Cython - implementing callbacks

Question

I have been working with Cython in an attempt to interface with a library written in c++. So far things are going pretty good, and I can effectively use MOST functions within th

Answer 1

I've recently been in the situation where I also had to interface an existing C++ library with Python using Cython, making an intensive use of events/callbacks. It was not that easy to find sources about this and I would like to put all of this together here :

First of all, the wrapping C++ callback class (based on 'double (METHOD)(void)' prototype, but it could have been templatized, since Cython can handle templates) :

ALabCallBack.h :

#ifndef ALABCALLBACK_H_
#define ALABCALLBACK_H_


#include 

using namespace std;

namespace elps {

//template < typename ReturnType, typename Parameter >
class ALabCallBack {
public:

    typedef double (*Method)(void *param, void *user_data);

    ALabCallBack();
    ALabCallBack(Method method, void *user_data);
    virtual ~ALabCallBack();

    double cy_execute(void *parameter);

    bool IsCythonCall()
    {
        return is_cy_call;
    }

protected:

    bool is_cy_call;

private:

    //void *_param;
    Method _method;
    void *_user_data;

};


} /* namespace elps */
#endif /* ALABCALLBACK_H_ */

ALabCallBack.cpp :

#include "ALabCallBack.h"

namespace elps {


ALabCallBack::ALabCallBack() {
    is_cy_call = true;
};

ALabCallBack::~ALabCallBack() {
};

ALabCallBack::ALabCallBack(Method method, void *user_data) {
    is_cy_call = true;
    _method = method;
    _user_data = user_data;
};

double ALabCallBack::cy_execute(void *parameter)
{
    return _method(parameter, _user_data);
};


} /* namespace elps */

Where :

• 'callback' :: The pattern/converter method to fire a Python (=Method) object method from C typed infos

• 'method' :: The effective method passed by the Python user (=user_data)

• 'parameter' :: The parameter to be passed to the 'method'

Now, we need to implement the .pyx file...

Our base prototype :

ctypedef double (*Method)(void *param, void *user_data)

Then, we provide a Cython wrapper for the C++ class :

cdef extern from "../inc/ALabCallBack.h" namespace "elps" :
    cdef cppclass ALabCallBack:
        ALabCallBack(Method method, void *user_data)
        double cy_execute(void *parameter)

The pattern/converter method to be used for translating C typed prototype to a Python object call :

cdef double callback(void *parameter, void *method):
    return (