Get access to C++ class created in python from C++

Question

How to do this

c++ -> Python -> c++
 ^               |
 |               |
 -----------------

C++ app is hosting python.
Python creates a class, which is actually a wrapping to c/c++ object
How to get access from hosting c++ to c/c++ pointer of this object created by python?

Example with code:

Imagine I have a C++ class wrapped for python (e.g. using boost.python)

// foo.cpp
#include <boost/python.hpp>

struct Cat {
   Cat (int fur=4): _fur{i} { }
   int get_fur() const { return _fur; }
private:
   int _fur;
};

BOOST_PYTHON_MODULE(foo) {
using namespace boost::python;

class_<Cat>("Cat", init<int>())
   .def("get_fur", &A::get_fur, "Density of cat's fur");

}

Now I'm hosting a python in C++. A python script creates Cat => A c++ Cat instance is created underneath. How to get a pointer to c++ instance of Cat from hosting c++ (from C++ to C++)?

#include <Python.h>

int
main(int argc, char *argv[])
{
  Py_SetProgramName(argv[0]);  /* optional but recommended */
  Py_Initialize();
  PyRun_SimpleString("from cat import Cat \n"
                     "cat = Cat(10) \n");

  // WHAT to do here to get pointer to C++ instance of Cat
   ... ??? ...

  std::cout << "Cat's fur: " << cat->get_fur() << std::endl

  Py_Finalize();
  return 0;
}

Real application The real problem is this: we have a c++ framework which has pretty complex initialization and configuration phase where performance is not critical; and then processing phase, where performance is everything. There is a python wrapping for the framework. Defining things in python is very convenient but running from python is still slower than pure c++ code. It is tempting for many reasons to do this configuration/initialization phase in python, get pointers to underneath C++ objects and then run in "pure c++ mode". It would be easy if we could write everything from scratch, but we have already pretty much defined (30 years old) c++ framework.

the class is from c++ but the instance is from python and you want to access that instance in c++? I am not sure if it is just my confusion or if you confused class/instance in the title/question — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented Nov 8, 2019 at 16:37

Brandon · Accepted Answer · 2019-11-08 19:07:09Z

#include <Python.h>

int main(int argc, char *argv[])
{
  Py_SetProgramName(argv[0]);
  Py_Initialize();

  object module = import("__main__");
  object space = module.attr("__dict__");

  exec("from cat import Cat \n"
       "cat = Cat(10) \n",
       space);

  Cat& cat = extract<Cat&>(space["cat"]);
  std::cout<<"Cat's fur: "<< cat.get_fur() <<"\n";

  //or:

  Cat& cat2 = extract<Cat&>(space.attr("cat"));
  std::cout<<"Cat's fur: "<< cat2.get_fur() <<"\n";

  //or:

  object result = eval("cat = Cat(10)");
  Cat& cat3 = extract<Cat&>(result);
  std::cout<<"Cat's fur: "<< cat3.get_fur() <<"\n";

  Py_Finalize();
  return 0;
}

yeoman · Accepted Answer · 2019-11-08 17:17:27Z

In case the wrapper is open source, use the wrapper's python object struct from C++. Cast the PyObject * to that struct which should have a PyObject as its first member iirc, and simply access the pointer to the C++ instance.

Make sure that the instance is not deleted while you're using it by keeping the wrapper instance around in Python. When it is released, it will probably delete the wrapped C++ instance you now still are holding a pointer to.

And keep in mind that this approach is risky and fragile, as it depends on implementation details of the wrapper that might change in future versions.

Starting point: cpython/object.h

Could you provide some code. E.g. based on the classes in the example?

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