Passing C++ objects to python with boost

Question

I'm trying to figure out how to use boost python to create an object in c++ and pass it into Python. I've managed to do this but then can't get garbage collection to occur.

Imagine the class A is defined somewhere in C++. The passNewAToPython() function is called from somewhere else in the code which creates an A object and then passes it to a callback function in Python. I want that specific instance passing to python and not a copy hence the use of ptr()

static PyObject * pythonCallbacks;

void passNewAToPython()
{
  A * a = new A();
  PyGILState_STATE _GILState = PyGILState_Ensure();
  //Should really use a try catch here too but lets ignore that for now
  boost::python::call_method<void>(pythonCallbacks, "newA", boost::python::ptr(a));
  PyGILState_Release(_GILState);
}

void initmodule(PyObject* userCallCallbacks_)
{
   PyEval_InitThreads();

   pythonCallbacks = userCallCallbacks_;
}

BOOST_PYTHON_MODULE(mymodule)
{
    def("initmodule", initmodule);

    class_<A, boost::noncopyable>("A", init<>());
}

The Python code

import mymodule

class pythonCallbacks(object):
    a_list = [];
    def newA(self, a):
         self.a_list.append(a)

callbacks = pythonCallbacks()
mymodule.initmodule(callbacks)

Now imagine some time later after the newA callback is called. The only place I expect the a instance to be stored is in a_list. So if I delete a from a_list then I would expect the c++ delete to be called on the object I created with new. That never happens and so I leak objects.

I've tried many variants of technique to do this but never managed to make everything work. I would really appreciate a complete example perhaps showing how to modify the above example.

I'm not familiar with boost::python, but where are you releasing the python object reference count on the C++ side? — Arafangion
– Arafangion, Commented Dec 4, 2012 at 23:56
How do you release the python object reference count on the C++ side? I've seen both BOOST and Python API commands for increasing/decreasing the reference count but they only operate on python objects. Here the python object is being created in the call_method function. After call_method is complete I want the only reference to the object to exist through Python so that when all Python references are gone the object gets deleted. I did try creating a boost::python::object out of the pointer and then calling increase / decrease on that but I usually just get a crash. — Dave
– Dave, Commented Dec 5, 2012 at 16:47
So you're specifically transferring ownership of the object, in that case you should decrement the reference count unless the target method does not increment the reference count. — Arafangion
– Arafangion, Commented Dec 5, 2012 at 23:51
Have you ever take a look to PythonQt? It's quite simple to wrap c++ objects to python! — linello
– linello, Commented Dec 16, 2012 at 20:31

Nathan Binkert · Accepted Answer · 2012-12-16 20:25:49Z

1

I'm pretty sure that ptr() has nothing to do with the lifetime of the object being pointed to. It's totally up to you to manage the lifetime if you use it. (I believe using it is like using a PyCObject.)

What you can do is define the pointer type of A to be a shared_ptr (either boost or std, doesn't matter). Something like this:

class_<A, shared_ptr<A>, boost:noncopyable>("A", init<>());

Then you can have your factory function do this:

void passNewAToPython()
{
    A * a = make_shared<A>();
    PyGILState_STATE _GILState = PyGILState_Ensure();     
    //Should really use a try catch here too but lets ignore that for now
    boost::python::call_method<void>(pythonCallbacks, "newA", a);
    PyGILState_Release(_GILState);
 }

answered Dec 16, 2012 at 20:25

Nathan Binkert

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