How can I implement a C++ class in Python, to be called by C++?

I have a class interface written in C++. I have a few classes that implement this interface also written in C++. These are called in the context of a larger C++ program, which essentially implements "main". I want to be able to write implementations of this interface in Python, and allow them to be used in the context of the larger C++ program, as if they had been just written in C++.

There's been a lot written about interfacing python and C++ but I cannot quite figure out how to do what I want. The closest I can find is here: http://www.cs.brown.edu/~jwicks/boost/libs/python/doc/tutorial/doc/html/python/exposing.html#python.class_virtual_functions, but this isn't quite right.

To be more concrete, suppose I have an existing C++ interface defined something like:

// myif.h
class myif {
   public:
     virtual float myfunc(float a);
};

What I want to be able to do is something like:

// mycl.py
... some magic python stuff ...
class MyCl(myif):
  def myfunc(a):
    return a*2

Then, back in my C++ code, I want to be able to say something like:

// mymain.cc
void main(...) {
  ... some magic c++ stuff ...
  myif c = MyCl();  // get the python class
  cout << c.myfunc(5) << endl;  // should print 10
}

I hope this is sufficiently clear ;)

There's two parts to this answer. First you need to expose your interface in Python in a way which allows Python implementations to override parts of it at will. Then you need to show your C++ program (in main how to call Python.

Exposing the existing interface to Python:

The first part is pretty easy to do with SWIG. I modified your example scenario slightly to fix a few issues and added an extra function for testing:

// myif.h
class myif {
   public:
     virtual float myfunc(float a) = 0;
};

inline void runCode(myif *inst) {
  std::cout << inst->myfunc(5) << std::endl;
}

For now I'll look at the problem without embedding Python in your application, i.e. you start excetion in Python, not in int main() in C++. It's fairly straightforward to add that later though.

First up is getting cross-language polymorphism working:

%module(directors="1") module

// We need to include myif.h in the SWIG generated C++ file
%{
#include <iostream>
#include "myif.h"
%}

// Enable cross-language polymorphism in the SWIG wrapper. 
// It's pretty slow so not enable by default
%feature("director") myif;

// Tell swig to wrap everything in myif.h
%include "myif.h"

To do that we've enabled SWIG's director feature globally and specifically for our interface. The rest of it is pretty standard SWIG though.

I wrote a test Python implementation:

import module

class MyCl(module.myif):
  def __init__(self):
    module.myif.__init__(self)
  def myfunc(self,a):
    return a*2.0

cl = MyCl()

print cl.myfunc(100.0)

module.runCode(cl)

With that I was then able to compile and run this:

swig -python  -c++ -Wall myif.i 
g++ -Wall -Wextra -shared -o _module.so myif_wrap.cxx -I/usr/include/python2.7 -lpython2.7

python mycl.py 
200.0
10

Exactly what you'd hope to see from that test.

Embedding the Python in the application:

Next up we need to implement a real version of your mymain.cc. I've put together a sketch of what it might look like:

#include <iostream>
#include "myif.h"
#include <Python.h>

int main()
{
  Py_Initialize();

  const double input = 5.0;

  PyObject *main = PyImport_AddModule("__main__");
  PyObject *dict = PyModule_GetDict(main);
  PySys_SetPath(".");
  PyObject *module = PyImport_Import(PyString_FromString("mycl"));
  PyModule_AddObject(main, "mycl", module);

  PyObject *instance = PyRun_String("mycl.MyCl()", Py_eval_input, dict, dict);
  PyObject *result = PyObject_CallMethod(instance, "myfunc", (char *)"(O)" ,PyFloat_FromDouble(input));

  PyObject *error = PyErr_Occurred();
  if (error) {
    std::cerr << "Error occured in PyRun_String" << std::endl;
    PyErr_Print();
  }

  double ret = PyFloat_AsDouble(result);
  std::cout << ret << std::endl;

  Py_Finalize();
  return 0;
}

It's basically just standard embedding Python in another application. It works and gives exactly what you'd hope to see also:

g++ -Wall -Wextra -I/usr/include/python2.7 main.cc -o main -lpython2.7
./main
200.0
10
10

The final piece of the puzzle is being able to convert the PyObject* that you get from creating the instance in Python into a myif *. SWIG again makes this reasonably straightforward.

