a PyObject and when returned to python won't be a numpy array. The 2, is the number of dimensions in the array. dims, is the shape of the array. This has to be of type npy_intp NPY_UINT16 is the data type that the array will be in python. you then use testVector.data() to get the data of the array, cast this to either void* or a pointer of the same data type as your vector.

Hope this helps anyone else who may need this.

(Also if you don't need pure speed I would advise avoiding using the C-API, it causes quite a few problems and cython or swig are still probably your best choices. There is also c types which can be quite helpful.