Converting c++ array to python list using SWIG

Question

I would like to have a method in C++ that returns an array of uint8_t, and then using SWIG make the function available in python as a list, something like:

class MyClass
{
uint8_t array_return() {
uint8_t random_array[100];
return random_array;
}
};

and then in Python:

object = MyClass
my_list = object.array_return()

I am quite new to SWIG, and not super proficient in C++, and other issues on stack overflow didn't seem to be working.

How to convert a C++ array to a Python list using SWIG? This answer didn't seem to be working (using std::vector)

Also tried mapping directly in the .i file:

%typemap(out) uint8_t *array_return %{
  $result = PyList_New(100);
  for (int i = 0; i < 100; ++i) {
    PyList_SetItem($result, i, PyFloat_FromDouble($1[i]));
  }
%}

In the end I would always get a SwigPyObject and it would not be subscriptable/iterable

SWIG is a great project. However if you are only targeting Python, you would usually be better off using pybind11 (and std library types like vector rather than C arrays). Documentation for that project is excellent. — TooTone
– TooTone, Commented Sep 14, 2023 at 14:27

Criminal_Affair_At_SO · Accepted Answer · 2023-09-14 12:57:20Z

0

std::vector has an official SWIG example: https://github.com/swig/swig/tree/master/Examples/python/std_vector

The resulting object will behave the same as a built-in Python list.

When it comes to your example, there is a large number of problems with it, but none related to the typemap itself which is completely correct.

Here it is a correct working version:

%module myarray

%typemap(out) uint8_t *array_return %{
  $result = PyList_New(100);
  for (int i = 0; i < 100; ++i) {
    PyList_SetItem($result, i, PyLong_FromLong($1[i]));
  }
%}

%inline %{
class MyClass
{
  uint8_t random_array[100];
public:
  uint8_t *array_return() {
    return random_array;
  }
};
%}

Python test:

import myarray
o = myarray.MyClass()
print(o.array_return())

answered Sep 14, 2023 at 12:57

Criminal_Affair_At_SO

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