First we need to ask SWIG to expose its runtime in a headerfile for us. We do this with an extra call to SWIG:

swig -Wall -c++ -python -external-runtime runtime.h

Next we need to re-compile our SWIG module, explicitly giving the table of types SWIG knows about a name so we can look it up from within our main.cc. We recompile the .so using:

g++ -DSWIG_TYPE_TABLE=myif -Wall -Wextra -shared -o _module.so myif_wrap.cxx -I/usr/include/python2.7 -lpython2.7

Then we add a helper function for converting the PyObject* to myif* in our main.cc:

#include "runtime.h"
// runtime.h was generated by SWIG for us with the second call we made

myif *python2interface(PyObject *obj) {
  void *argp1 = 0;
  swig_type_info * pTypeInfo = SWIG_TypeQuery("myif *");

  const int res = SWIG_ConvertPtr(obj, &argp1,pTypeInfo, 0);
  if (!SWIG_IsOK(res)) {
    abort();
  }
  return reinterpret_cast<myif*>(argp1);
}

Now this is in place we can use it from within main():

int main()
{
  Py_Initialize();

  const double input = 5.5;

  PySys_SetPath(".");
  PyObject *module = PyImport_ImportModule("mycl");

  PyObject *cls = PyObject_GetAttrString(module, "MyCl");
  PyObject *instance = PyObject_CallFunctionObjArgs(cls, NULL);

  myif *inst = python2interface(instance);
  std::cout << inst->myfunc(input) << std::endl;

  Py_XDECREF(instance);
  Py_XDECREF(cls);

  Py_Finalize();
  return 0;
}

Finally we have to compile main.cc with -DSWIG_TYPE_TABLE=myif and this gives:

./main
11

Minimal example; note that it is complicated by the fact that Base is not pure virtual. There we go:

1. baz.cpp:

   #include<string>
   #include<boost/python.hpp>
   using std::string;
   namespace py=boost::python;
   
   struct Base{
     virtual string foo() const { return "Base.foo"; }
     // fooBase is non-virtual, calling it from anywhere (c++ or python)
     // will go through c++ dispatch
     string fooBase() const { return foo(); }
   };
   struct BaseWrapper: Base, py::wrapper<Base>{
     string foo() const{
       // if Base were abstract (non-instantiable in python), then
       // there would be only this->get_override("foo")() here
       //
       // if called on a class which overrides foo in python
       if(this->get_override("foo")) return this->get_override("foo")();
       // no override in python; happens if Base(Wrapper) is instantiated directly
       else return Base::foo();
     }
   };
   
   BOOST_PYTHON_MODULE(baz){
     py::class_<BaseWrapper,boost::noncopyable>("Base")
       .def("foo",&Base::foo)
       .def("fooBase",&Base::fooBase)
     ;
   }
   

2. bar.py

   import sys
   sys.path.append('.')
   import baz
   
   class PyDerived(baz.Base):
     def foo(self): return 'PyDerived.foo'
   
   base=baz.Base()
   der=PyDerived()
   print base.foo(), base.fooBase()
   print der.foo(), der.fooBase()
   

3. Makefile

   default:
          g++ -shared -fPIC -o baz.so baz.cpp -lboost_python `pkg-config python --cflags`
   

And the result is:

Base.foo Base.foo
PyDerived.foo PyDerived.foo

where you can see how fooBase() (the non-virtual c++ function) calls virtual foo(), which resolves to the override regardless whether in c++ or python. You could derive a class from Base in c++ and it would work just the same.

EDIT (extracting c++ object):

PyObject* obj; // given
py::object pyObj(obj); // wrap as boost::python object (cheap)
py::extract<Base> ex(pyObj); 
if(ex.check()){ // types are compatible
  Base& b=ex(); // get the wrapped object
  // ...
} else {
  // error
}

// shorter, thrwos when conversion not possible
Base &b=py::extract<Base>(py::object(obj))();

Construct py::object from PyObject* and use py::extract to query whether the python object matches what you are trying to extract: PyObject* obj; py::extract<Base> extractor(py::object(obj)); if(!extractor.check()) /* error */; Base& b=extractor();