Python SWIG: convert C++ return parameter to return value, and convert raw C++ type to Python type

Question

I'm trying to modify an existing SWIG Python interface for a C++ library, to add Python wrappers for more functions, and I would greatly appreciate some help from someone experienced with SWIG.

Specifically I'm working on a function with a signature like this:

void execute(int x, double y, ResultType& result1, ResultType& result2);

This function accepts two empty ResultType objects and fills them in as a output parameters. In Python, this has to translate to a function that takes only x and y, then returns a tuple of result1 and result2.

ResultType is a container type that is used widely throughout the library.

typemap(in)

From research, I think I understand that I need to add a typemap "in" for result1 and result2, which swallows the arguments and saves them to temporary variables. I also found that the reference is converted to a pointer by SWIG, hence &temp instead of temp. Here is my typemap "in":

typemap(in, numinputs=0) ResultType& result1 (ResultType temp) {
    $1 = &temp;
}

typemap(in, numinputs=0) ResultType& result2 (ResultType temp) {
    $1 = &temp;
}

typemap(argout)

Next, I added a typemap "argout" that appends the values to a return tuple:

%typemap(argout) ResultType& result1 {
    $result = SWIG_Python_AppendOutput($result, temp$argnum);
}

%typemap(argout) ResultType& result2 {
    $result = SWIG_Python_AppendOutput($result, temp$argnum);
}

However, this obviously won't work, because temp$argnum will be of the raw C++ type ResultType, whereas I need to have a PyObject * in order to append to a tuple. ResultType already has a working SWIG wrapper. So, in Python I can call ResultType() to construct an instance of it without a problem. Assuming that I am on the right track so far, how do I convert the raw C++ ResultType object to a PyObject * belonging to the SWIG-generated wrapper for ResultType? (Sorry if too much detail, I'm trying to avoid the "XY Problem")

Answer 1

Just like $1 is a reference to the Python input object in the input typemap, $1 is a reference to the C++ output variable in the argout typemap. Using this, you can generate a Python object for that data and append it to the result.

Here's a functional example for Windows:

test.h

#ifdef EXPORT
#define API __declspec(dllexport)
#else
#define API __declspec(dllimport)
#endif

struct ResultType
{
    int x;
    double y;
};

API void execute(int x, double y, ResultType& result1, ResultType& result2);

test.cpp

#define EXPORT
#include "test.h"

API void execute(int x, double y, ResultType& result1, ResultType& result2)
{
    result1.x = 2 * x;
    result1.y = 2 * y;
    result2.x = 3 * x;
    result2.y = 3 * y;
}

test.i

%module test

%{
#include "test.h"
%}

%include <windows.i>

%typemap(in,numinputs=0) ResultType& %{
    // Create a persistent object to hold the result;
    $1 = new ResultType;
%}

%typemap(argout) ResultType& (PyObject* tmp) %{
    // Store the persistent object in a PyObject* that will be destroyed
    // when it goes out of scope.
    tmp = SWIG_NewPointerObj($1, $1_descriptor, SWIG_POINTER_OWN);
    $result = SWIG_Python_AppendOutput($result, tmp);
%}

%include "test.h"

Output

>>> import test
>>> r = test.execute(2,3)
>>> r[0].x
4
>>> r[0].y
6.0
>>> r[1].x
6
>>> r[1].y
9.0