Pybind11: Passing a Python list to C-style array

Question

I would like to pass a Python list to a constructor which takes C-style arrays. How should that work. The problem is that the C-style array is essentially a pointer. Furthermore, the array has dimension n x n, i.e. it is a multi-dimensional array.

     PYBIND11_MODULE(matrix_class_bind, m){
         py::class_<matrix_class<double>>(m, "matrix_class")
         .def(py::init([](double x[3][3]){
          matrix_class<double> new_class(x);
          return new_class;}));
    }

On the python side it should be something like:

import matrix_class_bind as mcb    
a = [[1,2,3], [3,4,5], [1,1,1]]
mcb.matrix_class(a)

Check out xtensor-python. You'd need to write an interface function that takes an xt::pyarray<double> reference and pass to your C-style function by taking the address of the first element similar to a vector/STL array. — rjsberry
– rjsberry, Commented Mar 15, 2018 at 14:11
Did you have a look at the documentation about custom contructors? pybind11.readthedocs.io/en/stable/advanced/…. This would allow you to write a wrapper that converts e.g. list to raw array. — NOhs
– NOhs, Commented Mar 15, 2018 at 14:22

jackw11111 · Accepted Answer · 2018-03-18 09:15:55Z

1

Instead of passing the matrix as a pointer, you could pass it using py::list, if your aim is to access the matrix as a C array.

class matrix_class {
    public:
        static const int n = 3;
        int carray[n][n];
        py::list list;
        matrix_class(const py::list &list) : list(list) {
            for (int i = 0; i < n; i++) {
                py::list l = list[i].cast<py::list>();
                for (int j = 0; j < n; j++){
                    int p = l[j].cast<int>();
                    carray[i][j] = p;
                }
            }
    }
}

PYBIND11_MODULE(matrix_class_bind, m) {
    py::class_<matrix_class>(m, "matrix_class")
        .def(py::init<const py::list &>());
}

answered Mar 18, 2018 at 9:15

jackw11111

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