Can I use generated swig code to convert C++ object to PyObject?

Question

I'm working on embedding python into my C++ program using swig. At the moment I have a object written in C++ which I want to pass to a python function. I've created the swig interface to wrap the class.

What I'm trying to do is take this C++ object which I've created and pass it to a python function with the ability to use it like I would in C++. Is it possible for me to use code generate by swig to do this? If not how can I approach this?

Flexo - Save the data dump · Accepted Answer · 2011-05-02 15:02:05Z

2

You can use PyObject_CallMethod to pass a newly created object back to python. Assuming ModuleName.object is a python object with a method called methodName that you want to pass a newly created C++ object to you want to roughly (from memory, I can't test it right now) do this in C++:

int callPython() {
   PyObject* module = PyImport_ImportModule("ModuleName");
   if (!module)
      return 0;

   // Get an object to call method on from ModuleName
   PyObject* python_object = PyObject_CallMethod(module, "object", "O", module);
   if (!python_object) {
      PyErr_Print();
      Py_DecRef(module);
      return 0;
   }

   // SWIGTYPE_p_Foo should be the SWIGTYPE for your wrapped class and
   // SWIG_POINTER_NEW is a flag indicating ownership of the new object
   PyObject *instance = SWIG_NewPointerObj(SWIG_as_voidptr(new Foo()), SWIGTYPE_p_Foo, SWIG_POINTER_NEW);

   PyObject *result = PyObject_CallMethod(python_object, "methodName", "O", instance);
   // Do something with result?

   Py_DecRef(instance);
   Py_DecRef(result);  
   Py_DecRef(module);

   return 1;
}

I think I've got the reference counting right for this, but I'm not totally sure.

edited May 2, 2011 at 15:02

answered May 1, 2011 at 14:56

Flexo - Save the data dump♦

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6 Comments

Nexus Over a year ago

Thanks for the speedy reply. I was more interested in understanding how you could extend swig to do this. The fact that the object is using the factory design pattern is actually not important. I'll remove reference to it in the question.

Flexo - Save the data dump Over a year ago

Do you want to extend swig itself? Or "inject" custom code? This is a fairly good example of the power of swig - you can splice lots of mixed language code together to solve a fairly complex problem without having to dive into the internals.

Nexus Over a year ago

Inject custom code. I'm just wanting the object that is created in C++ to be passed into Python, to be modified in Python and for that change to be reflected in the C++ object.

Flexo - Save the data dump Over a year ago

That's (more or less) what happens anyway. Can you be more specific in the question perhaps? E.g. simplest testcase with a note on observed vs expected behaviour?

Flexo - Save the data dump Over a year ago

In that case it sounds like you want PyObject_CallMethodObjArgs in C++ to dispatch the call and SWIG_NewPointerObj to wrap the pointer to the new object before making the call.

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