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[ Inheriting Base from C++ in Python, call abstract method using SWIG ]

I'm having some trouble coupling C++ (98) with python 3. I have some base classes in C++ which I'd like to extend in Python. Some methods in question are pure virtual on the C++ side and will thus be implemented on the Python side.

Currently, I can call the abstract methods from C++ and, over swig, the specialization gets called in Python. Cool. I'm having trouble handing over parameters to Python..

Minimal complete example to simplify my problem:

// iBase.h
#pragma once
#include <memory>

typedef enum EMyEnumeration{


class FooBase{
    int a;
    virtual int getA() = 0 ; 

class Foo : public FooBase{
    Foo() {a = 2;}
    int getA(){return a;}

class iBase{

    virtual void start() =0;
    virtual void run(std::shared_ptr<FooBase> p, TEMyEnumeration enumCode) = 0;

On the swig side:

// myif.i
%module(directors="1") DllWrapper

#include <iostream>
#include "iBase.h"

%include <std_shared_ptr.i>
%feature("director") FooBase;
%feature("director") iBase;
%include "iBase.h"

Run swig as:

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

Compile myif_wrap.cxx -> _DllWrapper.pyd

Create an *.exe with the following code, it will load up the _DllWrapper.pyd library (make sure it's in the same directory!). Also, copy DllWrapper.py generated by swig to the exe directory.

#include "stdafx.h"
#include <Python.h>
#include <windows.h> 
#include <string>
#include <memory>
#include "iBase.h"
#include "runtime.h"
using namespace std;

int _tmain(int argc, _TCHAR* argv[])
    string moduleName = "ExampleSmartPtr";

    // load *.pyd  (actually a dll file which implements PyInit__<swigWrapperName>)
    auto handle =LoadLibrary("_DllWrapper.pyd");

    // getting an instance handle..
    PyObject *main = PyImport_AddModule("__main__");
    PyObject *dict = PyModule_GetDict(main);

    PyObject *module = PyImport_Import(PyString_FromString(moduleName.c_str()));
    PyModule_AddObject(main, moduleName.c_str(), module);
    PyObject *instance = PyRun_String(string(moduleName+string(".")+moduleName+string("()")).c_str(), Py_eval_input, dict, dict);

    //calling start() in the Python derived class..
    //PyObject *result = PyObject_CallMethod(instance, "start", (char *)"()");

    // trying to call run in the Python derived class..

    shared_ptr<Foo> foo = make_shared<Foo>();
    EMyEnumeration enumCode = EMyEnumeration_Two;

    string  typeName1 = "std::shared_ptr <FooBase> *"; 
    swig_type_info* info1 = SWIG_TypeQuery(typeName1.c_str());
    auto swigData1 = SWIG_NewPointerObj((void*)(&foo), info1, SWIG_POINTER_OWN);

    string  typeName2 = "TEMyEnumeration *"; 
    swig_type_info* info2 = SWIG_TypeQuery(typeName2.c_str());
    auto swigData2 = SWIG_NewPointerObj((void*)(&enumCode), info2, SWIG_POINTER_OWN);

    auto result = PyObject_CallMethod(instance, "run", (char *)"(O)(O)", swigData1, swigData2);

    return 0;

Create a new Python file and put it in the exe's directory:

import DllWrapper

class ExampleSmartPtr(DllWrapper.iBase):
    def __init__(self): # constructor

    def start(self):
        return 0

    def run(self, data, enumCode):
        print("-> data: "+str(data))
        print("-> enumCode: "+str(enumCode))

        print (data.getA())
        return 1

The output of running the exe is :

-> data: (<DllWrapper.FooBase; proxy of <Swig Object of type 'std::shared_ptr< FooBase > *' at 0x00000000014F8B70> >,)
-> enumCode: (<Swig Object of type 'TEMyEnumeration *' at 0x00000000014F89F0>,)

How can one 'dereference' the enumCode to a simple int? How does one call print (data.getA()) in python class run()? In its current form it doesn't print anything..

Answer 1

This is not really an answer, but I read the discussion Discussion from 2005 and it makes sense, that it shouldn't be possible. If you on the Python side, do the following, you get your enumeration 'dereferenced' to a simple integer.

import ExampleSmartPtr

instance = ExampleSmartPtr.ExampleSmartPtr()
swigData1 = ExampleSmartPtr.DllWrapper.Foo()
swigData2 = ExampleSmartPtr.DllWrapper.EMyEnumeration_Two

This will print

-> data: <DllWrapper.Foo; proxy of <Swig Object of type 'std::shared_ptr< Foo > *' at 0x7f8825c0b7e0> >
-> enumCode: 2

I think that the issue is that two different vtables are into play. The original C++ vtable and that of the Swig Object. Just curious, in what scenario is it of interest to use a Python descendant of a C++ class from within C++?

Answer 2

It seems somebody else tried the exact same thing !

What I did was compile the *.pyd with -DSWIG_TYPE_TABLE=iBase.

Then I added this to the main application on the cpp side:

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

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

and called the implementation form python like this:

auto foo = make_shared<Foo>();
TEMyEnumeration enumCode = EMyEnumeration_Two;
python2interface(instance)->run(foo, enumCode);

To finish off, I compiled the C++ implementation again with -DSWIG_TYPE_TABLE=iBase.

Works like a charm!