Question

What is the SWIG equivalent to storing a copy of an arbitrary python object?

I’m pretty sure what I’m asking is possible because it would work with boost::python (see below), but I can’t see a way to do this using SWIG.

#include <boost/python.hpp>

using namespace boost::python;

class MyClass
{
public:
    // other operations
    object get_info() { return info_; }
    void set_info(object info) { info_ = info; }

private:
    object info_;
};

BOOST_PYTHON_MODULE( mymodule )
{
    class_<MyClass>("MyClass")
        .def("get_info", &MyClass::get_info )
        .def("set_info", &MyClass::set_info )
    ;
}

Answer 1

The simplest example is:

%module test

%inline %{
class MyClass
{
public:
    // other operations
    PyObject *get_info() { return info_; }
    void set_info(PyObject *info) { info_ = info; }

private:
    PyObject *info_;
};
%}

e.g.:

swig -python -Wall -c++ test.i
g++ -Wall -Wextra test_wrap.cxx -I/usr/include/python2.6 -o _test.so -shared
python
Python 2.6.6 (r266:84292, Dec 27 2010, 00:02:40)
[GCC 4.4.5] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import test
>>> foo = test.MyClass()
>>> str = "hi"
>>> foo.set_info(str)
>>> print foo.get_info()
hi
>>>

But note that in this case the ownership of the object has not been changed. If I’d done foo.set_info("hi") instead then it would have been released because no references were retained by the time it called foo.get_info().

You can fix that by adding a call to:

Py_INCREF(info);

inside set_info(), but then you’ll need a corresponding DECREF in the destructor, or if set_info() is called when a reference is already held, or for copy construction or assignment. (Or some nice RAII type to do all that for you…)